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Haloe nitric oxide measurements in view of ionospheric data
\ PERGAMON
Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0334Ð0346
HALOE nitric oxide measurements in view of ionospheric data M[ Friedricha\ \ D[ E[ Siskind b\ K[ M[ Torkarc a
Department of Communications and Wave Propagation\ Technical University Graz\ Inffeldgasse 01\ A!7909 Graz\ Austria b Naval Research Laboratory\ Washington DC 19264\ USA c Space Research Institute of the Austrian Academy of Sciences\ Inffeldgasse 01\ A!7909 Graz\ Austria Received 06 December 0886^ received in revised form 00 August 0887^ accepted 14 August 0887
Abstract The nitric oxide densities obtained by the HALOE instrument aboard the UARS satellite present a unique set of measurements which can practically only be checked against the results of theoretical models or\ indirectly\ by comparison to ionospheric data[ Long!term averages of mesospheric nitric oxide "NO# data are contrasted to D!region electron densities from an empirical model which is based on a large number of appropriate in situ measurements[ Electron densities calculated with an ion!chemical model\ which uses HALOE ðNOŁ as an input\ tend to underestimate empirical electron densities below 79 km and overestimate it above 79 km[ The puzzling diurnal asymmetry of ðNOŁ may also be re~ected in the electron densities[ Þ 0887 Elsevier Science Ltd[ All rights reserved[
0[ Introduction The production of free electrons in the daytime lower ionosphere "D! and E!region# is largely determined by the density of nitric oxide "NO# with its ionisation threshold low enough to be ionised by the prominent and stable solar Lyman!a line[ At latitudes outside the auroral zone\ this process is generally believed to be the main source of free electrons in the daytime[ Hence the actual density of this minor constituent is of prime interest for the understanding of the variation of the density of free electrons[ Hitherto\ ionospheric modelers had to rely lar! gely on theoretical densities of NO which\ in turn\ depend on assumptions concerning transport by poorly known eddy di}usion[ As examples\ Figs 0 and 1 show the most important production processes for low solar activity "F09[6 64 Jy# and two solar zenith angles at the equator[ The calculations were performed according to the pro! cedure described by Torkar and Friedrich "0872#\ and the mean NO densities from the HALOE instrument
Author to whom all correspondence should be addressed[ Tel[] ¦9932!205!7626338^ Fax] ¦9932!205!352586^ E!mail] friedrichÝinw[tu!graz[ac[at[
from equatorial data at low solar activity "F09[6³019 Jy# were used "mean of the curves shown in Fig[ 2#[ One can see that the region where NO dominates the ion production depends on the solar zenith angle\ x[ In the case of x 64>\ this domination ranges from 61 to 88 km\ whereas at 34>\ that range is restricted to 55 to 67 km[ Ionisation by O1 "0Dg# takes over from 67 to 77 km[ The contribution by O1 "0Dg#\ which is ionised by solar photons from 092 to 000[7 nm "Hu}man et al[\ 0860#\ is larger than usually assumed\ but is based on fairly recent global satellite measurements of that minor constituent "Thomas et al[\ 0873#[ Ionisation in the lowest D!region is provided by galactic cosmic rays whose ~uxes are inde! pendent of x "Heaps\ 0867#[ The lifetime of charged particles in the lower iono! sphere is short compared to changes in the ionisation[ Hence\ for practical purposes\ one always assumes a bal! ance between ion production\ q\ and e}ective electron recombination\ c[ Electron density\ Ne\ thus depends on the ion!pair production rate and is given as zq:c[ For the e}ective recombination rate\ one can either use empirically determined values such as those established by Torkar and Friedrich "0877# from a limited number "09# of daytime rocket measurements\ or use an ion! chemical model in which direct recombination as well as loss via the formation of secondary ions or via negative
0253Ð5715:87:, ! see front matter Þ 0887 Elsevier Science Ltd[ All rights reserved PII] S 0 2 5 3 ! 5 7 1 5 " 8 7 # 9 9 9 8 0 ! 0
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Fig[ 0[ Ionisation processes at the equator for low solar activity conditions "F09[6 64 Jy#\ a solar zenith angle of 34> and the mean ðNOŁ pro_le from HALOE of Fig[ 2[ One can see that at this zenith angle between 55 and 77 km the concentration of the trace constituents NO and O1 "0Dg# is crucial\ whereas at other altitudes only the intensity of the ionising ~ux and not the atmosphere|s composition matters[
ions is calculated[ The empirical values are lower than the theoretical ones by factors of 0[4 in the cluster ion region and between 0[5 and 3 above 89 km[ For the ensu! ing arguments\ this di}erence is not large enough to impact on the conclusions and the ion chemical c is employed throughout "Torkar and Friedrich\ 0872#[
1[ Available data The Halogen Occultation Experiment "HALOE# on the Upper Atmosphere Research Satellite "UARS# has been observing the middle and upper atmosphere for the last _ve years and has provided an unprecedented global
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Fig[ 1[ As Fig[ 0\ but for a zenith angle of 64>[
