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D-region processes at equatorial latitudes
D-region processes at equatorial latitudes P. A. J. RATNASIRI Ceylon Institute of Scientific and Industrial Research. P.O. Box 787. Colombo, Sri Lanka Abstract-Our current understanding of the ionization production and loss processes, and electron and ion density distributions in the undisturbed D-region are reviewed with special reference to the equatorial latitudes. The importance of combining ground-based measurements with rocket-borne measurements in the develonment of D-region electron densitv orofile models is stressed. The need for coordinated experiments- at low latitudes for investigating outstanding problems related to the quiet D-region is emphasized. 1. INTRODIJCIION
The D-region is the least understood region of the ionosphere. The presence of complex positive ions and negative ions, both of molecular and cluster types, and the unusual behaviour of its electron concentration, both in time and space, are among the features that are of special interest in the D-region. The direct measurement’ of the electron concentration, or of its ion composition, can only be done by means of rocket-borne instrumentation. The region is too low for satellites and too high for balloons. Various types of ground-based radio experiments have provided information about the behaviour of its electron concentration. The results of these experiments, carried out over a number of years, have not yet been satisfactorily interpreted in terms of existing theories concerning the D-region (THOMAS, 1974a). This is mainly because of the non-availability of accurate data on such important factors as the concentrations of minor neutral constituents, reaction rates and solar fluxes and also because of our lack of understanding of basic processes such as the ionpair production and electron loss rates. In this paper, an attempt is made to review the current status of our knowledge regarding these basic ionospheric processes, as applicable to the D-region, and to identify outstanding problems, with special emphasis on equatorial latitudes. It does not cover the processes taking place at night-time or under disturbed conditions. Attention is also drawn to the importance of combining ground-based measurements with rocket experiments, in order to obtain a better understanding of the normal day-time D-region. 2. IONIZING
SOURCES
It is now generally agreed that the dominant source of ionization in the daytime D-region under quiet
conditions is the solar Lyman-alpha line (1216 A). ionizing NO. This is the major source between about 62 km and 85 km at low zenith angles. Above this height, 30-40A X-rays become the major source ionizing N, and 0,, while below 62 km galactic cosmic rays become the major source. 2. I. NO ionization The intensity of the solar Lyman-alpha line has been measured over several years, and it has been observed that it is a fairly stable line, with the intensity varying between 2.4 x IO” and 3.6 x 10’ ’ photons cm- ’ set- ’ during the solar cycle (WEEKS, 1967; HALL and HINTEREGGER,1970; TIMOTHYand TIMOTHY,1970; VIDAL-MADJARet al., 1973). There is uncertainty, however, in the calculation of the production function due to Lyman-alpha, arising mainly from the uncertainty in the measured height distribution of [NO]. The profiles obtained by MEIRA(1971) at mid-latitudes under winter conditions have been widely used over the past few years to calculate the production function. However, the production rates calculated on the basis of these profiles were found to be too excessive to explain the measured electron densities, as well as the positive-ion composition in the middle D-region (DONAHUE,1972). Fortunately, this discrepancy, has now been reduced to a certain degree, in consequence of an important correction made to the specific fluorescence rates used in the processing of data from the gamma-band rocket experiment (TOHMATSUand IWAGAMI. 1976). This revision has resulted in a reduction of the previously measured rocket profiles of [NO] by a factor of 2. Meira’s profile is shown in Fig. I along with the profile measured by TI~~NE (1973) and the latest measurements of TOHMATSUand IWAGAM~(1976) obtained at the geomagnetic equator. These latter two profiles incorporate the corrections mentioned above
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/
501 lo6
1
I
lo8
lo7
10'
[NO] cd
from rocket measurements by MEIRA(1971), TK~NE (1973) and TOHMATSU and IWAGAMI (1976).
Fig. I. Height distributions of NO concentration
and, when compared with Meira’s profile, have the same shape while differing in magnitude by a factor of about 4. A nitric oxide profile for the D-region, applicable at equatorial latitudes can therefore be constructed by taking the average of the two profiles of TOHMATSJ and IWAGAMI(1976) for the height range 80-100 km, and below this height range by extending the profiles assuming constant mixing ratio. The ion-pair production rate, calculated using this profile and a solar Lyman-alpha intensity of 3 x 10” photons IS shown in Fig. 2 for zenith angles 10 cm-‘set-‘, and 60”. It is seen here that a significant solar zenith angle variation in the production due to Lyman-alpha occurs only below about 85 km. 2.2. X-ray ionization The X-ray band which is important for D-region ionization around 90 km under quiet solar conditions
is the 3WOA
band. The major contribution to the energy in this band comes from the C VI line at 33.7 A. Unfortunately, there is much uncertainty in the measurement of this line. MANS~N(1972) reports a value of 4 milliergs cm- ’ set- ’ while FREEMANand JONES (1970) report two measurements, 7 and 9 milliergs cm- ’ set- ‘, both taken during periods of high solar activity. On the other hand, ARGO et al. (1970) have obtained line intensities in the range 2&60 milliergs cmm2 se-‘, under a variety of solar conditions. More recently, SCHMIDTKEet al. (1975) have obtained a value of 49 milliergs cm- ’ set-’ using satellite instrumentation. Considering the flux values in the neighbouring bands and the accuracies claimed by MAN-N (1972), and also the recommendations of HINTEREGGER (1970), it has been the practice to adopt the lower values for this line intensity. However, it should be mentioned that rocket measured electron densities around 90 km are more
GCR 50 10'
I
16'
I
I
loo PRODUCTION
10' RATE
I
lo2
10'
cni's-'
Fig. 2. Ion-pair production rates due to Lyman-alpha radiation with line intensity of 3 x IO” photons cm-2sec- I, 30-40A X-rays with 33.7 8, line intensity of 8 milliergs cm-’ SK-‘, and EUV radiations with fluxes as given by MAN~~N(1972).
1001
D-region processes at equatorial latitudes
/ i10-Z
I
Id’
,
IO0 PRODUCTION
Fig. 3. Ion-pair production
10' RATE
lo2
IO'
cm-‘<’
rates due to l-lO A X-rays under different solar conditions;
the rates
for flare conditions are from MITRA(1975).
consistent with the higher production rates than with those given by Manson’s data. For purpose of comparison, the 3&4OA X-ray production rates calculated using a C VI line intensity of 8 milliergs cmm2 set-’ are also shown in Fig. 2 for zenith angles lo” and 60”. For the smaller zenith angle this source dominates the region between 88 and 95 km. Solar X-rays below 10 A also ionize the D-region, but this band of X-rays is generally considered a minor source under quiet conditions. The calculation of its production function poses two problems; firstly, the uncertainty in the exact spectral distribution of energy over the band, and secondly, the difficulty in determining correctly the true flux values from the satellite detector currents. Both these factors depend on the coronal temperature of the X-ray emitting regions, and it has been the practice in the past to calculate the flux values using arbitrarily fixed temperatures for different portions of the band. It was only recently that DERE et al. (1974) described a method to work out the true flux values from the satellite-measured detector currents, which takes into consideration the variability of the coronal temperature. The spectral energy at shorter wavelengths shows a strong variability with solar activity, particularly during disturbed conditions, which in turn is shown up in the production rates at lower altitudes. In Fig. 3 are shown the production rates due to these X-rays for three different solar conditions. Under quiet conditions the l-10 A X-rays are only a minor source with the production function vanishing completely below about 70 km. For disturbed conditions, however, their ionization could dominate the entire D-region, with values sometimes increasing by more than lOO-fold (MITRA. 1975).
2.3. O,(’ 4) ionization Solar radiations which are incapable of ionizing the molecules at ground-state could still cause ionization of molecules at excited states, particularly the longlived metastables. Among such species, O,(‘$) forms an important source, ionized by the 1027-1118 a band. The penetration of these wavelengths into lower heights, through the windows in the O2 absorption spectrum, is governed by absorption due to atmospheric CO1 (HUFFMAN et al., 1971). According to a formula derived by PAULSEN et al. (1972), its ionization profile at low zenith angles cuts off sharply below 70 km, after reaching a peak near 88 km. However, IWA~AKE(1973) has recently shown that the reduction of COZ concentration by photodissociation and transport could enhance the O,(‘AJ ionization rate below 85 km by at least a factor of 2. The rates for small zenith angles, based on high latitude values for the O,(‘$) profile and equatorial values for the columnar density of Oz, are shown in Fig. 4. At D-region heights, O,(‘$) is produced during daytime mainly by photodissociation of 0,. According to the model calculations of FUKUYAMA (1974), 0s is expected to have higher concentration at low latitudes, particularly above 80km. Hence, the concentration of O&As) at low latitudes could be significantly higher than it is at high latitudes. This increase in concentration, together with the enhanced ionizing flux resulting from the reduced attenuation by CO, mentioned previously, could make the O,(‘Ag) ionization at low latitudes dominate the region between 80 and 90 km, as indicated in Fig. 4. The confirmation of this enhanced ionization rates, however, has to await direct measurements of the distributions of CO2 and 0, or O,(‘AJ concentrations at D-region heights near the equator.
