- Email: [email protected]
Nitric oxide densities and their diurnal asymmetry in the upper middle atmosphere as revealed by ionospheric measurements
Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
www.elsevier.nl/locate/jastp
Nitric oxide densities and their diurnal asymmetry in the upper middle atmosphere as revealed by ionospheric measurements J. Lastovicka ∗ Institute of Atmospheric Physics, BoÄcni II, 141 31 Prague 4, Czech Republic Received 6 June 1999; received in revised form 1 June 2000; accepted 23 June 2000
Abstract The nitric oxide (NO) density is of principal importance for the lower ionosphere as it is the source of the main ionized component. The mesospheric NO density climatology based on HALOE=UARS measurements (Siskind et al., Advances in Space Research 21 (1998) 1353–1362) and a comparison of the HALOE NO density data with some ionospheric data (Friedrich et al., Journal of Atmospheric and Solar-Terrestrial Physics 60 (1998) 1445 –1457) revealed, among others, a large “puzzling” diurnal asymmetry of the NO density. In this paper, the existence of a diurnal asymmetry of the NO density is con rmed by independent data and is extended from the sunrise=sunset HALOE data to the entire daytime. We analyzed multi-frequency radio wave absorption measurements in the lower ionosphere over Central Europe, partly together with solar Lyman- and X-ray uxes, in order to estimate the NO density. The results show that our “ionospheric” NO densities are comparable to, or somewhat higher than, the climatological NO densities of Siskind et al. (1998). They also show that the diurnal asymmetry in summer is a stable and regular feature of the lower ionosphere throughout the daytime, and that there is a substantial diurnal asymmetry in the NO density in the upper middle atmosphere that coincides with that revealed by the HALOE data and which c 2000 Elsevier Science Ltd. All rights reserved. is responsible for the asymmetry in the lower ionosphere. Keywords: Nitric oxide density; Diurnal asymmetry; Lower ionosphere; Upper middle atmosphere
1. Introduction The nitric oxide (NO) density largely determines the ionization in the daytime in the lower ionosphere due to the prominent solar hydrogen Lyman- line, which ionizes only NO. Recently, Siskind et al. (1998) have published the climatology of NO in the mesosphere and thermosphere (50 –160 km) based on SME and HALOE=UARS satellite measurements, while Friedrich et al. (1998) have published the results of comparisons of HALOE=UARS nitric oxide density measurements with other ionospheric data. The rst objective of this brief communication is to provide support to the results of Friedrich et al. (1998) on the diurnal asymmetry of NO density through the results of independent analyses of further ionospheric data. The second objective is to ∗ Corresponding author. Tel.: +420-2-67103055; fax: +4202-72762528. E-mail address: [email protected] (J. Lastovicka).
compare our “ionospheric” NO densities with the climatological NO densities of Siskind et al. (1998). Another objective is to extend the knowledge of the diurnal asymmetry from the sunrise=sunset HALOE data to the entire daytime. Our analyses of ionospheric data were done in the 1970s, 1980s and early 1990s and were based either on simultaneous time development of ionospheric data and solar X-ray and Lyman- uxes, or on solar are e ects in the lower ionosphere, i.e. they were based on experimental ionospheric data. These ionospheric analyses revealed a large diurnal asymmetry in the lower ionosphere in summer, and a corresponding consistent diurnal asymmetry in the NO densities. The initial results of these ionospheric analyses were mostly published in less-known journals (e.g. Studia geophysica et geodaetica) or in journals which later changed their scope (e.g. Pure and Applied Geophysics), and they have never been summarized. The ionospheric data used here are the radio-wave absorption data measured in the lower ionosphere by the A3
c 2000 Elsevier Science Ltd. All rights reserved. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 0 ) 0 0 1 7 7 - 2
22
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
method (oblique incidence on the ionosphere; continuous wave) in Central Europe along the following radio paths: 1539 kHz (Deutschlandfunk — Panska Ves); re ection point 50:3◦ N, 11:8◦ E, L = 2:2; transmitter–receiver distance 390 km; equivalent vertical frequency feq = 0:65–0:7 MHz. 2775 kHz (Kiel — Panska Ves); re ection point 52:45◦ N, 12:45◦ E, L = 2:3; transmitter–receiver distance 517 km; feq = 1 MHz. The solar Lyman-ÿ and other typical E-region ionization agents did not a ect the absorption due to the suciently low feq . The absorption was evaluated at various xed solar-zenith angles, . While the absorption asymmetry analyses were made throughout the year, the NO asymmetry analyses were done for summer, or at least non-winter periods, because in winter the non-solar meteorological control largely dominates the behavior of the lower ionosphere (e.g., Lastovicka et al., 1994). The diurnal asymmetry of absorption was studied at more radio wave frequencies (e.g., Lastovicka, 1978; Lastovicka and Boska, 1982; Boska, 1984) with some data series being longer than the solar cycle. These investigations showed that the asymmetry was quite stable from year to year, at least as regards its sign, that it was large in summer and absent in winter, and that it was con ned to a range of altitudes. The upper boundary of the asymmetry region was coincident with the upper boundary of the region controlled by the solar ionizing Lyman- radiation. 2. NO density under undisturbed conditions Investigations for undisturbed conditions are based on the comparison of simultaneous time development of the radio wave absorption data and the solar X-ray and Lyman-
uxes. As the rst step in this comparison, the dependence of absorption at both frequencies on the solar X-ray and Lyman- uxes was established by the regression analysis. It was done for xed = 40; 50; 60 and 70◦ (e.g., Lastovicka, 1974; Lastovicka and Boska, 1982; Boska, 1984; Lastovicka, 1987) in the form L = AFxm + BF m + C = Lx + L + C;
(1)
where L is the absorption, Fx is the X-ray ux, F is the Lyman- ux, and Lx and L are the corresponding contributions to the total absorption, L. Computations were made for m = 0:3–2. As C was very sensitive to m, the optimum m was selected on the basis of the requirement that C was 5 –10% of the total absorption (contribution of cosmic rays and other minor sources). The 2775 kHz absorption was analyzed for the summers (June–August) of 1969, 1970 and 1972 (Lastovicka, 1974; Lastovicka and Boska, 1982) with di erent satellite data: Lyman- ux from OSO-5 (Vidal-Madjar, 1975) and X-ray ux from SOLRAD-9. The summertime 1539 kHz absorption was analyzed by Boska (1984) for the period 1978–1980 with the Lyman- ux from the AE-E satellite, somewhat corrected
by the AE-E solar Lyman-ÿ data, and satellite X-ray data. The solar activity was medium to high during both periods studied. The correlation of daily values of Fx with F was suciently weak, about r = 0:3, to enable a fairly reliable separation of Lx and L and, so, utilize this approach for estimating the NO density (e.g. Lastovicka and Boska, 1982). The Lx and L have been determined from time variation of absorption, X-ray ux and Lyman- ux. The accuracy of determination of relative variations of the 2775 kHz absorption is better than 2–3% and it is also reasonably good for ionizing uxes; thus, the accuracy of experimental determination of the Lx =L ratio is expected to be several percent. On the other hand, the absolute accuracy of absorption measurements is much worse than the accuracy of variations of absorption due to problems with the zerolevel determination, it is by a factor of 3–5 larger and systematic, not random error. This is the reason, why the Lx =L ratio, which is not a ected by the accuracy of determination of absolute magnitude of absorption, is used for inferring the NO density. Lastovicka (1976a, 1982) has estimated NO densities from analyses of the 2775 kHz absorption from comparison of the observed Lx =L ratio obtained from Eq. (1) with the Lx =L ratio obtained from the model computations of absorption height pro le. These model computations were based on the ionization rate, q, height pro les calculated from the observed intensities of solar ionizing radiation. Above the cosmic ray-dominated level, the model Lx =L at a xed height was considered to be equal to (qx =q )m , the value of m was taken from Eq. (1). This method is described in detail by Lastovicka (1982). Three di erent rocket-measured NO density pro les were used for the initial NO pro le: those of Baker et al. (1977), Meira (1971) extrapolated downward to t the Pontano and Hale (1970) NO density at 60 km, and Tisone (1973). In the present paper, the HALOE pro le of NO density by Siskind et al. (1998) for equinox, 50◦ N, has also been used. The e ect of O2 (1 g ) ionization was neglected in model Lx =L ratio calculations, because it contributed quite insigni cantly (Lastovicka, 1976a, 1982) to the total ionization rate at heights important for absorption. The model Lx =L ratio was compared with the observed Lx =L ratio. The di erence between them was attributed to wrong NO-pro les and therefore the NO-pro les were shifted towards higher or lower NO densities, without changing pro le shape, until there was a t of the model and the observed Lx =L ratios. The accuracy of model computations is dicult to estimate. The method of the NO density determination, described by Lastovicka, (1976a, 1982), has several potential sources of inaccuracy in model computations. The accuracy is not of statistical nature (scatter of data). For instance, the best representative electron density pro les for the given solar zenith angles were selected from available rocket-measured pro les as a basis for model computations of the absorption height pro le and the radio wave re ection height. However, some uncertainty remains whether
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
23
three “old” pro les and the HALOE NO pro le. However, it should be noted that the HALOE NO data between 70 and 80 km are the least certain of the entire data set (Gordley et al., 1996). The estimated values of NO density for =70◦ are slightly lower than those for = 60◦ , while those for =40◦ are higher by a factor of 2 (Lastovicka, 1982). There may be some error in the NO density estimate at = 40◦ as it is based on a smaller data set and could be arti cially enhanced because the e ect of O2 (1 g ) ionization, which increases with decreasing , was neglected. Due to the shape of the height pro le of the 2775 kHz absorption, the NO density determination is only reliable in two height ranges, the primary region is around 91 km and the secondary region is near 77–78 km (Fig. 1). Siskind et al. (1998) presented only winter and equinoctial NO densities. Their equinox, 50◦ N value of NO density near 91 km is by a factor 3– 4 lower than our summer value for = 60◦ while their value at 77–78 km is even lower, a factor of 10. Two e ects can contribute to this di erence: Fig. 1. The initial NO pro les, B, M, S, T (full lines) after Baker et al. (1977), Meira (1971; Pontano and Hale, 1970), Siskind et al. (1998; 50◦ N, equinox) and Tisone (1973), respectively, and their corrected versions B0 , M0 , S0 , T0 for = 60◦ (dashed lines). NO density is in 1013 m−3 . 1, 2 – the two height ranges of the maximum contribution to the total absorption at = 60◦ . Adopted from Lastovicka (1982) and updated.