ðNOŁ dataset which can be used to constrain the sources of D!region ionisation[ The experiment technique is to observe the Earth|s limb and measure the absorption of sunlight by various atmospheric gases "including NO\ NO1\ CH3\ HF\ H1O\ O2 and CO1 and aerosols# during both sunrise and sunset[ This is discussed in detail by Russell et al[ "0882#[ More speci_c error analysis of the NO dataset is given by Gordley et al[ "0885#[ The altitude range covered by the HALOE dataset is from 19 to 014 km[ For the purposes of the HALOE data\ to under! stand the ionospheric D!region\ several considerations
are important[ First\ since up to 05 occultations for either sunrise or sunset are seen in a given day\ the data we use is close to a true zonal mean[ However\ because HALOE uses the solar occultation technique\ it only samples a single narrow latitude band each day[ To obtain a com! plete sweep of latitudes "typically about 019># takes about 39 days[ For this reason\ plus the fact that most of the ionospheric measurements were made prior to the UARS mission\ we use a climatology that is derived from HALOE data and sorted according to latitude\ season and solar activity[ The details of this analysis have
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Fig[ 2[ Equatorial HALOE NO densities "24>#\ zonal average\ all seasons and low solar activity "F09[6³019 Jy#[ Data deduced at sunrise and sunset "SR and SS\ respectively# are indicated[
recently been published by Siskind et al[ "0887#[ A sample of data from that climatology for low solar activity "F09[6³019 Jy# and for 24> from the equator is shown in Fig[ 2[ The curves in this _gure are averages of 20 " for sunrise# and 29 " for sunset# days of data\ or 379 "SR# and 385 "SS# individual pro_les[ Random errors are esti! mated to be only perhaps 09)\ however especially below 099 km\ the uncertainties in the retrieval are assumed to
be considerably larger than any random error[ The _gure shows the large thermospheric peak which is due to the ionisation and dissociation of N1 by solar EUV\ soft X!ray photons and photoelectrons[ At these altitudes\ Siskind et al[ "0886# compared averaged HALOE data with a climatology of NO derived from Solar Meso! spheric Explorer "SME# data[ In general\ both datasets show the same relative behaviour but the peak HALOE
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values are consistently larger than those from SME[ Below the thermospheric layer\ the density decreases due to photo!dissociation "Siskind\ 0883#[ The minimum is reached between 69 and 79 km^ below that region\ the density increases again due to upward transport from the stratospheric odd nitrogen layer[ The latitudinal vari! ation "see later\ Figs[ 6Ð7# is consistent with the presence of a source of NO at high latitudes due to charged particle precipitation[ The latitudinal gradient of mesospheric NO has been discussed in depth by Siskind et al[ "0887#[ An unexpected feature of the HALOE data shown in Fig[ 2 is a diurnal asymmetry[ Thus\ between 59 and 89 km\ the sunset "SS# values are generally larger than the sunrise "SR# values[ Above 89 km\ the situation is reversed\ SR×SS[ Siskind et al[ "0887# discuss the lati! tudinal variation of the SR:SS ratio and show that it is largest at the equator\ i[e[\ in excess of one order of magnitude\ compared to less than a factor of two at 46> latitude[ This diurnal variation is puzzling\ particularly below 89 km\ because simple photo!chemical theory indi! cates that the lifetime of NO is generally much larger than a day "Siskind\ 0883#[ Unfortunately\ the NO data between 69 and 79 km are the least certain of the entire dataset "Gordley et al[\ 0885#[ This is due to the di.culty of measuring small values in the presence of a large fore! ground contribution from the thermospheric values[ Indeed\ the signal!to!noise ratio for individual pro_les is of the order of 9[0 for NO densities ³095 cm−2^ although\ because we have averaged about 499 pro_les together\ random e}ects are unlikely to be the cause of the SR:SS di}erence[ Since the work by Gordley et al[ "0885#\ the HALOE science team has continued to investigate this question\ but without _rm results[ A diurnal asymmetry does appear in the measured gas correlation signal for the HALOE NO channel[ However\ the HALOE team cannot rule out an uncertainty in modelling the instru! ment spectral response as the possible cause "L[ Gordley\ 0887\ private communication#[ Unfortunately\ unlike the case of the thermosphere\ there are no other global meso! spheric NO datasets with which to validate these data[ For these reasons\ the details of the mesospheric mini! mum should be treated with caution[ Instead\ in this paper\ we suggest "as an alternative# an indirect approach using ionospheric data that may shed light on this question[
2[ Ionospheric data The obvious question is how well electron densities computed with HALOE NO densities agree with mea! sured electron densities[ The most reliable electron den! sities in the lower ionosphere are based on rocket!borne radio wave propagation measurements\ because probe measurements are prone to be disturbed by aerodynamic e}ects and may be in~uenced in their absolute values