P. A. J. RA~-N~IRI
1002
“Oi;-----I
NO
\ 60-
50 16’
16’ PRODUCTION
IO0 RATE
IO’
cm-k-
Fig. 4. Ion-pair production rates due to ionization of O&h) by EUV radiations and O2 and N, by Galactic Cosmic Rays. Broken lines indicate possible enhancements due to increased EUV radiation level and increased O,(‘A& concentration at low latitudes, respectively.
2.4. Energetic particle ionization
At middle and high latitudes, precipitating electrons with energies greater than 40 keV contribute to D-region ionization. At low latitudes, however, precipitating electrons do not have a significant flux and their contribution is considered unimportant even during disturbed conditions. Galactic cosmic rays, on the other hand, provide a source of ionization at all latitudes and at all times of the day. These are particularly important in the lowest part of the D-region. According to measurements carried out at stratospheric heights using balloon-borne detectors, the ionization rates due to galactic cosmic rays were found to have a strong latitudinal and an inverse solar-cycle variation (NEHER and ANDERSON, 1962). Since the production rates are proportional to the total number density in the atmosphere, their values corresponding to any altitude can be obtained directly by multiplying the number density by a suitable factor. SWIDER (1969) has tabulated these factors for different latitudes and solar activity, based on stratospheric measurements. The value corresponding to low latitudes lies in the range (2-3) x IO-‘*set-‘, the actual value depending on the solar epoch. As shown in Fig. 4, its contribution becomes important only below 62 km.
3. ELECTRON DENSITY PROFILES The various types of production rates discussed above do not have much of a latitudinal variation under quiet conditions, except the contributions due to galactic cosmic rays. The attenuation of the ionizing solar radiations is governed by atmospheric O2
and N2 densities, and their latitudinal variations are too small to cause significant changes in the production rates. However, as mentioned earlier, the two equatorial [NO] profiles of TOHMATWand IWAGAMI (1976) are about half that of the revised MEW (1971) [NO] profile for mid-latitudes. Hence, one would expect low densities in the D-region near the Equator, provided the recombination processes do not have any marked latitudinal variation. A reduced electron density profile would also be consistent with the low values of radiowave absorption observed near the magnetic dip-equator (GEORGE, 197 1). In situ measurements of D-region electron densities near the equator are sparse; the few currently available measurements have been reviewed by SomuaJULU et al. (1973) and THOMAS(1974b). The direct comparison of low-latitude and high-latitude electron density profiles is complicated by the fact that these measurements were carried out under different solar conditions and different zenith angles. In Fig. 5, the low-latitude profiles at high zenith angles are compared with two mid-latitude profiles proposed by MECHTLY et al. (1972a) for quiet-sun and active-sun conditions. These profiles, obtained at widely different geographic locations, exhibit large variability among them so that any systematic difference between the high- and low-latitude profiles does not become so apparent. This is also the conclusion of HIRAO (1974) who examined a large number of D-region profiles. In Fig. 6, the low latitude profiles for small zenith angles are compared with a mid-latitude profile. The variability in the low latitude profiles is seen to be much less than that in the previous set of profiles. There is a close similarity between the high-latitude and low-latitude profiles, particularly above the ledge.
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D-region processesat equatorial latitudes,
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LOW
LAT.
60 1
4.
50110"
1
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Id
Id ELECTRON
>
IO' DENSITY
uf
10‘
lo5
cni’
Fig. 5. Low-latitude electron density profiles measured at high zenith angles, compared with two highlatitude median electron density profiles representing active and quiet solar conditions: Profiles 1 and 2 by MECHTLY et al. (1969b); profiles 3 and 4 by AIKINet al. (1972); Median profiles are from MEcHlXYf?t01.(t972a).
But, below the ledge, the mid-latitude values are slightly larger than those at the low latitudes down to about 70 km, in agreement with the suggested latitude variation of [NO]. Between 60 and 65 km, the mid-latitide values are seen to be lower by more than an order of magnitude. This behaviour is opposite to what is expected from theory, and the reason is not clear. Also shown in Fig. 6 is a reference electron density profile deduced by combining several years of multifrequency absorption measurements carried out at Colomho near the magnetic equator, with results of a rocket-home experiment (GNANALINGAM and KANE, 1975). There is good agreement between this profile
110
100
I
and the rocket profiles. The slight differences between the profiles cannot be considered sign&ant because they fall within the known limits of uncertainty of the rocket measurementa Unlike rocket profiles the profile based on absorp tion is representative of ambient conditions averaged over several years. Furthermore, absorption measurements are helpful in understanding the behaviour of the D-region electron density distribution, through various correlative studies GNANALINGAM and KANE (1975) have found that the calculated enhancements in the electron density profiles, and hence in absorp tion, as the solar l-8 A flux increases by moderate amounts is not consistent with the empirically estab-
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X r
E go .x t
1-z 2. --3, -.-.-
26 14.
4. 5. _..-.-
le’ 4dN G&K
,,L-----loo
10’
lo2 ELECTRON
d DENSITY
10’
105
cni’
Fig. 6. Electron density profiles measured at low zenith angles compared with a profile deduced from radio-wave absorption measurements: Profile No. 1 by MFCHTLY et al. (1969a), No. 2 by AWN et al. (1972), No. 3 by KANE(1974) and No. 4 by ME~HTLYand SWTH (1970); Profile G and K by GNANALINQAM and KANE(1975). 5
P. A. J. RATNASIRI
ELECTRON
DENSITY
cm?
Fig. 7. Solar zenith angle variation of the D-region electron concentration deduced from absorption measurements (GNANALINGAM and KANE,1975).
lished relationship between absorption and X-ray flux (GNANALINGAM, 1974). In order to remove this discrepancy, they have suggested a reduction of the ME~RA (1971) [NO] profile by a factor of about 5 below 95 km. As mentioned earlier, the latest equatorial measurements of the [NO] profiles by TOHMATSU and IWAGAMI(1976) give values about one fourth Meira’s values and thus provide independent support of the conclusions reached by GNANALINGAMand KANE (1975) from their study of long-term absorption measurements. In Fig. 7 is shown a set of electron density profiles deduced for low latitudes and for zenith angles between 10” and 75”. These profiles are consistent with the diurnal measurements of absorption and virtual height on selected days during the equinoctial months of 1968-70, when non-flaring conditions prevailed throughout the day (GANANLINGAM and KANE, 1975). An interesting feature of these profiles is the small variation of the electron density with zenith angle, near the ledge. This weak dependence on zenith angle has also been found in profiles derived from model calculations (RATNASIRIand &CHRIST, 1975). 4. POSITIVEION COMPOSITION Equatorial measurements
of the D-region positive by NARCISI et al. (1972a) in 1966 in Brazil and later by GOLDBERGand AWN (1971) in 1970 in India. These measurements exhibit the same general features found in similar measurements made at middle and high latitudes, particularly the occurrence of two distinct regions above and below about 82-85 km, with molecular ions NO+ and 0: dominating the upper region and hydrated cluster ions of the type H’(H20), domination composition
were first made
ing the lower region (NARCISI and BAILEY, 1965; JOHANNE~~EN and KRANKOWSKY,1972). There are however, small differences in the actual distributions of different ion species, especially of the hydrated cluster type. Some of these differences may be attributed to differences in the instrumentation and sampling techniques, while others are possibly due to real differences in the composition between equatorial and high latitudes. A noteworthy feature of the equatorial results is the presence of 0: in significant amounts down to about 70 km, whereas the theoretical predictions indicate complete absence of 0: in this part of the D-region, as shown in Fig. 8 (GOLDBERGand AIKIN, 1971). Another feature is the occurrence of substantial amounts of the hydrate of NO’ in the high latitude measurements. In the equatorial measurement, this is either absent or found only in small amounts. There are two currently accepted reaction paths for the formation of hydrated cluster ions H+(H,O),, which appear to dominate the lower D-region. One of these paths begins from 0: and the other from NO+ (FF.HSENFELD et al., 1971a, b). The 0; reaction path commences with the three-body reaction which converts 0: to 02 with a time constant of about 4 set at 80 km. Subsequent reactions leading up to the formation of H+(H20), are binary reactions, and all have very short time constants. Hence, the overall process is governed by the above 3-body reaction, which still is fast enough to account for the formation of cluster ions. Unfortunately, the 0: production rates are not high enough to make this process important in the undisturbed D-region (SECHRIST,1972). The NO+ scheme leads to the formation of H+(H,O)s and the higher order clusters, principally in two stages; first by clustering of NO+ to form
1005
D-region processes at equatorial latitudes I- ’ ‘1”“”
ION DENSITY
cr?