they really represent typical conditions or not. The ionization and absorption cross-sections of ionizing radiation in model computations are not quite certain. The CIRA model of the neutral atmosphere was applied, which may add another (although rather small) error. There may be also some other sources of systematic errors in model computations. On the other hand, relatively good consistency of the results obtained for di erent solar zenith angles, for di erent periods and di erent heliogeophysical conditions, indicates that the error in NO determination is not extremely high. Moreover, any of the above sources of uncertainty can hardly in uence the diurnal asymmetry, the study of which is the main purpose of this paper. Fig. 1 shows the estimated NO density for = 60◦ . In each case the NO density associated with each pro le was increased. Although shapes and NO densities of the initial NO pro les are quite di erent, the three corrected NO pro les intersect within both height ranges of the maximum contributions to the total absorption. More than 50% of the total absorption is due to the upper-height range and therefore all four corrected NO pro les are in reasonable agreement in this height range. The absorption method only gives a reliable estimate of the NO in the height range of the maximum contribution to the total absorption, where most of Lx and L are created. Therefore, as shown in Fig. 1, the estimate of NO density is almost independent of the assumed initial NO pro le. There is some disagreement between the NO density estimates in the lower height range based on the
(1) Higher level of solar activity for absorption data (1969, 1970, 1972) compared to the Siskind et al. (1998) equinoctial NO data (1992–1995). (2) The use of summer data. In summer the NO density is expected to be somewhat higher than in equinoxes. This is supported by the seasonal variation of absorption which exhibits equinoctial minima. However, these e ects can explain only a part of the difference, particularly at 77–78 km. Rocket measurements at comparable northern middle latitudes (Volgograd) in December 1985 gave NO densities that were even slightly larger than those inferred from the 2775 kHz absorption analysis (Tuchkov et al., 1990). The rocket NO densities, at 90 km, were 1 × 1014 and 1:8 × 1014 m−3 for two rocket
ights under normal winter conditions and higher for winter anomaly conditions, compared to the absorption-derived NO densities (Fig. 1) of (0:9–1:0) × 1014 m−3 . Lastovicka (1987) has also estimated NO densities from the analysis of the 1539 kHz absorption using the same method as for the 2775 kHz absorption. However, in this case only data for = 60 and 70◦ were used. The 1539 kHz signal was sometimes entirely absorbed at = 50 and 40◦ . The a priori NO pro les in this case were from Baker et al. (1977) and Meira (1971) extrapolated downward to t the Pontano and Hale (1970) NO density at 60 km. The analysis was made for the summers of 1978–1980; for more details see Lastovicka (1987). Again, due to the shape of the height pro le of the 1539 kHz absorption, the NO density determination is only reliable in two height ranges. The primary range is around 90 km and the secondary one near 76 –77 km (Lastovicka, 1987). The estimated morning NO densities near 90 km are comparable with those of Siskind et al. (1998) for equinox at 50◦ N. The afternoon NO densities are twice as high. Near 77 km, the NO densities of Siskind et al. (1998) for equinox at 50◦ N are between the morning and afternoon NO densities from the absorption
24
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
analysis. Given the di erence in solar activity level and season, which makes our NO densities slightly higher than those of Siskind et al. (1998), the agreement between them may be considered satisfactory. Friedrich et al. (1998) have calculated electron densities with an ion-chemical model using HALOE NO densities, and compared the calculated and experimental electron densities. The model results suggest that the NO densities are larger below 80 km and smaller above this height than the HALOE NO densities. The ionospheric absorption NO densities also require larger increases of HALOE NO densities below 80 km than above; however, at all altitudes they are comparable to or larger than the HALOE NO densities. There are several possible sources of error in the NO density estimated from the 2775 kHz absorption and three of these are discussed below. First, the SOLRAD-9 solar X-ray ux must be corrected with the gray body spectrum (Dere et al., 1974). Boska and Lastovicka (1983) have found that this correction causes a signi cant change in the ratio Lx =L . However, Lastovicka and Boska (1982) found that the combination of the corrected experimental Lx =L ratio and the corrected model Lx =L ratio results in NO density estimates that are almost identical with the NO density estimates without correction. Thus, the error from this e ect can be neglected. Second, the uncertainty and variability in the absorption cross-section, ionization cross-section and the solar Lyman- line shape can a ect the Lyman- ionization-rate calculations. This problem was treated by Lastovicka (1976b) who found that e ects of ionization cross-section and line shape itself are essentially negligible but the e ect of absorption cross-section can be very signi cant at lower altitudes. Due to the non-constant absorption cross-section across the solar Lyman- line width, the shape of the transmitted line changes with decreasing altitude (i.e. increasing absorption) and so the e ective absorption cross-section also changes with altitude. However, this e ect has been included in the present calculations. A third possibility is that the O2 (1 g ) pro les used in computations of the O2 (1 g ) ionization rate (Hu man et al., 1971; Evans and Llewellyn, 1970) underestimate O2 (1 g ) densities. The model calculations of the Lx =L ratio do not include ionization due to O2 (1 g ) and the analysis of the observational data does not explicitly include the ionization due to O2 (1 g ) in the Lx =L ratio calculations. However, the time variations of the Lyman- ux and O2 (1 g )-ionizing ux are very similar. Therefore, the absorption method cannot distinguish between the two and so implicitly includes the ionization due to O2 (1 g ) in the experimental Lx =L ratio. In this way, the O2 (1 g ) density is transformed into an arti cial additional e ective NO density. However, the O2 (1 g ) ionization rate computed by Friedrich et al. (1998) for the measured O2 (1 g ) densities of Thomas et al. (1984) is only slightly higher than that computed by us (e.g., Lastovicka, 1982). It should be noted that Friedrich et al. (1998) computed the ionization rates for the equator, where the NO density in the upper mesosphere is more than a factor of 2 lower
than that at 50◦ N (data of Siskind et al., 1998) and, therefore, obtained a very important role of O2 (1 g ) at lower solar zenith angles. At 50◦ N, the ionization contribution from O2 (1 g ) is substantially smaller and there could be some overestimation of NO density from the absorption method at small (40◦ ), but almost none at large (70◦ ). This is qualitatively consistent with the observed dependence of the estimated NO densities on (Lastovicka, 1982). 3. NO density under disturbed conditions Morozova and Lastovicka (1985) have analyzed the sudden ionospheric disturbances (SIDs) caused by three moderate ares in the summer of 1978 at = 55–60◦ . They used the 1539 kHz absorption data, i.e. SIDs in the form of SSWFs (sudden short-wave fadeouts), and the AE-E satellite solar Lyman- ux corrected with the AE-E solar Lyman-ÿ and the satellite 0.1– 0.8 nm and 0.05 – 0.3 nm X-ray uxes. The comparison of modelled ionization rates due to hard and soft X-rays, galactic cosmic rays, Lyman-ÿ
ux, Lyman- ux with the di erent NO-pro les by Meira (1971) and Baker et al. (1977), and the O2 (1 g ) ionization rate for quiet (pre- are) and are conditions yielded the model percentage enhancement of the overall ionization rate. Only the X-ray ux enhancement was considered for