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by payload charging "Mechtley\ 0863^ Jacobsen and Friedrich\ 0868#[ The resolution and sensitivity of ground!based measurements "e[g[\ incoherent scatter radar# is usually insu.cient\ particularly in the D!region and the comparison of the merits and limits of the various techniques made by Thrane "0863# generally still holds[ Because any expected seasonal variation at the equator is small\ HALOE ðNOŁ can be averaged for the whole duration of the UARS mission and thus yield statistically the most meaningful data[ In the literature\ there are only 14 electron pro_les based on rocket borne wave propagation experiments from within 201> from the equator[ Most are from the Indian range Thumba "7[5>N#\ four from Alca¼ntara\ Brazil "1>S# and the rest were launched from a research vessel in the Atlantic at 01>S\ covering solar zenith angles from nearly overhead to tropical midnight and solar activities from quiet to very active[ Comparison with any of these 14 pro_les cannot therefore be very conclusive\ given the restricted geophysical conditions at the times of the HALOE NO measurement[ Because of the likewise very limited amount of appropriate D!region electron density pro_les\ the empirical\ non!auroral D!region model by Friedrich and Torkar "0887#\ which is based on rocket borne wave propagation data only\ will be used for further compari! sons[ Approximate variance factors "input data vs[ results of the statistical model# are typically 0[5 or better below 79 and above 84 km\ whereas the largest discrepancies of up to a factor of 0[8 occur between 74 and 89 km[ The few daytime pro_les which were actually measured near the equator were carefully compared to the model results and found to yield similar agreement[ The electron density pro_les resulting from the ion! chemical model for the conditions of Figs[ 0Ð1 "i[e\ at the equator and for x 34> and 64>\ respectively# and the NO densities "SR and SS# shown in Fig[ 2 are plotted in Figs 3 and 4\ respectively[ These zenith angles were chosen because\ at smaller angles\ X!rays may dominate the ion production down to say 74 km\ whereas at very large solar zenith angles\ such as those used for the HALOE measurements\ galactic cosmic rays "GCR# can be the dominating source of ionisation up to perhaps 79 km[ For the present comparison\ the above empirical electron density model was established using AM and PM data separately in Fig[ 3 "x 34>#\ whereas both AM and PM electron density data are entered into the model to establish the empirical curve in Fig[ 4[ The resulting pro_les are also plotted in Figs[ 3Ð4[ The appar! ent asymmetry in the D!region electron densities is clearly seen in the radio wave absorption\ which is based on more measurements than the above electron densities data base[ Figure 5 shows the signal strength vs[ solar zenith angle of a 1[72 MHz signal over a 499 km path in central Europe "Coburg\ Germany\ to Graz\ Austria\ between 0863 and 0865\ i[e[\ at a time of generally low solar activity[ Two features are apparent]
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Fig[ 3[ Equatorial electron densities calculated using the mean NO densities of Fig[ 2 for a solar zenith angle of 34> "dashed lines# and the corresponding empirical values from the empirical model by Friedrich and Torkar "0887# " full lines#[ Also indicated on the right is the variance in the densities of the empirical D!region model[
"a# in Winter the signal during the day is above the annual average "indicated by the dotted line# and "b# in Summer the hysteresis is much larger[ "The signal strength variation at night is due to the height of the re~ection level in the E!region and not to absorption in the D!region#[ The results in Fig[ 3 reveal an asymmetry in the empiri! cal electron densities similar to the result of the cal! culation using the HALOE SR and SS data[ Below about 71 km\ the modelled densities are consistently too low\ implying erroneous results from the HALOE instrument\ although less so with the higher sunset values[ However\ the higher sunset pro_le Fig[ 4 increases the factor between calculated and empirical electron densities in the 74 to 89 km region[ In the E!region\ no conclusions
pertaining to the NO densities can be drawn because\ there\ the ionisation by X!rays clearly dominates over that due to NO[ Taubenheim "0866# employed this pro! cedure in reverse\ i[e[\ he deduced ðNOŁ from inferred electron densities pro_les assuming "a# that the e}ective recombination rate could be assumed and "b# that ionisation processes other than by NO and Lyman!a "qrest# were known[ ðNOŁ
Ne1 c−qrest \ sFLyman−a
where s is the ionisation cross section of NO and FLyman!a is the local ~ux of Lyman!a radiation[
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Fig[ 4[ As Fig[ 3\ but for a zenith angle of 64>[
With the empirical D!region model now available\ one should be able to apply this procedure more successfully[ However\ the low empirical values between 74 and 89 km would lead to ne`ative NO densities because\ there\ the ion production from O1 "0Dg# alone provides Ne larger than that observed[ The fact that the computed Ne exceeds the empirical values can have several expla! nations]
0[ The computed Ne is correct\ but the empirical electron densities are too low[ This height region has the largest variance "about a factor of 0[8 larger or smaller than the curves shown in Figs[ 3Ð4#[ 1[ The empirical Ne values are basically correct\ but the computed Ne values are too high[ For this hypothesis\ there are two explanations] "a# The computed ion!pair production\ by processes