’ ’ “““1
ION DENSITY
Fig. 8. Rocket measured D-region positive ion distribution at the dipequator ion distribution (GOLDBERG and AKIN, 1972). the series NO+(H20),, (n = I, 2,3), and next by converting the third-order hydrate to H+(H20)3. Since the direct hydration of NO+ with H,O to form NO+(H20) is too slow, faster mechanisms have been suggested where NO+ first clusters with more abundant constituents such as CO, and N2 to form either NO+(COJ or NO+(N,), which subsequently switches with HZ0 to form NO+(H20) (DUNKIN et al., 1971; HEIMJZRL and VANDERHOFF, 1974). However, the continuation of these clustering-switching reactions involving N2 and Hz0 to form the NO+(H,O)s cluster, will have to compete with collisional break-up reactions of the intermediate N, clusters (FERGUSON, 1974). The direct hydration of NO+(H,0)3 to form H+(H20), has a time constant of about 15 set at 80 km. However, this has the drawback that the lighter clusters H+(H20) and H+(H20)2 are not formed in the process. To overcome this problem BURKE (1970) and HEIMERLet al. (1972) have suggested reactions involving minor neutral species H, OH and HOz for the conversion of NO+(H20) into H+(H*O), which subsequently could yield H+(H,0)2. However, the rate coefficients of these reactions have been found too slow to make them important in D-region ion chemistry (FEHSENFELD et al., 1975). In the absence of a suitable mechanism to convert NO+ into H+(H20), clusters, one would tend to reexamine the sources of 0: in the D-region. This is
’ “““1
’ “““”
’ ““1
c~I-I‘~
compared with predicted
particularly important because recent measurements have indicated reduced concentrations of atmospheric NO. In this respect, it is also important to reassess the role played by O,(‘AJ in the production of O:, for reasons discussed previously. It should be mentioned that NORTONand REID (1972) have examined the possibility of 0; being produced by the ionization of excited 02, and have concluded that vibrationally or electronically excited O2 (in other than the ‘$ state) cannot provide an appreciable source of primary 0: ions in the D-region. Besides, there is evidence to suggest that sources other than Lyman-alpha could provide significant ionization in the middle D-region. Even though NO+ production rates between 80 and 90 km are not expected to show an appreciable solar zenith angle variation because the unit optical depth of Lymanalpha is around 75 km, direct measurements of D-region electron density profiles, partial reflection and cross-modulation results all indicate a larger zenith angle variation than predicted by theory (MECHTLYand SMITH, 1970; THRANE,1969). Further, GNANALINGAM(1974) observed that absorption enhancements occur on certain days on several frequencies, without being accompanied by any X-ray enhancements. These increases were found to obey nondeviative conditions, indicating that they were caused by increases in the electron density below about 90 km. Such day-to-day variations could be explained
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1006
better in terms of a variation in the O,(‘$) ionization rather than the NO ionization for the reason that the O,(’ AJ reactions involved have shorter time constant than the NO reactions. Mention should also be made of a recent rocket measurement carried out at mid-latitudes during a period of anomalous winter absorption, when it was observed that the D-region down to about 70 km was dominated by NO+, and not by hydrated cluster ions as observed at other times (BEYNONet al., 1976). On this occasion, the general level of ionization had increased by about an order of magnitude between 70 and 80 km, inferred as due to an increase in the NO concentration in the mesosphere. This observation may be compared with a similar situation when solar disturbances cause enhanced 0: concentrations, while at the same time depleting the level of hydrated cluster ions. However, it should be noted that in order to cause such depletion of the cluster ions relative to Oi, the ionization level has to be much stronger than that observed during the winter anomaly period mentioned above (MITRA and ROW, 1972). This indicates that the observed destruction of cluster ions has not been caused solely by increased electron densities. On the other hand, if there existed a fast path for converting NO+ to hydrated cluster ions, a measurable quantity of these ions would have been present. The absence of these cluster ions shows that no such conversion of NO+ to cluster ions has taken place. On normal days when the NO level is low the primary ion that gives rise to cluster ions would therefore appear to be 0,’ rather than NO+. This hypothesis is also supported by the fact that the observed abundance of 0: in the lower D-region is higher than that predicted by theory. In order to make further progress in our understanding of the D-region it is essential that more data should be obtained on the concentrations of minor neutral constituents such as NO, O,(‘A&, q3P) and HzO, which are important for D-region chemistry. Moreover, to minimize the influence of extraneous effects, such measurements should preferably be made at the equator where the effects of solar protons, precipitating particles. and seasonal variations are relatively small. Further, a simple ground-based absorption experiment on a sufficiently low frequency, such that reflection of the radio wave occurs near the ledge. would also furnish valuable information on the validity of the predictions concerning the diurnal behaviour of the electron density. 5. NEGATIVEION COMPOSlllON The presence of negative ions in the D-region is one of its unique features. As the neutral particle den-
RATNASIRI
sity increases with decreasing height, the attachment reaction
becomes more efficient causing the negative ion concentration to build up below about 70 km during daytime. The 0; ions so formed give rise to a series of charge-transfer and charge-rearrangement reactions forming ions like O;, 0;. CO;, CO;, NO; and NO; (FEHSENFELD et al., 1967, 1969). The reaction chain beginning from 0; takes place in two paths. In one, 0; is formed which in turn yields CO; and NO; successively. The other path initiates with a 3-body reaction forming O;, which in turn yields CO; and NO;, successively. In addition to these links in the main paths, reactions with atomic oxygen cause re-formation of 0; from 0; and CO;, and conversion of 0; to 0; and CO; to CO;, respectively. Hence the ratio [O]/[O,] controls to a large extent the negative-ion distribution in the D-region. According to this reaction scheme, the terminal ion is NO;. Its main loss processes during daytime are the mutual neutralization and photo-detachment. Recent laboratory work has indicated the formation of hydrated clusters such as O;(H,O), O;(H,O), CO;(H,O), but these hydration processes, being slow, are unlikely to have any important effect in the D-region. Further, FEHSNFELDand FERGIJ~ON(1974) have found that these hydrates break up on reacting with neutral molecules. During daytime, hydration of NO; will be important if it exceeds the photo-detachment of NO;. In the absence of such data, the above simple scheme incorporating only the molecular ions, appears to be adequate to explain the observed electron density in the lower part of the D-region (THOMASet al., 1973). Recent rocket measurements during nighttime have indicated the presence of NO; hydrates up to NO;(H,0)5 and also some hydrates of CO; (NARCISI et al., 1971). Again during nighttime but under slight enhancements in ionization, ARNOLD et al. (1971) found that only the molecular ions O;, NO;, CO;, CO; and some other clusters are present in the D-region, but not the hydrates of NO;. Model calculations by ARNOLD and KRANKOWSKY(1971) were able to explain the presence of lighter ions but not the heavier cluster ions, some of which were identified as NO;(HNO,). Under a PCA event during daytime, NARCISI et al. (1972b) observed 0; to be the dominant negative ion in the upper part of the D-region while CO; appeared to be dominant in the lower part.
1007
D-region processes at equatorial latitudes Unfortunately, no measurements are available at present for middle and low latitudes under normal daytime conditions. Hence it seems rather difficult to understand the role played by these negative ions in the ionization balance in the undisturbed D-region. In order to interpret properly the results of any such measurements that will be made in the future, it is equally important to have data concerning reaction rates, photo-detachment and mutual recombination rates as well as other relevant information such as the distribution of neutral species which are important for negative-ion
6.
chemistry.
ELECTRON
LOSS
In a region where the negative-ion concentration is negligible, the electron loss is essentially a recombination process, the r~mbination rate being proportional to the product of the concentrations of electrons and positive ions. When the dominant ion species is of a single type, ie. either molecular or cluster, the recombination coefficient will have a constant value irrespective of the ionization level. On the other hand, when both types of ions are present, the average r~omb~ation coefficient wili depend on the relative abundance of the two ion species. For 0; and NO+ an average recombination coeflicient is approximately 5 x lo-‘cm3 set-’ at D-region temperatures, and the corresponding value for the heavy clusters is about 1 x lo-’ cm3 see-‘. Based on these values, the average value of the recombination coefficient has been worked out for different values of the molecular to cluster ion ratios, and these are shown in Fig. 9. Analysing a large number of positive ion composition data, DANILOV and SIMONOV (1975) have shown that typically, the molecular ion to cluster ion ratio varies from about l/10 to 10, ie. by a factor of 100, across the transition region, between 80 and 85 km. The corresponding variation in the recombination coefficient, as given in Fig. 9, is only about 7, even though the individual values vary by about 20. Therefore, an abrupt change in the ion composition alone will not be able to explain the change in the electron density as observed across the ledge. According to the D-region two-ion model of HAUG and LANDMARK (1970) the electron density at a given height has a linear relationship with the production rate, and this relationship baas been established both by ground-based and rocket experiments for heights below about 85 km (THRANE,1972; MECHTLYet al.. 1972b; DANILOVand SIMONOV,1975). This apparent attachment-like behaviour, which is generally restricted to the region where
both
types of positive
ions
Fig. 9. Variation of the average recombination coefficients with cluster ion to molecular ion ratio. are present, can be simply interpreted in terms of a recombination coefficient which has an inverse dependence on the electron density. The larger the electron density, the faster will be the loss of cluster ions because of their high electron recombination rate. This causes the relative abundance of molecular ions to increase, and hence to decrease the average recombination coefficient, thus explaining the inverse relationship between the electron density and the average recombination coefficient. During intense ionization events, MITRA and ROWE(1972) have found that the percentage of hydrated ions decreases with increasing production rates, for a wide range of values of minor constituents such as Hz0 and q3P). 8. CONCLUSION
Our ~ders~nding of ®ion processes, though it has progressed to some extent over the past few years, cannot be considered as satisfactory. In particular, the paucity of in situ measurements at equatorial latitudes has made it difficult to establish firmly the latitudinal variation in D-region ionization and composition. Further, it has not been possibIe to construct a cfear picture of the un~stur~ daytime D-region, in view of the lack of systematic measurements of various D-region properties such as the ambient positive and negative ion composition, and the distribution of neutral species at middle and low latitudes. On the other hand, the disturbed D-region seems to have been investigated in much greater detail. The inability of current D-region theories to account for the observed positive ion composition emphasizes the need to re-evaluate the relative roles played by different ionization sources. It has also been
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shown that ground-based measurements of radiowave absorption conducted over a long period of time could provide vital information on the ambient D-region, particularly on the solar control of the electron concentration and its variations with time. Additional laboratory data on rate coefficients of both the neutral and ionic reactions, absorption cross-sections and also on solar fluxes and atmospheric parameters representing transport effects are needed for the realistic modelling of the D-region. It is hoped that such data will become available in the near future. More emphasis should also be laid on conducting co-ordinated D-region experiments, both
RATNASIRI
ground-based and in situ measurements, in order to avoid uncertainties due to time and space variability. It would also be beneficial to conduct long-term ground-based experiments such as radiowave absorption or partial reflection measurements at several stations along the same geographical longitude extending from equatorial to middle latitudes, so as to establish any latitudinal variation in the D-region. Acknowledgement-The author wishes to thank Dr. T. TOHMATSUand N. IWAGAMI for making available to him the results of their [NO] profile measurements, prior to publication.