are conditions. The increase of other ionizing uxes during these moderate ares was considered negligible. Morozova and Lastovicka (1985) used the relationship (Eqs. (2) and (3)) (valid at a xed height) between absorption L, electron density ne , ionization rate q and e ective recombination coecient e : q = e n2e ;
(2)
L ∼ ne
(3)
with a correction of Eq. (3) for the deviative absorption in the last ∼ 2 km below the re ection height. Taking into account the decrease of re ection height during the are maximum, which was almost 2 km for these moderate ares, Morozova and Lastovicka (1985) obtained the modelled increase of absorption from the modelled percentage ionization rate enhancement in are maximum and Eqs. (2) and (3). This increase was compared with the observed increase of absorption. The di erence between them was attributed to the wrong NO-pro le, thus NO-pro les were shifted towards higher or lower NO densities, until there was a t of the model and an observed increase of absorption. The modelled NO density was closer to the Meira (1971) values than to the Baker et al. (1977) values. It was somewhat lower than the Meira (1971) NO density for the early-morning are and somewhat higher for the afternoon
ares. The morning NO density was approximately a factor of 2 lower than the afternoon NO density. It should be repeated that due to the shape of the 1539 kHz absorption height pro le, the NO density determination for
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
quiet conditions is only reliable in two height ranges. The primary height range is around 90 km and the secondary near 76 –77 km (Lastovicka, 1987). For the modelled ares, the upper height range is shifted downwards by 1–1.5 km. The equinox, 50◦ N climatological value of NO density of Siskind et al. (1998) at 90 km is equal to the morning NO density of Morozova and Lastovicka (1985), while the values for 88.5 –89 km are in approximate agreement. It is possible that the O2 (1 g ) pro les used in computations (Hu man et al., 1971; Evans and Llewellyn, 1970) underestimate O2 (1 g ) densities but this does not have a large impact on the NO densities inferred for disturbed conditions. A strong solar proton are can signi cantly modify the mesospheric ion chemistry and such an event and a large ground-level enhancement of the cosmic ray ux was recorded in October 1989. Rocket measurements, in the southern Indian Ocean at high middle latitudes, detected a large enhancement of the NO density (Zadorozhny et al., 1992). In the upper mesosphere it was more than a factor of 4. An analysis of the daytime radio wave absorption enhancement at similar latitudes in Central Europe shows that the enhanced absorption can be explained basically in terms of the observed increase of NO density (Zadorozhny et al., 1992). 4. Diurnal asymmetry of NO density The results presented in Sections 2 and 3 indicate a possible diurnal asymmetry of NO densities with the afternoon values being higher than the morning values. Friedrich et al. (1998) also reported a “puzzling” diurnal asymmetry of NO densities measured by HALOE=UARS. The ionospheric radio wave absorption measurements in Central Europe along ve radio paths in the frequency range of 185 – 6090 kHz over more than ten years revealed a persistent and stable diurnal asymmetry in monthly values of absorption (Lastovicka, 1977, 1978; Lastovicka and Boska, 1982; Boska, 1984). In summer (April–August), the asymmetry is always present and large, with signi cantly larger absorption in the afternoon compared to the morning. Fig. 2 shows an example of such asymmetry for the 2775 kHz absorption. In the winter period (October– February), the mean asymmetry is small or zero, and in individual months it is always rather small and may have opposite signs. Thus the diurnal asymmetry appears to be a well-established phenomenon in the northern midlatitude lower ionosphere. The asymmetry seems to be con ned to a height range. It is probably developed best at altitudes of about 70 –90 km (it depends on solar zenith angle), and either disappears or shows reverse behavior before reaching a height of 100 km (Lastovicka, 1978; Monroe and Smith, 1975). Larger morning values as compared to the afternoon ones (reverse asymmetry) prevail in foE in southeastern Europe in summer (Pancheva and Mukhtarov, 1998).
25
Fig. 2. Mean diurnal variation of the 2775 kHz absorption in May for the period 1966 –1973. L = L − Lmax ; t = |tnoon − t|.
The behavior of the background neutral atmosphere density, temperature and pressure cannot account for the asymmetry in absorption (Lastovicka, 1978). The magnitude of the asymmetry depends on the equivalent vertical frequency of the individual radio paths, i.e. on their re ection heights. Lastovicka (1978) suggested that the changes of electron density, caused by diurnal asymmetry of NO density, might be the cause of the observed diurnal asymmetry in absorption. The reason was the coincidence of the height range of the pronounced asymmetry with the height range of the dominant nitric oxide ionization rate, independent of the solar zenith angle. The analysis of three solar are e ects (Section 3) indicated the possibility of substantially lower NO densities in the morning compared to the afternoon. An analysis of the 1539 kHz absorption during three summers (Boska, 1984; Lastovicka, 1987), brie y mentioned in Section 2, also indicated a signi cant diurnal asymmetry of NO density. Fig. 3 shows the estimated NO densities, inferred from the behavior of the 1539 kHz absorption, as a range of values of the corrected Baker et al. (1977) and Meira (1971) pro les, for both morning and afternoon. Each range is constructed from ve individual pro les for the summers of 1978–1980 for = 60◦ and 70◦ ; the sixth pro le was omitted as it was considered to be extreme and erroneous. Fig. 3 indicates a substantial asymmetry of about a factor of 2. As the ranges of NO curves are separated, the asymmetry must exist for all ve individual cases. Thus estimates of the NO density, based on ionospheric absorption data, provide an evident and substantial diurnal asymmetry of NO density. The width of the ranges of the NO curves in Fig. 3
26
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
Fig. 3. The range of corrected (Meira 1971; Pontano and Hale, 1970) and Baker et al. (1977) NO densities for forenoon (Mf ; Bf ) and afternoon (Ma ; Ba ). 1, 2 — the regions of the maximum contribution to the total absorption (mixed = 60◦ and 70◦ ). Adopted from Lastovicka (1987).
is partly caused by the signi cantly di erent level of solar activity in the summers of 1978, 1979 and 1980. The sunset=sunrise ratio in the HALOE=UARS equatorial NO density pro les (Friedrich et al., 1998) is approximately a factor of two near 90 km, equal to the asymmetry from the absorption “ionospheric” NO densities. The sunset=sunrise asymmetry in equatorial NO=HALOE is most developed (up to a factor of 10) at heights of 70 –85 km, the same heights as those inferred from the absorption analysis (Lastovicka, 1978). The results in Fig. 3 in this height range indicate an asymmetry close to a factor of 2. In general, various absorption data have an asymmetry and so indicate a nitric oxide asymmetry that is evidently less than a factor of 10. However, Siskind et al. (1998) found a decrease of the sunset=sunrise ratio (i.e. asymmetry) of NO densities towards high latitudes and its reduction to a factor of 2 at 57◦ N. This makes the NO asymmetry comparable to that based on the ionospheric absorption data. Thus although the HALOE NO data are for sunrise and sunset (occultation measurements), while absorption data are daytime data, the diurnal asymmetry in NO densities appears to be quite similar. However, the diurnal asymmetry of absorption could, in principle, be caused or signi cantly a ected by diurnal and semidiurnal tides. In that case the interpretation in terms of the asymmetry in the NO density would be wrong. Unfortunately, tidal e ects in the lower ionosphere have never been studied (Lastovicka, 1997). The tides in the neutral atmosphere at “absorption” heights (∼ 75–95 km) have been observed mostly in wind data and partly in temperature data; the density variations were deduced from the tides in temperature and winds (e.g., Forbes et al., 1994). The dominant tidal modes are the semidiurnal and diurnal tides (Forbes et al., 1994; Manson et al., 1989,1999).