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Fig[ 5[ Strength of a 1[72 MHz signal over a 499 km path in central Europe as a function of solar zenith angle[ Dotted line represents the annual average "after Torkar et al[\ 0867#[
other than Lyman!a and NO\ is too large[ The most important such process is O1 "0Dg# being ion! ised by solar radiation in the region from 092 to 009[4 nm[ The ~ux measured by Hall and Hin! teregger "0869# is probably still the most recent in this band and the absorption by O1 and CO1 was calculated for 64 discrete lines by Paulsen et al[ "0861#[ The densities of O1 "0Dg# used here are the ones obtained from the ozone experiment aboard the satellite SME "Thomas et al[\ 0873#[ "b# The e}ective electron recombination rate c of the theoretical model is too small[ Larger c "by orders of magnitude# occur at heights where the dom! inating ions are of the water cluster type[ The transition height "equal number of cluster and molecular ions# was found by Friedrich and Tor! kar "0877# to be a function of temperature and varies between 67 and 89 km " for 124 to 099 K\ respectively#[ Cluster ions do\ however\ disappear rather abruptly at the height where atomic oxygen exists with appreciable densities[ The ðOŁ values employed in the present context are idealised pro! _les based on rocket!borne data with a ledge at 71 km[ Only densities below that height are varied with the solar zenith angle beyond 74>[ The recent empirical model by Llewellyn and McDade "0885# is representative for diurnal averages and there! fore displays hardly any ledge[ However\ in the height region where too large electron densities are calculated "75 km#\ the Llewellyn and McDade model yields even larger O!densities "by a factor of three#\ hence the calculated cluster transition would be even lower "smaller recombination rate# and would increase the discrepancy[ Furthermore\ the empirical c "Friedrich and Torkar\ 0877# are somewhat lower than the results of the theoretical modelling\ but suggest the same transition height[
It appears from the above that electron densities in this height region "74 to 89 km# are the least predictable in the whole altitude range considered "49 to 049 km#\ but the data on the solar spectral range ionising O1 "0Dg# are also older than the more recent data on that minor spec! ies[ Let us assume the daytime electron densities given by the empirical D!region model to be representative and ignore all ionisation processes other than by Lyman!a and NO "qrest 9#[ By using recombination rates from the ion!chemical computation\ we can infer upper limits of NO densities for the three conditions for which we have representative results from HALOE "equator\ Fig[ 2 as well as 27> and 46>#[ Figure 6 shows the NO densities required to produce Ne at the equator\ as predicted by the empirical model in the absence of non!NO ionisation processes[ Naturally\ the NO thus required is di}erent for di}erent solar zenith angles\ x\ because for small x\ X!rays or Lyman!b radiation would contribute more to the ionisation and have to be replaced by excessively large NO densities[ On the other hand\ at low altitudes\ the ionisation by galactic cosmic rays can dominate for large x[ The low density envelope of the NO!densities thus computed for various x constitutes the upper limit of NO that can exist in order to reproduce the Ne of the empirical D!region model[ One can see that the HALOE NO!den! sities exceed the upper limit based on ionospheric con! siderations in the region between 79 and 84 km\ both for SR and SS[ Corresponding exercises were also carried out for the equinoxes at 27> and 46> "Figs 7 and 8#[ It is evident that at 27>\ the HALOE results exceed those conceivable on ionospheric grounds in only a limited height region "SS only#[ At 46>\ the electron density model largely hinges on rocket measurements from South Uist\ Scotland\ at a geomagnetic latitude of 59>\ whereas the same geographic latitude in the American sector is higher by about 09> in geomagnetic latitude[ This excess
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Fig[ 6[ Upper limits of NO densities assuming ion production solely due to NO and Lyman!a in order to reproduce the empirical electron densities[ The calculations were carried out for the equator and for di}erent solar zenith angles using electron loss rates due an ion!chemical scheme[ Also shown are the corresponding HALOE results "cf[ Figure 2#[ Note that between 79 and 89 km the NO densities "both SR and SS# should be considerably lower in order to be compatible with the ionospheric data[
density is signi_cant because the recombination rate\ c\ prevailing at these heights is the well established one of ¦ O¦ 1 and NO \ i[e[\ largely above the region of cluster ions with their much larger recombination rates[ During the day\ the transition from cluster to molecular ions was
found to be between 60 and 76 km\ depending on the temperature "Friedrich and Torkar\ 0877#[ The highest transition heights only occur when the mesosphere is extremely cold "high latitude Summer#^ here\ however\ only equinox conditions are considered[ The test shown