REFERENCES AIKIN A. C., GOLDBERGR. A., S~MAYAJULUY. V. and AVADHANULUM. B. ARCXI H. V.. BERGEYJ. A. and EVANSW. D. ARNOLD F. and KRANKOWSKYD. ARNOLD F., KI~~ELJ., KRANKOWSKYD., WIEDERH. and ZAHRINGERJ. BWNON W. J. G., WILLIAMSE. R., ARNOLD F.. KRANKOWSKYD., BAIN W. C. and DICKIN~~NP. H. G. BURKER. R. DANILOVA. D. and SIM~NOVA. G. DERE K. P., HORAN D. M. and KREPLINR. W. DONAHUET. M. DUNKIN D. B., FEHSENFELD F. C., S~HMELTEKOPF A. L. and FERGUWN E. E. FEHSENFELD F. C. and FERGLJ~~N E. E. FEHSEXFELD F. C., SCHMELTEKOPF A. L., SCHIFFH. 1. and FERGUXINE. E. FEHSENFELD F. C.. FERGUWN E. E. and BOHMED. K. FEHSENFELD F. C.. MOSESMANM. and FERGUXINE. E. FEHNSEFELD F. C., MOSESMANM. and FERGU~~NE. E. FEHSENFELD F. C., HOWARD C. J., HARROP W. J. and FERGU~~NE. E. FERGU~~NE. E. FREEMANF. F. and JONESB. B. FUKUYAMAK. GEORGEP. L. GNANALINGAMS. GOLDBERGR. A. and AIKIN A. C. HALL L. A. and HINTERECGER H. E. HAUG A. and LANDMARKB. HEIMERLJ. M. and VANDERHOFF J. A. HINTEREGGER H. E. HIRAO K. HUFFMANR. E., PAULSEND. E., LARRABEEJ. C. and CAIRNS R. B. IWA~AKA Y. JOHANESSEN A. and KRANKOWSKYD. KANE J. A. MAN~~N J. E. MECHTLY E. A. and SOUTHL. G. MECHTLY E. A., B~WHILL S. A. and SMITHL. G. MECHTLY E. A.. SECHRISTC. F. JR. and SMITHL. G. MECHTLY E. A., SEINOK. and SMITH L. G. MECHTL.Y E. A., MUKUNDA RAO M., SKAPERDASD. 0. and SMITH L. G.
1972
J. atmos. terr. Phys. 34, 1483.
1970 1971 1971
Astrophys. J. 160, 283. J. atmos. terr. Phys. 33, 1693. J. atmos terr. Phys. 33. I 169.
1976
Nature, bnd.
1970 1975 1974 1972 1971
J. geophys. Res. 15, 1345. Ind. J. Rad. Space Phys. 4. 233. J. atmos. terr. Phys. 36, 989. Radio Sci. 7, 73. J. them. Phys. 54, 3817.
1974 1967
J. them. Ph ys. 61, 3 18 1. Planet. Space Sci. 15. 373.
1969 1971a 197lb 1975
Planet. Space Sci. 17. 1759 J. them. Phys. 55, 2115. J. chum. Phys. 55, 2120. J. geophys. Res. 80, 2229.
1974 1970 1974 1971 1974 1971 1970 1970 1974 1970 1974 1971
Rev. Geophys. Space Phys. 12, 703 Solar Phys. 15, 288. J. atmos. terr. Phys. 36. 1297. J. atmos. terr. Phys. 33, 1893. J. atmos. terr. Phys. 36, 1334. J. geophys. Res. 76, 8352. J. geophys. Res. 75, 6959. J. atmos. terr. Phys. 32, 405 J. them. Phvs. 60. 4362. Annls GPopiys. 26, 547. lnd. J. Rad. Space Phys. 3. 109. J. geophys. Res. 76. 1028.
1973 1972 1974 1972 1970 l972a 1972b 1969a l969b
Rept. lonos. Space Res. 27, 115. J. geophys. Res. 77, 2888. Ind. J. Rad. Space Phys. 3, 99. Solar Phys. 27, 107. Radio Sci. 5, 1407. J. atmos. few. Phys. 34, 1899. J. atmos. terr. Phys. 34. 641. Radio Sci. 4, 37 I. Radio Sci. 4, 5 17.
261,
I 18
D-region processes at equatorial latitudes MEIRA L. G. JR. MITRA A. P. MITRA A. P. and ROWE J. N. NARCISIR. S. and BAILEYA. D. NARCISIR. S., BAILEYA. D., DELLA LUCCA L., SHERMANC. and THOMASD. M. NARCISIR. S., BAILEYA. D., WLODYKA L. E. and PHILBRICKC. R. NEHERH. V. and ANDERSONH. R. NORTONR. B. and Rae G. C. PAULSEND. E., HUFFMANR. E. and LARRABEEJ. C. SCHMIDTKE G., RAWER K., FISCHERW. and REL~T~CKC. SECHRIST C. F. JR. S~MAYAJULUY. V., AVADHANULUM. B. and ZALPURI K. S. SWIDERW. JR. THOMASL. THOMASL.. G~NDHALEKARP. M. and BOWMANM. R. THRANEE. V. THRANEE. V. TIMOTHYA. F. and TIMOTHYJ. G. TIS~NEG. C. TOHMATSUT. and IWAGAMI N. VIDAL-MADJARA., BLAMONTJ. E. and PHISSAMAYB. WEEKSL. H.
1971 1975 1972 1965 1971
J. J. J. J. J.
1972a
J. atmos. terr. Phys. 34, 647.
1962 1972 1972 1975
J. geophys. Res. 67, 1309. J. geophys. Res. 77, 6287. Radio Sci. 7. 51. Space Research XV, p. 345. Berlin. J. atmos. terr. Phys. 34, 1656. Space Research XIII, p. 477. Berlin. Reo. Geophys. 7, 573. Radio Sci. 9, 121. J. atmos. terr. Phys. 35, 397. J. geophys. Res. 74. 131 I. Radio Sci. 7. 143. J. geophys. Res. 75, 6950. J. geophys. Res. 78, 746. J. Geomag. Geolectr. (in press). J. geophys. Res. 78, 1115. Astophys. J. (left.) 147, 1203.
1972 1973 1969 1974a 1973 1969 1972 1970 1973 1976 1973 1967
geophys. Rex 76, atmos. terr. Phys. atmos. terr. Phys. geophys. Res. 70, atmos. tew. Phys.
1009 202. 37, 895. 34, 795. 3687. 33, 1147.
Akademie-Verlag,
Akademie-Verlag,
Reference IS also made to the following unpublished mattv-ial: GNANALINGAMS. and KANE J. A. HEIMERLJ M., VANDERHOFF J. A., PUCKEX-~ L. J. and NILESF. E. NARCISIR. S., SHERMANC., PHILBRICKC. R.. THOMASD. M., BAILEYA. D., WL~DYKA L. E., WWDYKA R. A., BAKERD. and FEDERIC~G. RATNASIRIP. A. J. and SECHRISTC. F. JR.
197s 1972
THOMASL.
1974b
1972b
1975
NASAIGSFC Rept. No. X-623-75-145. BRL Rept. No. 1570. Ballistic Res. Lab.. Aberdeen, Md. Proc. COSPAR Symp. Solar Particle Eamt of November 1969, AFCRL Rept. No. 144. p. 411. Aeronomy Rept. No. 67, University of Illinois, Urbana-Champaign, Illinois. Proc. COSPAR Symp. ‘Methods 01. Measurements und Results of Lower Ionosphere Structure’, Constance, FRG, p. 153.