A persistent diurnal asymmetry over a 14-h-long range, from 5 to 19 h (time 0 = 12 LT, i.e. noon) is shown in Fig. 2. The absolute value of maximum absorption (Lmax ) in Fig. 2 is 45 dB. Thus, L = −25 dB means the absolute value of absorption is 20 dB. The time 7 h corresponds to = 85◦ and taking this into account, Fig. 2 shows that the afternoon=morning ratio even slightly increases towards sunset ( = 90◦ ). It is impossible to explain such an asymmetry, persistent over at least 14 h by the e ect of the semidiurnal (12-h) tide. The results in Fig. 2 do not display a detectable signature of a 12-h oscillation. However, if the asymmetry were solely of tidal origin with tidal maxima at 0 and 6 h (in terms of Fig. 2) it would result in no asymmetry. A maximum at 3 h would result in large asymmetry at 3 h, no asymmetry at 6 h, and reverse asymmetry at 7 h. The maximum at 1 h would result in no asymmetry at 6 h. A strong e ect of the diurnal tide on the asymmetry can hardly be expected because a detectable signature of the semidiurnal tide has not been observed. Moreover, Fig. 12 of Manson et al. (1999) consistently provides the eastward maximum (= phase) for the zonal wind diurnal tide in the afternoon, even if earlier in winter and later in summer, whereas in the absorption there is substantial asymmetry in summer and none in winter. As far as I know, the seasonal variation of the diurnal tide phase in atmospheric density and pressure is not known from observations at the heights in question (75 –95 km). Consequently, the tides cannot be the direct cause of the asymmetry. 5. Conclusions A review of our earlier results on the estimate of the NO density from the lower ionospheric measurements has been presented. These results have been extended with the application of the HALOE NO density pro le as another initial NO pro le for our analyses. The results from Central Europe yield NO densities that are comparable to or higher than the relatively high HALOE=UARS climatological NO densities of Siskind et al. (1998). This di erence may be partly attributed to di erent seasons and solar activity levels of ionospheric and HALOE measurements. There may be some uncertainty due to the uncertain role of the O2 (1 g ) ionization in the lower ionosphere and its omission in the “ionospheric” NO density estimating procedure. The most important result of this study is support for the existence of a substantial diurnal asymmetry, as observed in HALOE=UARS NO densities (Friedrich et al., 1998), and its extension to the entire daytime by ionospheric data. Both the NO densities inferred from the variability of radio wave absorption in the lower ionosphere and the ionospheric data themselves reveal a consistent substantial diurnal asymmetry in summer. This asymmetry is not con ned to sunset and sunrise (HALOE measurements are solar occultation measurements), but is well developed throughout the whole
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
daytime period (Fig. 2; Lastovicka, 1977, 1978). The ionospheric data show that the asymmetry is a regular and stable phenomenon over at least one solar cycle. In other words, the diurnal asymmetry reported by Friedrich et al. (1998) is not con ned to the several years of HALOE observations but it is a regular feature of the upper mesosphere. Ionospheric data cannot provide information about the NO densities in winter due to the dominance of meteorological control in winter (e.g., Lastovicka et al., 1994). The support for the HALOE NO density asymmetry is signi cant, because the HALOE NO data between 70 and 80 km are the least certain of the entire data set (Gordley et al., 1996). However, the explanation of this NO asymmetry presents a serious challenge for middle atmosphere modellers. References Baker, K.D., Nagy, A.F., Olsen, R.O., Oran, E.S., Randhawa, J., Strobel, D.F., Tohmatsu, T., 1977. Measurement of the nitric oxide altitude distribution in the midlatitude mesosphere. Journal of Geophysical Research 82, 3281–3286. Boska, J., 1984. Ionospheric absorption on 1539 kHz in relation to solar ionizing radiation. Handbook for MAP 10, 31–33. Boska, J., Lastovicka, J., 1983. On the role of solar Lyman alpha radiation in radio-wave absorption in the D-region. Studia geophysica et geodaetica 27, 85–99. Dere, K.P., Horan, D.M., Kreplin, R.W., 1974. The spectral dependence of solar soft X-ray ux values obtained by SOLRAD 9. Journal of Atmospheric and Terrestrial Physics 36, 989–994. Evans, W.F.J., Llewellyn, E.J., 1970. Molecular oxygen emission in the airglow. Annales de Geophysique 26, 167–178. Forbes, J.M., Manson, A.H., Vincent, R.A., Fraser, G.J., Vial, F., Wand, R., Avery, S.K., Clark, R.R., Johnson, R., Roper, R., Schminder, R., Tsuda, T., Kazimirovsky, E.S., 1994. Semidiurnal tide in the 80 –150 km region: an assimilative data analysis. Journal of Atmospheric and Terrestrial Physics 56, 1237–1249. Friedrich, M., Siskind, D.E., Torkar, K.M., 1998. HALOE nitric oxide measurements in view of ionospheric data. Journal of Atmospheric and Solar-Terrestrial Physics 60, 1445–1457. Gordley, L.L., Russell III, J.M., Mickley, J.L., Frederick, J.E., Park, J.H., Stone, K.A., Beaver, G.M., McInerney, J.M., Deaver, L.E., Toon, G.C., Murcray, F.J., Blatherwik, R.D., Gunson, M.R., Abbatt, J.D., Mauldin III, R.L., Mount, G.H., Sen, B., Blavier, J.-F., 1996. Validation of nitric oxide and nitrogen dioxide measurements made by the halogen occultation experiment for UARS platform. Journal of Geophysical Research 101 (D6), 10241–10266. Hu man, R.E., Paulsen, D.E., Larrabee, J.C., Cairns, R.D., 1971. Decrease in D-region O2 (1g ) photoionization rates resulted from CO2 absorption. Journal of Geophysical Research 76, 1028–1038. Lastovicka, J., 1974. On the role of the solar Lyman- radiation in ionospheric absorption at 2775 kHz. Pure and Applied Geophysics 112, 655–663. Lastovicka, J., 1976a. The dependence of the ionospheric absorption at 2775 kHz on the intensity of ionizing radiation — ionospheric implications. Pure and Applied Geophysics 114, 321–331.
27
Lastovicka, J., 1976b. On some sources of uncertainty in the Lyman- ionization-rate calculation. Studia geophysica et geodaetica 20, 273–283. Lastovicka, J., 1977. Seasonal variation in the asymmetry of diurnal variation of absorption in the lower ionosphere. Journal of Atmospheric and Terrestrial Physics 39, 891–894. Lastovicka, J., 1978. Diurnal asymmetry in the lower ionosphere — seasonal variability. Studia geophysica et geodaetica 22, 309–313. Lastovicka, J., 1982. Nitric oxide concentration near the mesopause as deduced from ionospheric absorption measurements. Pure and Applied Geophysics 120, 607–614. Lastovicka, J., 1987. Diurnal asymmetry of NO concentration in the summer upper mesosphere in Central Europe. Advances in Space Research 7 (9), 111–114. Lastovicka, J., 1997. Observations of tides and planetary waves in the atmosphere-ionosphere system. Advances in Space Research 20, 1209–1222. Lastovicka, J., Alberca, L.F., Benediktov, E.E., Boska, J., Bremer, J., Entzian, G., Fiser, V., Jodogne, J.-C., Knyazev, A.K., Marcz, F., de la Morena, B.A., Mukhtarov, Pl., Ogawa, T., Pancheva, D., Ranta, H., Rapoport, Z.Ts., Xiong Nianlu, 1994. Lower ionosphere at middle latitudes: its morphology and response to meteorological and solar-terrestrial activity during the DYANA campaign 1990. Journal of Atmospheric and Terrestrial Physics 56, 1947–1962. Lastovicka, J., Boska, J., 1982. The dominant role of solar Lyman- in D-region radio wave absorption. Journal of Atmospheric and Terrestrial Research 44, 793–795. Manson, A., Meek, C., Hagan, M., Hall, C., Hocking, W., MacDougall, J., Franke, S., Riggin, D., Fritts, D., Vincent, R., Burrage, M., 1999. Seasonal variations of the semi-diurnal and diurnal tides in the MLT: multi-year MF radar observations from 2 to 70◦ N, and the GSWM tidal model. Journal of Atmospheric and Terrestrial Physics 61, 809–828. Manson, A.H., Meek, C.E., Teitelbaum, H., Vial, F., Schminder, R., Kurschner, D., Smith, M.J., Fraser, G.J., Clark, R.R., 1989. Climatologies of the semidiurnal and diurnal tides in the middle atmosphere (70 –100 km) at middle latitudes (45 –55◦ ). Journal of Atmospheric and Terrestrial Physics 51, 573–579. Meira, L.G., 1971. Rocket measurements of upper atmospheric nitric oxide and their consequences to the lower ionosphere. Journal of Geophysical Research 76, 201–212. Monroe, P.E., Smith, L.G., 1975. Determination of nitric oxide concentration from sunrise E-region electron density. Radio Science 10, 317–329. Morozova, L.P., Lastovicka, J., 1985. Estimates of nitric oxide density in the mesosphere based on changes of radio wave absorption during sudden ionospheric disturbances. Geomagnetizm i Aeronomiya 25, 322–324 (in Russian). Pancheva, D., Mukhtarov, P., 1998. A diurnal asymmetry of the monthly median E-region critical frequency. Advances in Space Research 22, 771–774. Pontano, B.A., Hale, L.C., 1970. Measurements of N ionizable constituents of the low ionosphere using a Lyman- source and blunt probe. Space Research 10, 208–214. Siskind, D.E., Barth, C.A., Russell III, J.M., 1998. A climatology of nitric oxide in the mesosphere and thermosphere. Advances in Space Research 21, 1353–1362. Thomas, R.J., Barth, C.A., Rusch, D.W., Sanders, R.W., 1984. Solar mesospheric explorer near-infrared spectrometer: measurements of 1:27 m radiances and the inference of
28
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
mesospheric ozone. Journal of Geophysical Research 89, 9569–9580. Tisone, C.C., 1973. Measurements of NO densities during sunrise at Kauai. Journal of Geophysical Research 78, 746–750. Tuchkov, G.A., Zadorozhny, M.A., Lastovicka, J., 1990. Nitric oxide density measurements at middle latitudes. Studia geophysica et geodaetica 34, 261–268.