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Fig[ 7[ Upper limits of ðNOŁ on ionospheric grounds for a zonal mean at a latitude of 27> and equinox[ Also indicated are the averaged NO densities from HALOE for the same conditions[ The satellite data appear to be generally compatible with the ionospheric data[
in Fig[ 8 is somewhat distorted because we compare upper ðNOŁ!limits based on non!auroral electron densities with NO densities partly gathered in the auroral zone where charged particle precipitation constitutes a very important source of NO[
3[ Conclusions We have shown that the ionospheric data are generally consistent with HALOE NO pro_les that show a mini! mum in the 69 to 79 km region[ There are signi_cant
M[ Friedrich et al[:J[ Atmos[ Solar!Terrestrial Physics 59 "0887# 0334Ð0346
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Fig[ 8[ As Fig[ 7\ but for 46> latitude[ The ionospheric data on which the conceivable upper limits of ðNOŁ are based are from geomagnetic latitudes ³50>\ whereas the NO data are partly from higher geomagnetic latitudes[ This may explain the NO densities exceeding the ionospheric limits in the upper D!region[
di}erences "up to a factor of 3# in the details of the magnitude and height of the minimum[ Given the di.! culties in obtaining reliable NO measurements in this region\ one should be cautious about over!interpreting these di}erences[ Whereas the unexpected large di}erence between sun! rise and sunset values is at least indirectly supported by
ionospheric measurements\ the magnitude and the height of the NO minimum is not re~ected in ionospheric data and may\ indeed\ be a problem of low signal!to!noise ratio[ The disagreement in the NO densities to reproduce the observed electron densities is never larger than a factor of four[ The exercise reported here points to the following problems]
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, The gross features of the comparison between HALOE NO and the ionospheric results indicate the need for reduced NO between 74 and 84 km[ , The ionisation contribution of O1 "0Dg# should be addressed to see if smaller values are conceivable[ , The possibility of a diurnal variation in theoretical models of NO should be investigated[ According to conventional wisdom\ the lifetime of an NO molecule in the mesosphere and thermosphere is of the order of days^ hence\ there should be no diurnal variation "e[g[ Solomon et al[\ 0871#[ However\ some asymmetry with respect to noon has long been observed in mid!latitude ionospheric absorption data[ Las³tovic³ka "0866# published values larger in the afternoon for the same solar zenith angles "e[g[\ 06 vs[ 6 dB#[ He also found that this asymmetry was most pronounced in Summer "when the Sun rises fast#\ whereas it was hardly noticeable in the Winter data[ From the absorption ratio between morning and afternoon\ one can conclude a cor! responding ratio in electron density or approximately that ratio squared in ionisation "which is roughly pro! portional to the NO!density#[ A mechanism which may\ at least in part\ explain D!region asymmetries is via a change of the electron loss rate due to diurnally asym! metric neutral temperatures[ The recent measurements by the CRISTA experiment aboard the Shuttle Pallet Satellite\ ~own in November 0883 "Preusse et al[\ 0886#\ revealed dayÐnight temperature di}erences of up to 29 K in the mesosphere\ centred at the equator[ Inserting these temperatures in the ion!chemical model yields electron density di}erences of up to 14)[ It is interesting to note that the HALOE NO data shows diurnal asymmetries in the same sense as the ionospheric data which have previously been reported without an explanation[ Unfor! tunately\ the large uncertainties inherent in quantifying the NO minimum at 69 km preclude further discussions at this point[ Opposite diurnal asymmetries "less in the afternoon# were found by Monro et al[ "0865# in the E! region in incoherent scatter data[ This observation also seems to agree with the present NO!data[ Sounding rock! ets instrumented to measure NO\ O1 "0Dg# and electron densities by the same payload should be launched on the same day at the same solar zenith angle in the morning and the evening[ One should thus gain an insight into the causes of the unquestionable diurnal asymmetries in radiowave absorption[ The preferred condition for such an experiment is a scenario with fast sunrise and sunset\ such as in the tropics or mid!latitudes in Summer[
References Barth\ C[A[\ 0885[ Reference models for thermospheric nitric oxide[ Advances in Space Research 07 "8:09#\ 068Ð197[ Friedrich\ M[\ Torkar\ K[M[\ 0877[ Empirical transition heights