The D-region is the least understood region of the ionosphere. The presence of complex positive ions and negative ions, both of molecular and cluster types, and the unusual behaviour of its electron concentration, both in time and space, are among the features that are of special interest in the D-region. The direct measurement’ of the electron concentration, or of its ion composition, can only be done by means of rocket-borne instrumentation. The region is too low for satellites and too high for balloons. Various types of ground-based radio experiments have provided information about the behaviour of its electron concentration. The results of these experiments, carried out over a number of years, have not yet been satisfactorily interpreted in terms of existing theories concerning the D-region (THOMAS, 1974a). This is mainly because of the non-availability of accurate data on such important factors as the concentrations of minor neutral constituents, reaction rates and solar fluxes and also because of our lack of understanding of basic processes such as the ionpair production and electron loss rates. In this paper, an attempt is made to review the current status of our knowledge regarding these basic ionospheric processes, as applicable to the D-region, and to identify outstanding problems, with special emphasis on equatorial latitudes. It does not cover the processes taking place at night-time or under disturbed conditions. Attention is also drawn to the importance of combining ground-based measurements with rocket experiments, in order to obtain a better understanding of the normal day-time D-region. 2. IONIZING
SOURCES
It is now generally agreed that the dominant source of ionization in the daytime D-region under quiet
conditions is the solar Lyman-alpha line (1216 A). ionizing NO. This is the major source between about 62 km and 85 km at low zenith angles. Above this height, 30-40A X-rays become the major source ionizing N, and 0,, while below 62 km galactic cosmic rays become the major source. 2. I. NO ionization The intensity of the solar Lyman-alpha line has been measured over several years, and it has been observed that it is a fairly stable line, with the intensity varying between 2.4 x IO” and 3.6 x 10’ ’ photons cm- ’ set- ’ during the solar cycle (WEEKS, 1967; HALL and HINTEREGGER,1970; TIMOTHYand TIMOTHY,1970; VIDAL-MADJARet al., 1973). There is uncertainty, however, in the calculation of the production function due to Lyman-alpha, arising mainly from the uncertainty in the measured height distribution of [NO]. The profiles obtained by MEIRA(1971) at mid-latitudes under winter conditions have been widely used over the past few years to calculate the production function. However, the production rates calculated on the basis of these profiles were found to be too excessive to explain the measured electron densities, as well as the positive-ion composition in the middle D-region (DONAHUE,1972). Fortunately, this discrepancy, has now been reduced to a certain degree, in consequence of an important correction made to the specific fluorescence rates used in the processing of data from the gamma-band rocket experiment (TOHMATSUand IWAGAMI. 1976). This revision has resulted in a reduction of the previously measured rocket profiles of [NO] by a factor of 2. Meira’s profile is shown in Fig. I along with the profile measured by TI~~NE (1973) and the latest measurements of TOHMATSUand IWAGAM~(1976) obtained at the geomagnetic equator. These latter two profiles incorporate the corrections mentioned above
999
P. A. J. RATNASIRI 110
/
501 lo6
1
I
lo8
lo7
10'
[NO] cd
from rocket measurements by MEIRA(1971), TK~NE (1973) and TOHMATSU and IWAGAMI (1976).
Fig. I. Height distributions of NO concentration
and, when compared with Meira’s profile, have the same shape while differing in magnitude by a factor of about 4. A nitric oxide profile for the D-region, applicable at equatorial latitudes can therefore be constructed by taking the average of the two profiles of TOHMATSJ and IWAGAMI(1976) for the height range 80-100 km, and below this height range by extending the profiles assuming constant mixing ratio. The ion-pair production rate, calculated using this profile and a solar Lyman-alpha intensity of 3 x 10” photons IS shown in Fig. 2 for zenith angles 10 cm-‘set-‘, and 60”. It is seen here that a significant solar zenith angle variation in the production due to Lyman-alpha occurs only below about 85 km. 2.2. X-ray ionization The X-ray band which is important for D-region ionization around 90 km under quiet solar conditions
is the 3WOA
band. The major contribution to the energy in this band comes from the C VI line at 33.7 A. Unfortunately, there is much uncertainty in the measurement of this line. MANS~N(1972) reports a value of 4 milliergs cm- ’ set- ’ while FREEMANand JONES (1970) report two measurements, 7 and 9 milliergs cm- ’ set- ‘, both taken during periods of high solar activity. On the other hand, ARGO et al. (1970) have obtained line intensities in the range 2&60 milliergs cmm2 se-‘, under a variety of solar conditions. More recently, SCHMIDTKEet al. (1975) have obtained a value of 49 milliergs cm- ’ set-’ using satellite instrumentation. Considering the flux values in the neighbouring bands and the accuracies claimed by MAN-N (1972), and also the recommendations of HINTEREGGER (1970), it has been the practice to adopt the lower values for this line intensity. However, it should be mentioned that rocket measured electron densities around 90 km are more
GCR 50 10'
I
16'
I
I
loo PRODUCTION
10' RATE
I
lo2
10'
cni's-'
Fig. 2. Ion-pair production rates due to Lyman-alpha radiation with line intensity of 3 x IO” photons cm-2sec- I, 30-40A X-rays with 33.7 8, line intensity of 8 milliergs cm-’ SK-‘, and EUV radiations with fluxes as given by MAN~~N(1972).
1001
D-region processes at equatorial latitudes
/ i10-Z
I
Id’
,
IO0 PRODUCTION
Fig. 3. Ion-pair production
10' RATE
lo2
IO'
cm-‘<’
rates due to l-lO A X-rays under different solar conditions;
the rates
for flare conditions are from MITRA(1975).
consistent with the higher production rates than with those given by Manson’s data. For purpose of comparison, the 3&4OA X-ray production rates calculated using a C VI line intensity of 8 milliergs cmm2 set-’ are also shown in Fig. 2 for zenith angles lo” and 60”. For the smaller zenith angle this source dominates the region between 88 and 95 km. Solar X-rays below 10 A also ionize the D-region, but this band of X-rays is generally considered a minor source under quiet conditions. The calculation of its production function poses two problems; firstly, the uncertainty in the exact spectral distribution of energy over the band, and secondly, the difficulty in determining correctly the true flux values from the satellite detector currents. Both these factors depend on the coronal temperature of the X-ray emitting regions, and it has been the practice in the past to calculate the flux values using arbitrarily fixed temperatures for different portions of the band. It was only recently that DERE et al. (1974) described a method to work out the true flux values from the satellite-measured detector currents, which takes into consideration the variability of the coronal temperature. The spectral energy at shorter wavelengths shows a strong variability with solar activity, particularly during disturbed conditions, which in turn is shown up in the production rates at lower altitudes. In Fig. 3 are shown the production rates due to these X-rays for three different solar conditions. Under quiet conditions the l-10 A X-rays are only a minor source with the production function vanishing completely below about 70 km. For disturbed conditions, however, their ionization could dominate the entire D-region, with values sometimes increasing by more than lOO-fold (MITRA. 1975).
2.3. O,(’ 4) ionization Solar radiations which are incapable of ionizing the molecules at ground-state could still cause ionization of molecules at excited states, particularly the longlived metastables. Among such species, O,(‘$) forms an important source, ionized by the 1027-1118 a band. The penetration of these wavelengths into lower heights, through the windows in the O2 absorption spectrum, is governed by absorption due to atmospheric CO1 (HUFFMAN et al., 1971). According to a formula derived by PAULSEN et al. (1972), its ionization profile at low zenith angles cuts off sharply below 70 km, after reaching a peak near 88 km. However, IWA~AKE(1973) has recently shown that the reduction of COZ concentration by photodissociation and transport could enhance the O,(‘AJ ionization rate below 85 km by at least a factor of 2. The rates for small zenith angles, based on high latitude values for the O,(‘$) profile and equatorial values for the columnar density of Oz, are shown in Fig. 4. At D-region heights, O,(‘$) is produced during daytime mainly by photodissociation of 0,. According to the model calculations of FUKUYAMA (1974), 0s is expected to have higher concentration at low latitudes, particularly above 80km. Hence, the concentration of O&As) at low latitudes could be significantly higher than it is at high latitudes. This increase in concentration, together with the enhanced ionizing flux resulting from the reduced attenuation by CO, mentioned previously, could make the O,(‘Ag) ionization at low latitudes dominate the region between 80 and 90 km, as indicated in Fig. 4. The confirmation of this enhanced ionization rates, however, has to await direct measurements of the distributions of CO2 and 0, or O,(‘AJ concentrations at D-region heights near the equator.
P. A. J. RA~-N~IRI
1002
“Oi;-----I
NO
\ 60-
50 16’
16’ PRODUCTION
IO0 RATE
IO’
cm-k-
Fig. 4. Ion-pair production rates due to ionization of O&h) by EUV radiations and O2 and N, by Galactic Cosmic Rays. Broken lines indicate possible enhancements due to increased EUV radiation level and increased O,(‘A& concentration at low latitudes, respectively.