Vidal-Madjar, A., 1975. Evolution of the solar Lyman alpha ux during four consecutive years. Solar Physics 40, 69–82. Zadorozhny, A.M., Tuchkov, G.A., Kikhtenko, V.N., Lastovicka, J., Boska, J., Novak, A., 1992. Nitric oxide and lower ionospheric quantities during solar particle events of October 1989 after rocket and ground-based measurements. Journal of Atmospheric and Terrestrial Physics 54, 183–192.
www.elsevier.nl/locate/jastp
Nitric oxide densities and their diurnal asymmetry in the upper middle atmosphere as revealed by ionospheric measurements J. Lastovicka ∗ Institute of Atmospheric Physics, BoÄcni II, 141 31 Prague 4, Czech Republic Received 6 June 1999; received in revised form 1 June 2000; accepted 23 June 2000
Abstract The nitric oxide (NO) density is of principal importance for the lower ionosphere as it is the source of the main ionized component. The mesospheric NO density climatology based on HALOE=UARS measurements (Siskind et al., Advances in Space Research 21 (1998) 1353–1362) and a comparison of the HALOE NO density data with some ionospheric data (Friedrich et al., Journal of Atmospheric and Solar-Terrestrial Physics 60 (1998) 1445 –1457) revealed, among others, a large “puzzling” diurnal asymmetry of the NO density. In this paper, the existence of a diurnal asymmetry of the NO density is con rmed by independent data and is extended from the sunrise=sunset HALOE data to the entire daytime. We analyzed multi-frequency radio wave absorption measurements in the lower ionosphere over Central Europe, partly together with solar Lyman- and X-ray uxes, in order to estimate the NO density. The results show that our “ionospheric” NO densities are comparable to, or somewhat higher than, the climatological NO densities of Siskind et al. (1998). They also show that the diurnal asymmetry in summer is a stable and regular feature of the lower ionosphere throughout the daytime, and that there is a substantial diurnal asymmetry in the NO density in the upper middle atmosphere that coincides with that revealed by the HALOE data and which c 2000 Elsevier Science Ltd. All rights reserved. is responsible for the asymmetry in the lower ionosphere. Keywords: Nitric oxide density; Diurnal asymmetry; Lower ionosphere; Upper middle atmosphere
1. Introduction The nitric oxide (NO) density largely determines the ionization in the daytime in the lower ionosphere due to the prominent solar hydrogen Lyman- line, which ionizes only NO. Recently, Siskind et al. (1998) have published the climatology of NO in the mesosphere and thermosphere (50 –160 km) based on SME and HALOE=UARS satellite measurements, while Friedrich et al. (1998) have published the results of comparisons of HALOE=UARS nitric oxide density measurements with other ionospheric data. The rst objective of this brief communication is to provide support to the results of Friedrich et al. (1998) on the diurnal asymmetry of NO density through the results of independent analyses of further ionospheric data. The second objective is to ∗ Corresponding author. Tel.: +420-2-67103055; fax: +4202-72762528. E-mail address: [email protected] (J. Lastovicka).
compare our “ionospheric” NO densities with the climatological NO densities of Siskind et al. (1998). Another objective is to extend the knowledge of the diurnal asymmetry from the sunrise=sunset HALOE data to the entire daytime. Our analyses of ionospheric data were done in the 1970s, 1980s and early 1990s and were based either on simultaneous time development of ionospheric data and solar X-ray and Lyman- uxes, or on solar are e ects in the lower ionosphere, i.e. they were based on experimental ionospheric data. These ionospheric analyses revealed a large diurnal asymmetry in the lower ionosphere in summer, and a corresponding consistent diurnal asymmetry in the NO densities. The initial results of these ionospheric analyses were mostly published in less-known journals (e.g. Studia geophysica et geodaetica) or in journals which later changed their scope (e.g. Pure and Applied Geophysics), and they have never been summarized. The ionospheric data used here are the radio-wave absorption data measured in the lower ionosphere by the A3
c 2000 Elsevier Science Ltd. All rights reserved. 1364-6826/01/$ - see front matter PII: S 1 3 6 4 - 6 8 2 6 ( 0 0 ) 0 0 1 7 7 - 2
22
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
method (oblique incidence on the ionosphere; continuous wave) in Central Europe along the following radio paths: 1539 kHz (Deutschlandfunk — Panska Ves); re ection point 50:3◦ N, 11:8◦ E, L = 2:2; transmitter–receiver distance 390 km; equivalent vertical frequency feq = 0:65–0:7 MHz. 2775 kHz (Kiel — Panska Ves); re ection point 52:45◦ N, 12:45◦ E, L = 2:3; transmitter–receiver distance 517 km; feq = 1 MHz. The solar Lyman-ÿ and other typical E-region ionization agents did not a ect the absorption due to the suciently low feq . The absorption was evaluated at various xed solar-zenith angles, . While the absorption asymmetry analyses were made throughout the year, the NO asymmetry analyses were done for summer, or at least non-winter periods, because in winter the non-solar meteorological control largely dominates the behavior of the lower ionosphere (e.g., Lastovicka et al., 1994). The diurnal asymmetry of absorption was studied at more radio wave frequencies (e.g., Lastovicka, 1978; Lastovicka and Boska, 1982; Boska, 1984) with some data series being longer than the solar cycle. These investigations showed that the asymmetry was quite stable from year to year, at least as regards its sign, that it was large in summer and absent in winter, and that it was con ned to a range of altitudes. The upper boundary of the asymmetry region was coincident with the upper boundary of the region controlled by the solar ionizing Lyman- radiation. 2. NO density under undisturbed conditions Investigations for undisturbed conditions are based on the comparison of simultaneous time development of the radio wave absorption data and the solar X-ray and Lyman-
uxes. As the rst step in this comparison, the dependence of absorption at both frequencies on the solar X-ray and Lyman- uxes was established by the regression analysis. It was done for xed = 40; 50; 60 and 70◦ (e.g., Lastovicka, 1974; Lastovicka and Boska, 1982; Boska, 1984; Lastovicka, 1987) in the form L = AFxm + BF m + C = Lx + L + C;
(1)
where L is the absorption, Fx is the X-ray ux, F is the Lyman- ux, and Lx and L are the corresponding contributions to the total absorption, L. Computations were made for m = 0:3–2. As C was very sensitive to m, the optimum m was selected on the basis of the requirement that C was 5 –10% of the total absorption (contribution of cosmic rays and other minor sources). The 2775 kHz absorption was analyzed for the summers (June–August) of 1969, 1970 and 1972 (Lastovicka, 1974; Lastovicka and Boska, 1982) with di erent satellite data: Lyman- ux from OSO-5 (Vidal-Madjar, 1975) and X-ray ux from SOLRAD-9. The summertime 1539 kHz absorption was analyzed by Boska (1984) for the period 1978–1980 with the Lyman- ux from the AE-E satellite, somewhat corrected
by the AE-E solar Lyman-ÿ data, and satellite X-ray data. The solar activity was medium to high during both periods studied. The correlation of daily values of Fx with F was suciently weak, about r = 0:3, to enable a fairly reliable separation of Lx and L and, so, utilize this approach for estimating the NO density (e.g. Lastovicka and Boska, 1982). The Lx and L have been determined from time variation of absorption, X-ray ux and Lyman- ux. The accuracy of determination of relative variations of the 2775 kHz absorption is better than 2–3% and it is also reasonably good for ionizing uxes; thus, the accuracy of experimental determination of the Lx =L ratio is expected to be several percent. On the other hand, the absolute accuracy of absorption measurements is much worse than the accuracy of variations of absorption due to problems with the zerolevel determination, it is by a factor of 3–5 larger and systematic, not random error. This is the reason, why the Lx =L ratio, which is not a ected by the accuracy of determination of absolute magnitude of absorption, is used for inferring the NO density. Lastovicka (1976a, 1982) has estimated NO densities from analyses of the 2775 kHz absorption from comparison of the observed Lx =L ratio obtained from Eq. (1) with the Lx =L ratio obtained from the model computations of absorption height pro le. These model computations were based on the ionization rate, q, height pro les calculated from the observed intensities of solar ionizing radiation. Above the cosmic ray-dominated level, the model Lx =L at a xed height was considered to be equal to (qx =q )m , the value of m was taken from Eq. (1). This method is described in detail by Lastovicka (1982). Three di erent rocket-measured NO density pro les were used for the initial NO pro le: those of Baker et al. (1977), Meira (1971) extrapolated downward to t the Pontano and Hale (1970) NO density at 60 km, and Tisone (1973). In the present paper, the HALOE pro le of NO density by Siskind et al. (1998) for equinox, 50◦ N, has also been used. The e ect of O2 (1 g ) ionization was neglected in model Lx =L ratio calculations, because it contributed quite insigni cantly (Lastovicka, 1976a, 1982) to the total ionization rate at heights important for absorption. The model Lx =L ratio was compared with the observed Lx =L ratio. The di erence between them was attributed to wrong NO-pro les and therefore the NO-pro les were shifted towards higher or lower NO densities, without changing pro le shape, until there was a t of the model and the observed Lx =L ratios. The accuracy of model computations is dicult to estimate. The method of the NO density determination, described by Lastovicka, (1976a, 1982), has several potential sources of inaccuracy in model computations. The accuracy is not of statistical nature (scatter of data). For instance, the best representative electron density pro les for the given solar zenith angles were selected from available rocket-measured pro les as a basis for model computations of the absorption height pro le and the radio wave re ection height. However, some uncertainty remains whether
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
23
three “old” pro les and the HALOE NO pro le. However, it should be noted that the HALOE NO data between 70 and 80 km are the least certain of the entire data set (Gordley et al., 1996). The estimated values of NO density for =70◦ are slightly lower than those for = 60◦ , while those for =40◦ are higher by a factor of 2 (Lastovicka, 1982). There may be some error in the NO density estimate at = 40◦ as it is based on a smaller data set and could be arti cially enhanced because the e ect of O2 (1 g ) ionization, which increases with decreasing , was neglected. Due to the shape of the height pro le of the 2775 kHz absorption, the NO density determination is only reliable in two height ranges, the primary region is around 91 km and the secondary region is near 77–78 km (Fig. 1). Siskind et al. (1998) presented only winter and equinoctial NO densities. Their equinox, 50◦ N value of NO density near 91 km is by a factor 3– 4 lower than our summer value for = 60◦ while their value at 77–78 km is even lower, a factor of 10. Two e ects can contribute to this di erence: Fig. 1. The initial NO pro les, B, M, S, T (full lines) after Baker et al. (1977), Meira (1971; Pontano and Hale, 1970), Siskind et al. (1998; 50◦ N, equinox) and Tisone (1973), respectively, and their corrected versions B0 , M0 , S0 , T0 for = 60◦ (dashed lines). NO density is in 1013 m−3 . 1, 2 – the two height ranges of the maximum contribution to the total absorption at = 60◦ . Adopted from Lastovicka (1982) and updated.