of cluster ions[ Advances in Space Research 7 "3#\ 124Ð 127[ Friedrich\ M[\ Torkar\ K[M[\ 0887[ Comparison between an empirical and a theoretical model of the D!region[ Advances in Space Research 10 "5#\ 784Ð893[ Gordley\ L[L[\ Russell III\ J[M[\ Mickley\ L[J[\ 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[\ 0885[ Validation of nitric oxide and nitrogen dioxide measurements made by the halogen occultation experiment for UARS platform[ Journal of Geo! physical Research 090 "D5#\ 09130Ð09155[ Hall\ L[A[\ Hinteregger\ H[E[\ 0869[ Solar radiation in the extreme ultraviolet and its variation with solar rotation[ Jour! nal of Geophysical Research 64\ 5848Ð5854[ Heaps\ M[G[\ 0867[ A parametrization of cosmic ray ionization[ Planetary and Space Science 15 "5#\ 402Ð406[ Hu}man\ R[E[\ Paulsen\ D[E[\ Larabee\ J[C[\ 0860[ Decrease in D!region O1 "0Dg# photoionization rates resulting from CO1 absorption[ Journal of Geophysical Research 65 "3#\ 0917Ð 0927[ Jacobsen\ T[A[\ Friedrich\ M[\ 0868[ Electron density measure! ments in the lower D!region[ Journal of Atmospheric and Terrestrial Physics 30 "01#\ 0084Ð0199[ Las³tovic³ka\ J[\ 0866[ Seasonal variation in the asymmetry of diurnal variation of absorption in the lower ionosphere[ Jour! nal of Atmospheric and Terrestrial Physics 28 "7#\ 780Ð 783[ Llewellyn\ E[J[\ McDade\ I[C[\ 0885[ A reference model for atomic oxygen in the terrestrial atmosphere[ Advances in Space Research 07 "8:09#\ 198Ð115[ Mechtley\ E[A[\ 0863[ Accuracy of rocket measurements of the lower ionosphere electron concentration[ Radio Science 8 "2#\ 262Ð267[ Monro\ P[E[\ Nisbet\ J[S[\ Stick\ T[L[\ 0865[ E}ects of tidal oscillations in the neutral atmosphere on electron densities in the E!region[ Journal of Atmospheric and Terrestrial Physics 27\ 412Ð418[ Paulsen\ D[E[\ Hu}man\ R[E[\ Larrabee\ J[C[\ 0861[ Improved photoionization rates of O1 "0Dg# in the D!region[ Radio Sci! ence 6 "0#\ 40Ð44[ Preusse\ P[\ Riese\ A[A[\ Oberheide\ J[\ Bittner\ M[\ Grossmann\ K[U[\ O}ermann\ D[\ 0886[ Evidence for zonally trapped diurnal tide in CRISTA temperatures[ Advances in Space Research 08\ 468Ð471[ Russell III\ J[M[\ Gordley\ L[L[\ Park\ J[H[\ Drayson\ S[R[\ Hesketh\ D[H[\ Cicerone\ R[J[\ Tuck\ A[F[\ Frederick\ J[E[\ Harries\ J[E[\ Crutzen\ P[J[\ 0882[ The halogen occultation experiment[ Journal of Geophysical Research 87 "D5#\ 09666Ð 09686[ Siskind\ D[E[\ 0883[ On the radiative coupling between meso! spheric and thermospheric nitric oxide[ Journal of Geo! physical Research 88 "D00#\ 11646Ð11655[ Siskind\ D[E[\ Bacmeister\ J[T[\ Summers\ M[E[\ Russell III\ J[M[\ 0886[ Two!dimensional model calculations of nitric oxide transport in the middle atmosphere and comparison with halogen occultation experiment data[ Journal of Geo! physical Research 091 "D2#\ 2416Ð2434[ Siskind\ D[E[\ Barth\ C[A[\ Russell III\ J[M[\ 0887[ A clima! tology of nitric oxide in the mesosphere and thermosphere[ Advances in Space Research 10 "09#\ 0242Ð0251[
M[ Friedrich et al[:J[ Atmos[ Solar!Terrestrial Physics 59 "0887# 0334Ð0346 Solomon\ S[\ Crutzen\ P[J[\ Roble\ R[G[\ 0871[ Photochemical coupling between the thermosphere and the lower atmosphere 0[ Odd nitrogen from 49 to 019 km[ Journal of Geophysical Research 76\ 6195Ð6119[ Taubenheim\ J[\ 0866[ The distribution of nitric oxide and its variation near the mesopause derived from ionospheric obser! vations[ Space Research 06\ 160Ð167[ Thomas\ R[J[\ Barth\ C[A[\ Rusch\ D[W[\ Sanders\ R[W[\ 0873[ Solar mesospheric explorer near!infrared spectrometer] Measurements of 0[16 mm radiances and the inference of meso! spheric ozone[ Journal of Geophysical Research 78 "D5#\ 8458Ð8479[ Thrane\ E[ V[ "0863# Ionospheric pro_les up to 059 km*A
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review of techniques and pro_les[ In COSPAR Methods of Measurements and Results of Lower Ionospheric Structure\ ed[ K[ Rawer\ pp[ 2Ð10[ Akademie Verlag\ Berlin[ Torkar\ K[M[\ Friedrich\ M[\ 0872[ Tests of an ion!chemical model of the D! and lower E!region[ Journal of Atmospheric and Terrestrial Physics 34 "5#\ 258Ð274[ Torkar\ K[M[\ Friedrich\ M[\ 0877[ Empirical electron recom! bination coe.cients in the D! and E!region[ Journal of Atmo! spheric and Terrestrial Physics 49 "7#\ 638Ð650[ Torkar\ K[M[\ Friedrich\ M[\ Wallner\ W[\ Rose\ G[\ Widdel\ H[U[\ 0867[ Preliminary results of absorption measurements of a central European A2 path[ Kleinheubacher Berichte 10\ 044Ð050[
Journal of Atmospheric and Solar!Terrestrial Physics 59 "0887# 0334Ð0346
HALOE nitric oxide measurements in view of ionospheric data M[ Friedricha\ \ D[ E[ Siskind b\ K[ M[ Torkarc a
Department of Communications and Wave Propagation\ Technical University Graz\ Inffeldgasse 01\ A!7909 Graz\ Austria b Naval Research Laboratory\ Washington DC 19264\ USA c Space Research Institute of the Austrian Academy of Sciences\ Inffeldgasse 01\ A!7909 Graz\ Austria Received 06 December 0886^ received in revised form 00 August 0887^ accepted 14 August 0887
Abstract The nitric oxide densities obtained by the HALOE instrument aboard the UARS satellite present a unique set of measurements which can practically only be checked against the results of theoretical models or\ indirectly\ by comparison to ionospheric data[ Long!term averages of mesospheric nitric oxide "NO# data are contrasted to D!region electron densities from an empirical model which is based on a large number of appropriate in situ measurements[ Electron densities calculated with an ion!chemical model\ which uses HALOE ðNOŁ as an input\ tend to underestimate empirical electron densities below 79 km and overestimate it above 79 km[ The puzzling diurnal asymmetry of ðNOŁ may also be re~ected in the electron densities[ Þ 0887 Elsevier Science Ltd[ All rights reserved[