2.4. Energetic particle ionization
At middle and high latitudes, precipitating electrons with energies greater than 40 keV contribute to D-region ionization. At low latitudes, however, precipitating electrons do not have a significant flux and their contribution is considered unimportant even during disturbed conditions. Galactic cosmic rays, on the other hand, provide a source of ionization at all latitudes and at all times of the day. These are particularly important in the lowest part of the D-region. According to measurements carried out at stratospheric heights using balloon-borne detectors, the ionization rates due to galactic cosmic rays were found to have a strong latitudinal and an inverse solar-cycle variation (NEHER and ANDERSON, 1962). Since the production rates are proportional to the total number density in the atmosphere, their values corresponding to any altitude can be obtained directly by multiplying the number density by a suitable factor. SWIDER (1969) has tabulated these factors for different latitudes and solar activity, based on stratospheric measurements. The value corresponding to low latitudes lies in the range (2-3) x IO-‘*set-‘, the actual value depending on the solar epoch. As shown in Fig. 4, its contribution becomes important only below 62 km.
3. ELECTRON DENSITY PROFILES The various types of production rates discussed above do not have much of a latitudinal variation under quiet conditions, except the contributions due to galactic cosmic rays. The attenuation of the ionizing solar radiations is governed by atmospheric O2
and N2 densities, and their latitudinal variations are too small to cause significant changes in the production rates. However, as mentioned earlier, the two equatorial [NO] profiles of TOHMATWand IWAGAMI (1976) are about half that of the revised MEW (1971) [NO] profile for mid-latitudes. Hence, one would expect low densities in the D-region near the Equator, provided the recombination processes do not have any marked latitudinal variation. A reduced electron density profile would also be consistent with the low values of radiowave absorption observed near the magnetic dip-equator (GEORGE, 197 1). In situ measurements of D-region electron densities near the equator are sparse; the few currently available measurements have been reviewed by SomuaJULU et al. (1973) and THOMAS(1974b). The direct comparison of low-latitude and high-latitude electron density profiles is complicated by the fact that these measurements were carried out under different solar conditions and different zenith angles. In Fig. 5, the low-latitude profiles at high zenith angles are compared with two mid-latitude profiles proposed by MECHTLY et al. (1972a) for quiet-sun and active-sun conditions. These profiles, obtained at widely different geographic locations, exhibit large variability among them so that any systematic difference between the high- and low-latitude profiles does not become so apparent. This is also the conclusion of HIRAO (1974) who examined a large number of D-region profiles. In Fig. 6, the low latitude profiles for small zenith angles are compared with a mid-latitude profile. The variability in the low latitude profiles is seen to be much less than that in the previous set of profiles. There is a close similarity between the high-latitude and low-latitude profiles, particularly above the ledge.
1003
D-region processesat equatorial latitudes,
-
LOW
LAT.
60 1
4.
50110"
1
!
Id
Id ELECTRON
>
IO' DENSITY
uf
10‘
lo5
cni’
Fig. 5. Low-latitude electron density profiles measured at high zenith angles, compared with two highlatitude median electron density profiles representing active and quiet solar conditions: Profiles 1 and 2 by MECHTLY et al. (1969b); profiles 3 and 4 by AIKINet al. (1972); Median profiles are from MEcHlXYf?t01.(t972a).
But, below the ledge, the mid-latitude values are slightly larger than those at the low latitudes down to about 70 km, in agreement with the suggested latitude variation of [NO]. Between 60 and 65 km, the mid-latitide values are seen to be lower by more than an order of magnitude. This behaviour is opposite to what is expected from theory, and the reason is not clear. Also shown in Fig. 6 is a reference electron density profile deduced by combining several years of multifrequency absorption measurements carried out at Colomho near the magnetic equator, with results of a rocket-home experiment (GNANALINGAM and KANE, 1975). There is good agreement between this profile
110
100
I
and the rocket profiles. The slight differences between the profiles cannot be considered sign&ant because they fall within the known limits of uncertainty of the rocket measurementa Unlike rocket profiles the profile based on absorp tion is representative of ambient conditions averaged over several years. Furthermore, absorption measurements are helpful in understanding the behaviour of the D-region electron density distribution, through various correlative studies GNANALINGAM and KANE (1975) have found that the calculated enhancements in the electron density profiles, and hence in absorp tion, as the solar l-8 A flux increases by moderate amounts is not consistent with the empirically estab-
I
I
I
X r
E go .x t
1-z 2. --3, -.-.-
26 14.
4. 5. _..-.-
le’ 4dN G&K
,,L-----loo
10’
lo2 ELECTRON
d DENSITY
10’
105
cni’
Fig. 6. Electron density profiles measured at low zenith angles compared with a profile deduced from radio-wave absorption measurements: Profile No. 1 by MFCHTLY et al. (1969a), No. 2 by AWN et al. (1972), No. 3 by KANE(1974) and No. 4 by ME~HTLYand SWTH (1970); Profile G and K by GNANALINQAM and KANE(1975). 5
P. A. J. RATNASIRI
ELECTRON
DENSITY
cm?
Fig. 7. Solar zenith angle variation of the D-region electron concentration deduced from absorption measurements (GNANALINGAM and KANE,1975).
lished relationship between absorption and X-ray flux (GNANALINGAM, 1974). In order to remove this discrepancy, they have suggested a reduction of the ME~RA (1971) [NO] profile by a factor of about 5 below 95 km. As mentioned earlier, the latest equatorial measurements of the [NO] profiles by TOHMATSU and IWAGAMI(1976) give values about one fourth Meira’s values and thus provide independent support of the conclusions reached by GNANALINGAMand KANE (1975) from their study of long-term absorption measurements. In Fig. 7 is shown a set of electron density profiles deduced for low latitudes and for zenith angles between 10” and 75”. These profiles are consistent with the diurnal measurements of absorption and virtual height on selected days during the equinoctial months of 1968-70, when non-flaring conditions prevailed throughout the day (GANANLINGAM and KANE, 1975). An interesting feature of these profiles is the small variation of the electron density with zenith angle, near the ledge. This weak dependence on zenith angle has also been found in profiles derived from model calculations (RATNASIRIand &CHRIST, 1975). 4. POSITIVEION COMPOSITION Equatorial measurements
of the D-region positive by NARCISI et al. (1972a) in 1966 in Brazil and later by GOLDBERGand AWN (1971) in 1970 in India. These measurements exhibit the same general features found in similar measurements made at middle and high latitudes, particularly the occurrence of two distinct regions above and below about 82-85 km, with molecular ions NO+ and 0: dominating the upper region and hydrated cluster ions of the type H’(H20), domination composition
were first made
ing the lower region (NARCISI and BAILEY, 1965; JOHANNE~~EN and KRANKOWSKY,1972). There are however, small differences in the actual distributions of different ion species, especially of the hydrated cluster type. Some of these differences may be attributed to differences in the instrumentation and sampling techniques, while others are possibly due to real differences in the composition between equatorial and high latitudes. A noteworthy feature of the equatorial results is the presence of 0: in significant amounts down to about 70 km, whereas the theoretical predictions indicate complete absence of 0: in this part of the D-region, as shown in Fig. 8 (GOLDBERGand AIKIN, 1971). Another feature is the occurrence of substantial amounts of the hydrate of NO’ in the high latitude measurements. In the equatorial measurement, this is either absent or found only in small amounts. There are two currently accepted reaction paths for the formation of hydrated cluster ions H+(H,O),, which appear to dominate the lower D-region. One of these paths begins from 0: and the other from NO+ (FF.HSENFELD et al., 1971a, b). The 0; reaction path commences with the three-body reaction which converts 0: to 02 with a time constant of about 4 set at 80 km. Subsequent reactions leading up to the formation of H+(H20), are binary reactions, and all have very short time constants. Hence, the overall process is governed by the above 3-body reaction, which still is fast enough to account for the formation of cluster ions. Unfortunately, the 0: production rates are not high enough to make this process important in the undisturbed D-region (SECHRIST,1972). The NO+ scheme leads to the formation of H+(H,O)s and the higher order clusters, principally in two stages; first by clustering of NO+ to form
1005
D-region processes at equatorial latitudes I- ’ ‘1”“”
ION DENSITY
cr?