they really represent typical conditions or not. The ionization and absorption cross-sections of ionizing radiation in model computations are not quite certain. The CIRA model of the neutral atmosphere was applied, which may add another (although rather small) error. There may be also some other sources of systematic errors in model computations. On the other hand, relatively good consistency of the results obtained for di erent solar zenith angles, for di erent periods and di erent heliogeophysical conditions, indicates that the error in NO determination is not extremely high. Moreover, any of the above sources of uncertainty can hardly in uence the diurnal asymmetry, the study of which is the main purpose of this paper. Fig. 1 shows the estimated NO density for = 60◦ . In each case the NO density associated with each pro le was increased. Although shapes and NO densities of the initial NO pro les are quite di erent, the three corrected NO pro les intersect within both height ranges of the maximum contributions to the total absorption. More than 50% of the total absorption is due to the upper-height range and therefore all four corrected NO pro les are in reasonable agreement in this height range. The absorption method only gives a reliable estimate of the NO in the height range of the maximum contribution to the total absorption, where most of Lx and L are created. Therefore, as shown in Fig. 1, the estimate of NO density is almost independent of the assumed initial NO pro le. There is some disagreement between the NO density estimates in the lower height range based on the
(1) Higher level of solar activity for absorption data (1969, 1970, 1972) compared to the Siskind et al. (1998) equinoctial NO data (1992–1995). (2) The use of summer data. In summer the NO density is expected to be somewhat higher than in equinoxes. This is supported by the seasonal variation of absorption which exhibits equinoctial minima. However, these e ects can explain only a part of the difference, particularly at 77–78 km. Rocket measurements at comparable northern middle latitudes (Volgograd) in December 1985 gave NO densities that were even slightly larger than those inferred from the 2775 kHz absorption analysis (Tuchkov et al., 1990). The rocket NO densities, at 90 km, were 1 × 1014 and 1:8 × 1014 m−3 for two rocket
ights under normal winter conditions and higher for winter anomaly conditions, compared to the absorption-derived NO densities (Fig. 1) of (0:9–1:0) × 1014 m−3 . Lastovicka (1987) has also estimated NO densities from the analysis of the 1539 kHz absorption using the same method as for the 2775 kHz absorption. However, in this case only data for = 60 and 70◦ were used. The 1539 kHz signal was sometimes entirely absorbed at = 50 and 40◦ . The a priori NO pro les in this case were from Baker et al. (1977) and Meira (1971) extrapolated downward to t the Pontano and Hale (1970) NO density at 60 km. The analysis was made for the summers of 1978–1980; for more details see Lastovicka (1987). Again, due to the shape of the height pro le of the 1539 kHz absorption, the NO density determination is only reliable in two height ranges. The primary range is around 90 km and the secondary one near 76 –77 km (Lastovicka, 1987). The estimated morning NO densities near 90 km are comparable with those of Siskind et al. (1998) for equinox at 50◦ N. The afternoon NO densities are twice as high. Near 77 km, the NO densities of Siskind et al. (1998) for equinox at 50◦ N are between the morning and afternoon NO densities from the absorption
24
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
analysis. Given the di erence in solar activity level and season, which makes our NO densities slightly higher than those of Siskind et al. (1998), the agreement between them may be considered satisfactory. Friedrich et al. (1998) have calculated electron densities with an ion-chemical model using HALOE NO densities, and compared the calculated and experimental electron densities. The model results suggest that the NO densities are larger below 80 km and smaller above this height than the HALOE NO densities. The ionospheric absorption NO densities also require larger increases of HALOE NO densities below 80 km than above; however, at all altitudes they are comparable to or larger than the HALOE NO densities. There are several possible sources of error in the NO density estimated from the 2775 kHz absorption and three of these are discussed below. First, the SOLRAD-9 solar X-ray ux must be corrected with the gray body spectrum (Dere et al., 1974). Boska and Lastovicka (1983) have found that this correction causes a signi cant change in the ratio Lx =L . However, Lastovicka and Boska (1982) found that the combination of the corrected experimental Lx =L ratio and the corrected model Lx =L ratio results in NO density estimates that are almost identical with the NO density estimates without correction. Thus, the error from this e ect can be neglected. Second, the uncertainty and variability in the absorption cross-section, ionization cross-section and the solar Lyman- line shape can a ect the Lyman- ionization-rate calculations. This problem was treated by Lastovicka (1976b) who found that e ects of ionization cross-section and line shape itself are essentially negligible but the e ect of absorption cross-section can be very signi cant at lower altitudes. Due to the non-constant absorption cross-section across the solar Lyman- line width, the shape of the transmitted line changes with decreasing altitude (i.e. increasing absorption) and so the e ective absorption cross-section also changes with altitude. However, this e ect has been included in the present calculations. A third possibility is that the O2 (1 g ) pro les used in computations of the O2 (1 g ) ionization rate (Hu man et al., 1971; Evans and Llewellyn, 1970) underestimate O2 (1 g ) densities. The model calculations of the Lx =L ratio do not include ionization due to O2 (1 g ) and the analysis of the observational data does not explicitly include the ionization due to O2 (1 g ) in the Lx =L ratio calculations. However, the time variations of the Lyman- ux and O2 (1 g )-ionizing ux are very similar. Therefore, the absorption method cannot distinguish between the two and so implicitly includes the ionization due to O2 (1 g ) in the experimental Lx =L ratio. In this way, the O2 (1 g ) density is transformed into an arti cial additional e ective NO density. However, the O2 (1 g ) ionization rate computed by Friedrich et al. (1998) for the measured O2 (1 g ) densities of Thomas et al. (1984) is only slightly higher than that computed by us (e.g., Lastovicka, 1982). It should be noted that Friedrich et al. (1998) computed the ionization rates for the equator, where the NO density in the upper mesosphere is more than a factor of 2 lower
than that at 50◦ N (data of Siskind et al., 1998) and, therefore, obtained a very important role of O2 (1 g ) at lower solar zenith angles. At 50◦ N, the ionization contribution from O2 (1 g ) is substantially smaller and there could be some overestimation of NO density from the absorption method at small (40◦ ), but almost none at large (70◦ ). This is qualitatively consistent with the observed dependence of the estimated NO densities on (Lastovicka, 1982). 3. NO density under disturbed conditions Morozova and Lastovicka (1985) have analyzed the sudden ionospheric disturbances (SIDs) caused by three moderate ares in the summer of 1978 at = 55–60◦ . They used the 1539 kHz absorption data, i.e. SIDs in the form of SSWFs (sudden short-wave fadeouts), and the AE-E satellite solar Lyman- ux corrected with the AE-E solar Lyman-ÿ and the satellite 0.1– 0.8 nm and 0.05 – 0.3 nm X-ray uxes. The comparison of modelled ionization rates due to hard and soft X-rays, galactic cosmic rays, Lyman-ÿ
ux, Lyman- ux with the di erent NO-pro les by Meira (1971) and Baker et al. (1977), and the O2 (1 g ) ionization rate for quiet (pre- are) and are conditions yielded the model percentage enhancement of the overall ionization rate. Only the X-ray ux enhancement was considered for