0[ Introduction The production of free electrons in the daytime lower ionosphere "D! and E!region# is largely determined by the density of nitric oxide "NO# with its ionisation threshold low enough to be ionised by the prominent and stable solar Lyman!a line[ At latitudes outside the auroral zone\ this process is generally believed to be the main source of free electrons in the daytime[ Hence the actual density of this minor constituent is of prime interest for the understanding of the variation of the density of free electrons[ Hitherto\ ionospheric modelers had to rely lar! gely on theoretical densities of NO which\ in turn\ depend on assumptions concerning transport by poorly known eddy di}usion[ As examples\ Figs 0 and 1 show the most important production processes for low solar activity "F09[6 64 Jy# and two solar zenith angles at the equator[ The calculations were performed according to the pro! cedure described by Torkar and Friedrich "0872#\ and the mean NO densities from the HALOE instrument
Author to whom all correspondence should be addressed[ Tel[] ¦9932!205!7626338^ Fax] ¦9932!205!352586^ E!mail] friedrichÝinw[tu!graz[ac[at[
from equatorial data at low solar activity "F09[6³019 Jy# were used "mean of the curves shown in Fig[ 2#[ One can see that the region where NO dominates the ion production depends on the solar zenith angle\ x[ In the case of x 64>\ this domination ranges from 61 to 88 km\ whereas at 34>\ that range is restricted to 55 to 67 km[ Ionisation by O1 "0Dg# takes over from 67 to 77 km[ The contribution by O1 "0Dg#\ which is ionised by solar photons from 092 to 000[7 nm "Hu}man et al[\ 0860#\ is larger than usually assumed\ but is based on fairly recent global satellite measurements of that minor constituent "Thomas et al[\ 0873#[ Ionisation in the lowest D!region is provided by galactic cosmic rays whose ~uxes are inde! pendent of x "Heaps\ 0867#[ The lifetime of charged particles in the lower iono! sphere is short compared to changes in the ionisation[ Hence\ for practical purposes\ one always assumes a bal! ance between ion production\ q\ and e}ective electron recombination\ c[ Electron density\ Ne\ thus depends on the ion!pair production rate and is given as zq:c[ For the e}ective recombination rate\ one can either use empirically determined values such as those established by Torkar and Friedrich "0877# from a limited number "09# of daytime rocket measurements\ or use an ion! chemical model in which direct recombination as well as loss via the formation of secondary ions or via negative
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Fig[ 0[ Ionisation processes at the equator for low solar activity conditions "F09[6 64 Jy#\ a solar zenith angle of 34> and the mean ðNOŁ pro_le from HALOE of Fig[ 2[ One can see that at this zenith angle between 55 and 77 km the concentration of the trace constituents NO and O1 "0Dg# is crucial\ whereas at other altitudes only the intensity of the ionising ~ux and not the atmosphere|s composition matters[
ions is calculated[ The empirical values are lower than the theoretical ones by factors of 0[4 in the cluster ion region and between 0[5 and 3 above 89 km[ For the ensu! ing arguments\ this di}erence is not large enough to impact on the conclusions and the ion chemical c is employed throughout "Torkar and Friedrich\ 0872#[
1[ Available data The Halogen Occultation Experiment "HALOE# on the Upper Atmosphere Research Satellite "UARS# has been observing the middle and upper atmosphere for the last _ve years and has provided an unprecedented global
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Fig[ 1[ As Fig[ 0\ but for a zenith angle of 64>[
ðNOŁ dataset which can be used to constrain the sources of D!region ionisation[ The experiment technique is to observe the Earth|s limb and measure the absorption of sunlight by various atmospheric gases "including NO\ NO1\ CH3\ HF\ H1O\ O2 and CO1 and aerosols# during both sunrise and sunset[ This is discussed in detail by Russell et al[ "0882#[ More speci_c error analysis of the NO dataset is given by Gordley et al[ "0885#[ The altitude range covered by the HALOE dataset is from 19 to 014 km[ For the purposes of the HALOE data\ to under! stand the ionospheric D!region\ several considerations
are important[ First\ since up to 05 occultations for either sunrise or sunset are seen in a given day\ the data we use is close to a true zonal mean[ However\ because HALOE uses the solar occultation technique\ it only samples a single narrow latitude band each day[ To obtain a com! plete sweep of latitudes "typically about 019># takes about 39 days[ For this reason\ plus the fact that most of the ionospheric measurements were made prior to the UARS mission\ we use a climatology that is derived from HALOE data and sorted according to latitude\ season and solar activity[ The details of this analysis have
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Fig[ 2[ Equatorial HALOE NO densities "24>#\ zonal average\ all seasons and low solar activity "F09[6³019 Jy#[ Data deduced at sunrise and sunset "SR and SS\ respectively# are indicated[
recently been published by Siskind et al[ "0887#[ A sample of data from that climatology for low solar activity "F09[6³019 Jy# and for 24> from the equator is shown in Fig[ 2[ The curves in this _gure are averages of 20 " for sunrise# and 29 " for sunset# days of data\ or 379 "SR# and 385 "SS# individual pro_les[ Random errors are esti! mated to be only perhaps 09)\ however especially below 099 km\ the uncertainties in the retrieval are assumed to