’ ’ “““1
ION DENSITY
Fig. 8. Rocket measured D-region positive ion distribution at the dipequator ion distribution (GOLDBERG and AKIN, 1972). the series NO+(H20),, (n = I, 2,3), and next by converting the third-order hydrate to H+(H20)3. Since the direct hydration of NO+ with H,O to form NO+(H20) is too slow, faster mechanisms have been suggested where NO+ first clusters with more abundant constituents such as CO, and N2 to form either NO+(COJ or NO+(N,), which subsequently switches with HZ0 to form NO+(H20) (DUNKIN et al., 1971; HEIMJZRL and VANDERHOFF, 1974). However, the continuation of these clustering-switching reactions involving N2 and Hz0 to form the NO+(H,O)s cluster, will have to compete with collisional break-up reactions of the intermediate N, clusters (FERGUSON, 1974). The direct hydration of NO+(H,0)3 to form H+(H20), has a time constant of about 15 set at 80 km. However, this has the drawback that the lighter clusters H+(H20) and H+(H20)2 are not formed in the process. To overcome this problem BURKE (1970) and HEIMERLet al. (1972) have suggested reactions involving minor neutral species H, OH and HOz for the conversion of NO+(H20) into H+(H*O), which subsequently could yield H+(H,0)2. However, the rate coefficients of these reactions have been found too slow to make them important in D-region ion chemistry (FEHSENFELD et al., 1975). In the absence of a suitable mechanism to convert NO+ into H+(H20), clusters, one would tend to reexamine the sources of 0: in the D-region. This is
’ “““1
’ “““”
’ ““1
c~I-I‘~
compared with predicted
particularly important because recent measurements have indicated reduced concentrations of atmospheric NO. In this respect, it is also important to reassess the role played by O,(‘AJ in the production of O:, for reasons discussed previously. It should be mentioned that NORTONand REID (1972) have examined the possibility of 0; being produced by the ionization of excited 02, and have concluded that vibrationally or electronically excited O2 (in other than the ‘$ state) cannot provide an appreciable source of primary 0: ions in the D-region. Besides, there is evidence to suggest that sources other than Lyman-alpha could provide significant ionization in the middle D-region. Even though NO+ production rates between 80 and 90 km are not expected to show an appreciable solar zenith angle variation because the unit optical depth of Lymanalpha is around 75 km, direct measurements of D-region electron density profiles, partial reflection and cross-modulation results all indicate a larger zenith angle variation than predicted by theory (MECHTLYand SMITH, 1970; THRANE,1969). Further, GNANALINGAM(1974) observed that absorption enhancements occur on certain days on several frequencies, without being accompanied by any X-ray enhancements. These increases were found to obey nondeviative conditions, indicating that they were caused by increases in the electron density below about 90 km. Such day-to-day variations could be explained
P. A. J.
1006
better in terms of a variation in the O,(‘$) ionization rather than the NO ionization for the reason that the O,(’ AJ reactions involved have shorter time constant than the NO reactions. Mention should also be made of a recent rocket measurement carried out at mid-latitudes during a period of anomalous winter absorption, when it was observed that the D-region down to about 70 km was dominated by NO+, and not by hydrated cluster ions as observed at other times (BEYNONet al., 1976). On this occasion, the general level of ionization had increased by about an order of magnitude between 70 and 80 km, inferred as due to an increase in the NO concentration in the mesosphere. This observation may be compared with a similar situation when solar disturbances cause enhanced 0: concentrations, while at the same time depleting the level of hydrated cluster ions. However, it should be noted that in order to cause such depletion of the cluster ions relative to Oi, the ionization level has to be much stronger than that observed during the winter anomaly period mentioned above (MITRA and ROW, 1972). This indicates that the observed destruction of cluster ions has not been caused solely by increased electron densities. On the other hand, if there existed a fast path for converting NO+ to hydrated cluster ions, a measurable quantity of these ions would have been present. The absence of these cluster ions shows that no such conversion of NO+ to cluster ions has taken place. On normal days when the NO level is low the primary ion that gives rise to cluster ions would therefore appear to be 0,’ rather than NO+. This hypothesis is also supported by the fact that the observed abundance of 0: in the lower D-region is higher than that predicted by theory. In order to make further progress in our understanding of the D-region it is essential that more data should be obtained on the concentrations of minor neutral constituents such as NO, O,(‘A&, q3P) and HzO, which are important for D-region chemistry. Moreover, to minimize the influence of extraneous effects, such measurements should preferably be made at the equator where the effects of solar protons, precipitating particles. and seasonal variations are relatively small. Further, a simple ground-based absorption experiment on a sufficiently low frequency, such that reflection of the radio wave occurs near the ledge. would also furnish valuable information on the validity of the predictions concerning the diurnal behaviour of the electron density. 5. NEGATIVEION COMPOSlllON The presence of negative ions in the D-region is one of its unique features. As the neutral particle den-
RATNASIRI
sity increases with decreasing height, the attachment reaction
becomes more efficient causing the negative ion concentration to build up below about 70 km during daytime. The 0; ions so formed give rise to a series of charge-transfer and charge-rearrangement reactions forming ions like O;, 0;. CO;, CO;, NO; and NO; (FEHSENFELD et al., 1967, 1969). The reaction chain beginning from 0; takes place in two paths. In one, 0; is formed which in turn yields CO; and NO; successively. The other path initiates with a 3-body reaction forming O;, which in turn yields CO; and NO;, successively. In addition to these links in the main paths, reactions with atomic oxygen cause re-formation of 0; from 0; and CO;, and conversion of 0; to 0; and CO; to CO;, respectively. Hence the ratio [O]/[O,] controls to a large extent the negative-ion distribution in the D-region. According to this reaction scheme, the terminal ion is NO;. Its main loss processes during daytime are the mutual neutralization and photo-detachment. Recent laboratory work has indicated the formation of hydrated clusters such as O;(H,O), O;(H,O), CO;(H,O), but these hydration processes, being slow, are unlikely to have any important effect in the D-region. Further, FEHSNFELDand FERGIJ~ON(1974) have found that these hydrates break up on reacting with neutral molecules. During daytime, hydration of NO; will be important if it exceeds the photo-detachment of NO;. In the absence of such data, the above simple scheme incorporating only the molecular ions, appears to be adequate to explain the observed electron density in the lower part of the D-region (THOMASet al., 1973). Recent rocket measurements during nighttime have indicated the presence of NO; hydrates up to NO;(H,0)5 and also some hydrates of CO; (NARCISI et al., 1971). Again during nighttime but under slight enhancements in ionization, ARNOLD et al. (1971) found that only the molecular ions O;, NO;, CO;, CO; and some other clusters are present in the D-region, but not the hydrates of NO;. Model calculations by ARNOLD and KRANKOWSKY(1971) were able to explain the presence of lighter ions but not the heavier cluster ions, some of which were identified as NO;(HNO,). Under a PCA event during daytime, NARCISI et al. (1972b) observed 0; to be the dominant negative ion in the upper part of the D-region while CO; appeared to be dominant in the lower part.
1007
D-region processes at equatorial latitudes Unfortunately, no measurements are available at present for middle and low latitudes under normal daytime conditions. Hence it seems rather difficult to understand the role played by these negative ions in the ionization balance in the undisturbed D-region. In order to interpret properly the results of any such measurements that will be made in the future, it is equally important to have data concerning reaction rates, photo-detachment and mutual recombination rates as well as other relevant information such as the distribution of neutral species which are important for negative-ion
6.
chemistry.
ELECTRON
LOSS
In a region where the negative-ion concentration is negligible, the electron loss is essentially a recombination process, the r~mbination rate being proportional to the product of the concentrations of electrons and positive ions. When the dominant ion species is of a single type, ie. either molecular or cluster, the recombination coefficient will have a constant value irrespective of the ionization level. On the other hand, when both types of ions are present, the average r~omb~ation coefficient wili depend on the relative abundance of the two ion species. For 0; and NO+ an average recombination coeflicient is approximately 5 x lo-‘cm3 set-’ at D-region temperatures, and the corresponding value for the heavy clusters is about 1 x lo-’ cm3 see-‘. Based on these values, the average value of the recombination coefficient has been worked out for different values of the molecular to cluster ion ratios, and these are shown in Fig. 9. Analysing a large number of positive ion composition data, DANILOV and SIMONOV (1975) have shown that typically, the molecular ion to cluster ion ratio varies from about l/10 to 10, ie. by a factor of 100, across the transition region, between 80 and 85 km. The corresponding variation in the recombination coefficient, as given in Fig. 9, is only about 7, even though the individual values vary by about 20. Therefore, an abrupt change in the ion composition alone will not be able to explain the change in the electron density as observed across the ledge. According to the D-region two-ion model of HAUG and LANDMARK (1970) the electron density at a given height has a linear relationship with the production rate, and this relationship baas been established both by ground-based and rocket experiments for heights below about 85 km (THRANE,1972; MECHTLYet al.. 1972b; DANILOVand SIMONOV,1975). This apparent attachment-like behaviour, which is generally restricted to the region where
both
types of positive
ions
Fig. 9. Variation of the average recombination coefficients with cluster ion to molecular ion ratio. are present, can be simply interpreted in terms of a recombination coefficient which has an inverse dependence on the electron density. The larger the electron density, the faster will be the loss of cluster ions because of their high electron recombination rate. This causes the relative abundance of molecular ions to increase, and hence to decrease the average recombination coefficient, thus explaining the inverse relationship between the electron density and the average recombination coefficient. During intense ionization events, MITRA and ROWE(1972) have found that the percentage of hydrated ions decreases with increasing production rates, for a wide range of values of minor constituents such as Hz0 and q3P). 8. CONCLUSION
Our ~ders~nding of ®ion processes, though it has progressed to some extent over the past few years, cannot be considered as satisfactory. In particular, the paucity of in situ measurements at equatorial latitudes has made it difficult to establish firmly the latitudinal variation in D-region ionization and composition. Further, it has not been possibIe to construct a cfear picture of the un~stur~ daytime D-region, in view of the lack of systematic measurements of various D-region properties such as the ambient positive and negative ion composition, and the distribution of neutral species at middle and low latitudes. On the other hand, the disturbed D-region seems to have been investigated in much greater detail. The inability of current D-region theories to account for the observed positive ion composition emphasizes the need to re-evaluate the relative roles played by different ionization sources. It has also been