are conditions. The increase of other ionizing uxes during these moderate ares was considered negligible. Morozova and Lastovicka (1985) used the relationship (Eqs. (2) and (3)) (valid at a xed height) between absorption L, electron density ne , ionization rate q and e ective recombination coecient e : q = e n2e ;
(2)
L ∼ ne
(3)
with a correction of Eq. (3) for the deviative absorption in the last ∼ 2 km below the re ection height. Taking into account the decrease of re ection height during the are maximum, which was almost 2 km for these moderate ares, Morozova and Lastovicka (1985) obtained the modelled increase of absorption from the modelled percentage ionization rate enhancement in are maximum and Eqs. (2) and (3). This increase was compared with the observed increase of absorption. The di erence between them was attributed to the wrong NO-pro le, thus NO-pro les were shifted towards higher or lower NO densities, until there was a t of the model and an observed increase of absorption. The modelled NO density was closer to the Meira (1971) values than to the Baker et al. (1977) values. It was somewhat lower than the Meira (1971) NO density for the early-morning are and somewhat higher for the afternoon
ares. The morning NO density was approximately a factor of 2 lower than the afternoon NO density. It should be repeated that due to the shape of the 1539 kHz absorption height pro le, the NO density determination for
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
quiet conditions is only reliable in two height ranges. The primary height range is around 90 km and the secondary near 76 –77 km (Lastovicka, 1987). For the modelled ares, the upper height range is shifted downwards by 1–1.5 km. The equinox, 50◦ N climatological value of NO density of Siskind et al. (1998) at 90 km is equal to the morning NO density of Morozova and Lastovicka (1985), while the values for 88.5 –89 km are in approximate agreement. It is possible that the O2 (1 g ) pro les used in computations (Hu man et al., 1971; Evans and Llewellyn, 1970) underestimate O2 (1 g ) densities but this does not have a large impact on the NO densities inferred for disturbed conditions. A strong solar proton are can signi cantly modify the mesospheric ion chemistry and such an event and a large ground-level enhancement of the cosmic ray ux was recorded in October 1989. Rocket measurements, in the southern Indian Ocean at high middle latitudes, detected a large enhancement of the NO density (Zadorozhny et al., 1992). In the upper mesosphere it was more than a factor of 4. An analysis of the daytime radio wave absorption enhancement at similar latitudes in Central Europe shows that the enhanced absorption can be explained basically in terms of the observed increase of NO density (Zadorozhny et al., 1992). 4. Diurnal asymmetry of NO density The results presented in Sections 2 and 3 indicate a possible diurnal asymmetry of NO densities with the afternoon values being higher than the morning values. Friedrich et al. (1998) also reported a “puzzling” diurnal asymmetry of NO densities measured by HALOE=UARS. The ionospheric radio wave absorption measurements in Central Europe along ve radio paths in the frequency range of 185 – 6090 kHz over more than ten years revealed a persistent and stable diurnal asymmetry in monthly values of absorption (Lastovicka, 1977, 1978; Lastovicka and Boska, 1982; Boska, 1984). In summer (April–August), the asymmetry is always present and large, with signi cantly larger absorption in the afternoon compared to the morning. Fig. 2 shows an example of such asymmetry for the 2775 kHz absorption. In the winter period (October– February), the mean asymmetry is small or zero, and in individual months it is always rather small and may have opposite signs. Thus the diurnal asymmetry appears to be a well-established phenomenon in the northern midlatitude lower ionosphere. The asymmetry seems to be con ned to a height range. It is probably developed best at altitudes of about 70 –90 km (it depends on solar zenith angle), and either disappears or shows reverse behavior before reaching a height of 100 km (Lastovicka, 1978; Monroe and Smith, 1975). Larger morning values as compared to the afternoon ones (reverse asymmetry) prevail in foE in southeastern Europe in summer (Pancheva and Mukhtarov, 1998).
25
Fig. 2. Mean diurnal variation of the 2775 kHz absorption in May for the period 1966 –1973. L = L − Lmax ; t = |tnoon − t|.
The behavior of the background neutral atmosphere density, temperature and pressure cannot account for the asymmetry in absorption (Lastovicka, 1978). The magnitude of the asymmetry depends on the equivalent vertical frequency of the individual radio paths, i.e. on their re ection heights. Lastovicka (1978) suggested that the changes of electron density, caused by diurnal asymmetry of NO density, might be the cause of the observed diurnal asymmetry in absorption. The reason was the coincidence of the height range of the pronounced asymmetry with the height range of the dominant nitric oxide ionization rate, independent of the solar zenith angle. The analysis of three solar are e ects (Section 3) indicated the possibility of substantially lower NO densities in the morning compared to the afternoon. An analysis of the 1539 kHz absorption during three summers (Boska, 1984; Lastovicka, 1987), brie y mentioned in Section 2, also indicated a signi cant diurnal asymmetry of NO density. Fig. 3 shows the estimated NO densities, inferred from the behavior of the 1539 kHz absorption, as a range of values of the corrected Baker et al. (1977) and Meira (1971) pro les, for both morning and afternoon. Each range is constructed from ve individual pro les for the summers of 1978–1980 for = 60◦ and 70◦ ; the sixth pro le was omitted as it was considered to be extreme and erroneous. Fig. 3 indicates a substantial asymmetry of about a factor of 2. As the ranges of NO curves are separated, the asymmetry must exist for all ve individual cases. Thus estimates of the NO density, based on ionospheric absorption data, provide an evident and substantial diurnal asymmetry of NO density. The width of the ranges of the NO curves in Fig. 3
26
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
Fig. 3. The range of corrected (Meira 1971; Pontano and Hale, 1970) and Baker et al. (1977) NO densities for forenoon (Mf ; Bf ) and afternoon (Ma ; Ba ). 1, 2 — the regions of the maximum contribution to the total absorption (mixed = 60◦ and 70◦ ). Adopted from Lastovicka (1987).
is partly caused by the signi cantly di erent level of solar activity in the summers of 1978, 1979 and 1980. The sunset=sunrise ratio in the HALOE=UARS equatorial NO density pro les (Friedrich et al., 1998) is approximately a factor of two near 90 km, equal to the asymmetry from the absorption “ionospheric” NO densities. The sunset=sunrise asymmetry in equatorial NO=HALOE is most developed (up to a factor of 10) at heights of 70 –85 km, the same heights as those inferred from the absorption analysis (Lastovicka, 1978). The results in Fig. 3 in this height range indicate an asymmetry close to a factor of 2. In general, various absorption data have an asymmetry and so indicate a nitric oxide asymmetry that is evidently less than a factor of 10. However, Siskind et al. (1998) found a decrease of the sunset=sunrise ratio (i.e. asymmetry) of NO densities towards high latitudes and its reduction to a factor of 2 at 57◦ N. This makes the NO asymmetry comparable to that based on the ionospheric absorption data. Thus although the HALOE NO data are for sunrise and sunset (occultation measurements), while absorption data are daytime data, the diurnal asymmetry in NO densities appears to be quite similar. However, the diurnal asymmetry of absorption could, in principle, be caused or signi cantly a ected by diurnal and semidiurnal tides. In that case the interpretation in terms of the asymmetry in the NO density would be wrong. Unfortunately, tidal e ects in the lower ionosphere have never been studied (Lastovicka, 1997). The tides in the neutral atmosphere at “absorption” heights (∼ 75–95 km) have been observed mostly in wind data and partly in temperature data; the density variations were deduced from the tides in temperature and winds (e.g., Forbes et al., 1994). The dominant tidal modes are the semidiurnal and diurnal tides (Forbes et al., 1994; Manson et al., 1989,1999).
A persistent diurnal asymmetry over a 14-h-long range, from 5 to 19 h (time 0 = 12 LT, i.e. noon) is shown in Fig. 2. The absolute value of maximum absorption (Lmax ) in Fig. 2 is 45 dB. Thus, L = −25 dB means the absolute value of absorption is 20 dB. The time 7 h corresponds to = 85◦ and taking this into account, Fig. 2 shows that the afternoon=morning ratio even slightly increases towards sunset ( = 90◦ ). It is impossible to explain such an asymmetry, persistent over at least 14 h by the e ect of the semidiurnal (12-h) tide. The results in Fig. 2 do not display a detectable signature of a 12-h oscillation. However, if the asymmetry were solely of tidal origin with tidal maxima at 0 and 6 h (in terms of Fig. 2) it would result in no asymmetry. A maximum at 3 h would result in large asymmetry at 3 h, no asymmetry at 6 h, and reverse asymmetry at 7 h. The maximum at 1 h would result in no asymmetry at 6 h. A strong e ect of the diurnal tide on the asymmetry can hardly be expected because a detectable signature of the semidiurnal tide has not been observed. Moreover, Fig. 12 of Manson et al. (1999) consistently provides the eastward maximum (= phase) for the zonal wind diurnal tide in the afternoon, even if earlier in winter and later in summer, whereas in the absorption there is substantial asymmetry in summer and none in winter. As far as I know, the seasonal variation of the diurnal tide phase in atmospheric density and pressure is not known from observations at the heights in question (75 –95 km). Consequently, the tides cannot be the direct cause of the asymmetry. 5. Conclusions A review of our earlier results on the estimate of the NO density from the lower ionospheric measurements has been presented. These results have been extended with the application of the HALOE NO density pro le as another initial NO pro le for our analyses. The results from Central Europe yield NO densities that are comparable to or higher than the relatively high HALOE=UARS climatological NO densities of Siskind et al. (1998). This di erence may be partly attributed to di erent seasons and solar activity levels of ionospheric and HALOE measurements. There may be some uncertainty due to the uncertain role of the O2 (1 g ) ionization in the lower ionosphere and its omission in the “ionospheric” NO density estimating procedure. The most important result of this study is support for the existence of a substantial diurnal asymmetry, as observed in HALOE=UARS NO densities (Friedrich et al., 1998), and its extension to the entire daytime by ionospheric data. Both the NO densities inferred from the variability of radio wave absorption in the lower ionosphere and the ionospheric data themselves reveal a consistent substantial diurnal asymmetry in summer. This asymmetry is not con ned to sunset and sunrise (HALOE measurements are solar occultation measurements), but is well developed throughout the whole
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
daytime period (Fig. 2; Lastovicka, 1977, 1978). The ionospheric data show that the asymmetry is a regular and stable phenomenon over at least one solar cycle. In other words, the diurnal asymmetry reported by Friedrich et al. (1998) is not con ned to the several years of HALOE observations but it is a regular feature of the upper mesosphere. Ionospheric data cannot provide information about the NO densities in winter due to the dominance of meteorological control in winter (e.g., Lastovicka et al., 1994). The support for the HALOE NO density asymmetry is signi cant, because the HALOE NO data between 70 and 80 km are the least certain of the entire data set (Gordley et al., 1996). However, the explanation of this NO asymmetry presents a serious challenge for middle atmosphere modellers. References Baker, K.D., Nagy, A.F., Olsen, R.O., Oran, E.S., Randhawa, J., Strobel, D.F., Tohmatsu, T., 1977. Measurement of the nitric oxide altitude distribution in the midlatitude mesosphere. Journal of Geophysical Research 82, 3281–3286. Boska, J., 1984. Ionospheric absorption on 1539 kHz in relation to solar ionizing radiation. Handbook for MAP 10, 31–33. Boska, J., Lastovicka, J., 1983. On the role of solar Lyman alpha radiation in radio-wave absorption in the D-region. Studia geophysica et geodaetica 27, 85–99. Dere, K.P., Horan, D.M., Kreplin, R.W., 1974. The spectral dependence of solar soft X-ray ux values obtained by SOLRAD 9. Journal of Atmospheric and Terrestrial Physics 36, 989–994. Evans, W.F.J., Llewellyn, E.J., 1970. Molecular oxygen emission in the airglow. Annales de Geophysique 26, 167–178. Forbes, J.M., Manson, A.H., Vincent, R.A., Fraser, G.J., Vial, F., Wand, R., Avery, S.K., Clark, R.R., Johnson, R., Roper, R., Schminder, R., Tsuda, T., Kazimirovsky, E.S., 1994. Semidiurnal tide in the 80 –150 km region: an assimilative data analysis. Journal of Atmospheric and Terrestrial Physics 56, 1237–1249. Friedrich, M., Siskind, D.E., Torkar, K.M., 1998. HALOE nitric oxide measurements in view of ionospheric data. Journal of Atmospheric and Solar-Terrestrial Physics 60, 1445–1457. Gordley, L.L., Russell III, J.M., Mickley, J.L., Frederick, J.E., Park, J.H., Stone, K.A., Beaver, G.M., McInerney, J.M., Deaver, L.E., Toon, G.C., Murcray, F.J., Blatherwik, R.D., Gunson, M.R., Abbatt, J.D., Mauldin III, R.L., Mount, G.H., Sen, B., Blavier, J.-F., 1996. Validation of nitric oxide and nitrogen dioxide measurements made by the halogen occultation experiment for UARS platform. Journal of Geophysical Research 101 (D6), 10241–10266. Hu man, R.E., Paulsen, D.E., Larrabee, J.C., Cairns, R.D., 1971. Decrease in D-region O2 (1g ) photoionization rates resulted from CO2 absorption. Journal of Geophysical Research 76, 1028–1038. Lastovicka, J., 1974. On the role of the solar Lyman- radiation in ionospheric absorption at 2775 kHz. Pure and Applied Geophysics 112, 655–663. Lastovicka, J., 1976a. The dependence of the ionospheric absorption at 2775 kHz on the intensity of ionizing radiation — ionospheric implications. Pure and Applied Geophysics 114, 321–331.
27
Lastovicka, J., 1976b. On some sources of uncertainty in the Lyman- ionization-rate calculation. Studia geophysica et geodaetica 20, 273–283. Lastovicka, J., 1977. Seasonal variation in the asymmetry of diurnal variation of absorption in the lower ionosphere. Journal of Atmospheric and Terrestrial Physics 39, 891–894. Lastovicka, J., 1978. Diurnal asymmetry in the lower ionosphere — seasonal variability. Studia geophysica et geodaetica 22, 309–313. Lastovicka, J., 1982. Nitric oxide concentration near the mesopause as deduced from ionospheric absorption measurements. Pure and Applied Geophysics 120, 607–614. Lastovicka, J., 1987. Diurnal asymmetry of NO concentration in the summer upper mesosphere in Central Europe. Advances in Space Research 7 (9), 111–114. Lastovicka, J., 1997. Observations of tides and planetary waves in the atmosphere-ionosphere system. Advances in Space Research 20, 1209–1222. Lastovicka, J., Alberca, L.F., Benediktov, E.E., Boska, J., Bremer, J., Entzian, G., Fiser, V., Jodogne, J.-C., Knyazev, A.K., Marcz, F., de la Morena, B.A., Mukhtarov, Pl., Ogawa, T., Pancheva, D., Ranta, H., Rapoport, Z.Ts., Xiong Nianlu, 1994. Lower ionosphere at middle latitudes: its morphology and response to meteorological and solar-terrestrial activity during the DYANA campaign 1990. Journal of Atmospheric and Terrestrial Physics 56, 1947–1962. Lastovicka, J., Boska, J., 1982. The dominant role of solar Lyman- in D-region radio wave absorption. Journal of Atmospheric and Terrestrial Research 44, 793–795. Manson, A., Meek, C., Hagan, M., Hall, C., Hocking, W., MacDougall, J., Franke, S., Riggin, D., Fritts, D., Vincent, R., Burrage, M., 1999. Seasonal variations of the semi-diurnal and diurnal tides in the MLT: multi-year MF radar observations from 2 to 70◦ N, and the GSWM tidal model. Journal of Atmospheric and Terrestrial Physics 61, 809–828. Manson, A.H., Meek, C.E., Teitelbaum, H., Vial, F., Schminder, R., Kurschner, D., Smith, M.J., Fraser, G.J., Clark, R.R., 1989. Climatologies of the semidiurnal and diurnal tides in the middle atmosphere (70 –100 km) at middle latitudes (45 –55◦ ). Journal of Atmospheric and Terrestrial Physics 51, 573–579. Meira, L.G., 1971. Rocket measurements of upper atmospheric nitric oxide and their consequences to the lower ionosphere. Journal of Geophysical Research 76, 201–212. Monroe, P.E., Smith, L.G., 1975. Determination of nitric oxide concentration from sunrise E-region electron density. Radio Science 10, 317–329. Morozova, L.P., Lastovicka, J., 1985. Estimates of nitric oxide density in the mesosphere based on changes of radio wave absorption during sudden ionospheric disturbances. Geomagnetizm i Aeronomiya 25, 322–324 (in Russian). Pancheva, D., Mukhtarov, P., 1998. A diurnal asymmetry of the monthly median E-region critical frequency. Advances in Space Research 22, 771–774. Pontano, B.A., Hale, L.C., 1970. Measurements of N ionizable constituents of the low ionosphere using a Lyman- source and blunt probe. Space Research 10, 208–214. Siskind, D.E., Barth, C.A., Russell III, J.M., 1998. A climatology of nitric oxide in the mesosphere and thermosphere. Advances in Space Research 21, 1353–1362. Thomas, R.J., Barth, C.A., Rusch, D.W., Sanders, R.W., 1984. Solar mesospheric explorer near-infrared spectrometer: measurements of 1:27 m radiances and the inference of
28
J. LaÄstoviÄcka / Journal of Atmospheric and Solar-Terrestrial Physics 63 (2001) 21–28
mesospheric ozone. Journal of Geophysical Research 89, 9569–9580. Tisone, C.C., 1973. Measurements of NO densities during sunrise at Kauai. Journal of Geophysical Research 78, 746–750. Tuchkov, G.A., Zadorozhny, M.A., Lastovicka, J., 1990. Nitric oxide density measurements at middle latitudes. Studia geophysica et geodaetica 34, 261–268.
Vidal-Madjar, A., 1975. Evolution of the solar Lyman alpha ux during four consecutive years. Solar Physics 40, 69–82. Zadorozhny, A.M., Tuchkov, G.A., Kikhtenko, V.N., Lastovicka, J., Boska, J., Novak, A., 1992. Nitric oxide and lower ionospheric quantities during solar particle events of October 1989 after rocket and ground-based measurements. Journal of Atmospheric and Terrestrial Physics 54, 183–192.