I008
P. A. J.
shown that ground-based measurements of radiowave absorption conducted over a long period of time could provide vital information on the ambient D-region, particularly on the solar control of the electron concentration and its variations with time. Additional laboratory data on rate coefficients of both the neutral and ionic reactions, absorption cross-sections and also on solar fluxes and atmospheric parameters representing transport effects are needed for the realistic modelling of the D-region. It is hoped that such data will become available in the near future. More emphasis should also be laid on conducting co-ordinated D-region experiments, both
RATNASIRI
ground-based and in situ measurements, in order to avoid uncertainties due to time and space variability. It would also be beneficial to conduct long-term ground-based experiments such as radiowave absorption or partial reflection measurements at several stations along the same geographical longitude extending from equatorial to middle latitudes, so as to establish any latitudinal variation in the D-region. Acknowledgement-The author wishes to thank Dr. T. TOHMATSUand N. IWAGAMI for making available to him the results of their [NO] profile measurements, prior to publication.
REFERENCES AIKIN A. C., GOLDBERGR. A., S~MAYAJULUY. V. and AVADHANULUM. B. ARCXI H. V.. BERGEYJ. A. and EVANSW. D. ARNOLD F. and KRANKOWSKYD. ARNOLD F., KI~~ELJ., KRANKOWSKYD., WIEDERH. and ZAHRINGERJ. BWNON W. J. G., WILLIAMSE. R., ARNOLD F.. KRANKOWSKYD., BAIN W. C. and DICKIN~~NP. H. G. BURKER. R. DANILOVA. D. and SIM~NOVA. G. DERE K. P., HORAN D. M. and KREPLINR. W. DONAHUET. M. DUNKIN D. B., FEHSENFELD F. C., S~HMELTEKOPF A. L. and FERGUWN E. E. FEHSENFELD F. C. and FERGLJ~~N E. E. FEHSEXFELD F. C., SCHMELTEKOPF A. L., SCHIFFH. 1. and FERGUXINE. E. FEHSENFELD F. C.. FERGUWN E. E. and BOHMED. K. FEHSENFELD F. C.. MOSESMANM. and FERGUXINE. E. FEHNSEFELD F. C., MOSESMANM. and FERGU~~NE. E. FEHSENFELD F. C., HOWARD C. J., HARROP W. J. and FERGU~~NE. E. FERGU~~NE. E. FREEMANF. F. and JONESB. B. FUKUYAMAK. GEORGEP. L. GNANALINGAMS. GOLDBERGR. A. and AIKIN A. C. HALL L. A. and HINTERECGER H. E. HAUG A. and LANDMARKB. HEIMERLJ. M. and VANDERHOFF J. A. HINTEREGGER H. E. HIRAO K. HUFFMANR. E., PAULSEND. E., LARRABEEJ. C. and CAIRNS R. B. IWA~AKA Y. JOHANESSEN A. and KRANKOWSKYD. KANE J. A. MAN~~N J. E. MECHTLY E. A. and SOUTHL. G. MECHTLY E. A., B~WHILL S. A. and SMITHL. G. MECHTLY E. A.. SECHRISTC. F. JR. and SMITHL. G. MECHTLY E. A., SEINOK. and SMITH L. G. MECHTL.Y E. A., MUKUNDA RAO M., SKAPERDASD. 0. and SMITH L. G.
1972
J. atmos. terr. Phys. 34, 1483.
1970 1971 1971
Astrophys. J. 160, 283. J. atmos. terr. Phys. 33, 1693. J. atmos terr. Phys. 33. I 169.
1976
Nature, bnd.
1970 1975 1974 1972 1971
J. geophys. Res. 15, 1345. Ind. J. Rad. Space Phys. 4. 233. J. atmos. terr. Phys. 36, 989. Radio Sci. 7, 73. J. them. Phys. 54, 3817.
1974 1967
J. them. Ph ys. 61, 3 18 1. Planet. Space Sci. 15. 373.
1969 1971a 197lb 1975
Planet. Space Sci. 17. 1759 J. them. Phys. 55, 2115. J. chum. Phys. 55, 2120. J. geophys. Res. 80, 2229.
1974 1970 1974 1971 1974 1971 1970 1970 1974 1970 1974 1971
Rev. Geophys. Space Phys. 12, 703 Solar Phys. 15, 288. J. atmos. terr. Phys. 36. 1297. J. atmos. terr. Phys. 33, 1893. J. atmos. terr. Phys. 36, 1334. J. geophys. Res. 76, 8352. J. geophys. Res. 75, 6959. J. atmos. terr. Phys. 32, 405 J. them. Phvs. 60. 4362. Annls GPopiys. 26, 547. lnd. J. Rad. Space Phys. 3. 109. J. geophys. Res. 76. 1028.
1973 1972 1974 1972 1970 l972a 1972b 1969a l969b
Rept. lonos. Space Res. 27, 115. J. geophys. Res. 77, 2888. Ind. J. Rad. Space Phys. 3, 99. Solar Phys. 27, 107. Radio Sci. 5, 1407. J. atmos. few. Phys. 34, 1899. J. atmos. terr. Phys. 34. 641. Radio Sci. 4, 37 I. Radio Sci. 4, 5 17.
261,
I 18
D-region processes at equatorial latitudes MEIRA L. G. JR. MITRA A. P. MITRA A. P. and ROWE J. N. NARCISIR. S. and BAILEYA. D. NARCISIR. S., BAILEYA. D., DELLA LUCCA L., SHERMANC. and THOMASD. M. NARCISIR. S., BAILEYA. D., WLODYKA L. E. and PHILBRICKC. R. NEHERH. V. and ANDERSONH. R. NORTONR. B. and Rae G. C. PAULSEND. E., HUFFMANR. E. and LARRABEEJ. C. SCHMIDTKE G., RAWER K., FISCHERW. and REL~T~CKC. SECHRIST C. F. JR. S~MAYAJULUY. V., AVADHANULUM. B. and ZALPURI K. S. SWIDERW. JR. THOMASL. THOMASL.. G~NDHALEKARP. M. and BOWMANM. R. THRANEE. V. THRANEE. V. TIMOTHYA. F. and TIMOTHYJ. G. TIS~NEG. C. TOHMATSUT. and IWAGAMI N. VIDAL-MADJARA., BLAMONTJ. E. and PHISSAMAYB. WEEKSL. H.
1971 1975 1972 1965 1971
J. J. J. J. J.
1972a
J. atmos. terr. Phys. 34, 647.
1962 1972 1972 1975
J. geophys. Res. 67, 1309. J. geophys. Res. 77, 6287. Radio Sci. 7. 51. Space Research XV, p. 345. Berlin. J. atmos. terr. Phys. 34, 1656. Space Research XIII, p. 477. Berlin. Reo. Geophys. 7, 573. Radio Sci. 9, 121. J. atmos. terr. Phys. 35, 397. J. geophys. Res. 74. 131 I. Radio Sci. 7. 143. J. geophys. Res. 75, 6950. J. geophys. Res. 78, 746. J. Geomag. Geolectr. (in press). J. geophys. Res. 78, 1115. Astophys. J. (left.) 147, 1203.
1972 1973 1969 1974a 1973 1969 1972 1970 1973 1976 1973 1967
geophys. Rex 76, atmos. terr. Phys. atmos. terr. Phys. geophys. Res. 70, atmos. tew. Phys.
1009 202. 37, 895. 34, 795. 3687. 33, 1147.
Akademie-Verlag,
Akademie-Verlag,
Reference IS also made to the following unpublished mattv-ial: GNANALINGAMS. and KANE J. A. HEIMERLJ M., VANDERHOFF J. A., PUCKEX-~ L. J. and NILESF. E. NARCISIR. S., SHERMANC., PHILBRICKC. R.. THOMASD. M., BAILEYA. D., WL~DYKA L. E., WWDYKA R. A., BAKERD. and FEDERIC~G. RATNASIRIP. A. J. and SECHRISTC. F. JR.
197s 1972
THOMASL.
1974b
1972b
1975
NASAIGSFC Rept. No. X-623-75-145. BRL Rept. No. 1570. Ballistic Res. Lab.. Aberdeen, Md. Proc. COSPAR Symp. Solar Particle Eamt of November 1969, AFCRL Rept. No. 144. p. 411. Aeronomy Rept. No. 67, University of Illinois, Urbana-Champaign, Illinois. Proc. COSPAR Symp. ‘Methods 01. Measurements und Results of Lower Ionosphere Structure’, Constance, FRG, p. 153.