be considerably larger than any random error[ The _gure shows the large thermospheric peak which is due to the ionisation and dissociation of N1 by solar EUV\ soft X!ray photons and photoelectrons[ At these altitudes\ Siskind et al[ "0886# compared averaged HALOE data with a climatology of NO derived from Solar Meso! spheric Explorer "SME# data[ In general\ both datasets show the same relative behaviour but the peak HALOE
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values are consistently larger than those from SME[ Below the thermospheric layer\ the density decreases due to photo!dissociation "Siskind\ 0883#[ The minimum is reached between 69 and 79 km^ below that region\ the density increases again due to upward transport from the stratospheric odd nitrogen layer[ The latitudinal vari! ation "see later\ Figs[ 6Ð7# is consistent with the presence of a source of NO at high latitudes due to charged particle precipitation[ The latitudinal gradient of mesospheric NO has been discussed in depth by Siskind et al[ "0887#[ An unexpected feature of the HALOE data shown in Fig[ 2 is a diurnal asymmetry[ Thus\ between 59 and 89 km\ the sunset "SS# values are generally larger than the sunrise "SR# values[ Above 89 km\ the situation is reversed\ SR×SS[ Siskind et al[ "0887# discuss the lati! tudinal variation of the SR:SS ratio and show that it is largest at the equator\ i[e[\ in excess of one order of magnitude\ compared to less than a factor of two at 46> latitude[ This diurnal variation is puzzling\ particularly below 89 km\ because simple photo!chemical theory indi! cates that the lifetime of NO is generally much larger than a day "Siskind\ 0883#[ Unfortunately\ the NO data between 69 and 79 km are the least certain of the entire dataset "Gordley et al[\ 0885#[ This is due to the di.culty of measuring small values in the presence of a large fore! ground contribution from the thermospheric values[ Indeed\ the signal!to!noise ratio for individual pro_les is of the order of 9[0 for NO densities ³095 cm−2^ although\ because we have averaged about 499 pro_les together\ random e}ects are unlikely to be the cause of the SR:SS di}erence[ Since the work by Gordley et al[ "0885#\ the HALOE science team has continued to investigate this question\ but without _rm results[ A diurnal asymmetry does appear in the measured gas correlation signal for the HALOE NO channel[ However\ the HALOE team cannot rule out an uncertainty in modelling the instru! ment spectral response as the possible cause "L[ Gordley\ 0887\ private communication#[ Unfortunately\ unlike the case of the thermosphere\ there are no other global meso! spheric NO datasets with which to validate these data[ For these reasons\ the details of the mesospheric mini! mum should be treated with caution[ Instead\ in this paper\ we suggest "as an alternative# an indirect approach using ionospheric data that may shed light on this question[
2[ Ionospheric data The obvious question is how well electron densities computed with HALOE NO densities agree with mea! sured electron densities[ The most reliable electron den! sities in the lower ionosphere are based on rocket!borne radio wave propagation measurements\ because probe measurements are prone to be disturbed by aerodynamic e}ects and may be in~uenced in their absolute values
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by payload charging "Mechtley\ 0863^ Jacobsen and Friedrich\ 0868#[ The resolution and sensitivity of ground!based measurements "e[g[\ incoherent scatter radar# is usually insu.cient\ particularly in the D!region and the comparison of the merits and limits of the various techniques made by Thrane "0863# generally still holds[ Because any expected seasonal variation at the equator is small\ HALOE ðNOŁ can be averaged for the whole duration of the UARS mission and thus yield statistically the most meaningful data[ In the literature\ there are only 14 electron pro_les based on rocket borne wave propagation experiments from within 201> from the equator[ Most are from the Indian range Thumba "7[5>N#\ four from Alca¼ntara\ Brazil "1>S# and the rest were launched from a research vessel in the Atlantic at 01>S\ covering solar zenith angles from nearly overhead to tropical midnight and solar activities from quiet to very active[ Comparison with any of these 14 pro_les cannot therefore be very conclusive\ given the restricted geophysical conditions at the times of the HALOE NO measurement[ Because of the likewise very limited amount of appropriate D!region electron density pro_les\ the empirical\ non!auroral D!region model by Friedrich and Torkar "0887#\ which is based on rocket borne wave propagation data only\ will be used for further compari! sons[ Approximate variance factors "input data vs[ results of the statistical model# are typically 0[5 or better below 79 and above 84 km\ whereas the largest discrepancies of up to a factor of 0[8 occur between 74 and 89 km[ The few daytime pro_les which were actually measured near the equator were carefully compared to the model results and found to yield similar agreement[ The electron density pro_les resulting from the ion! chemical model for the conditions of Figs[ 0Ð1 "i[e\ at the equator and for x 34> and 64>\ respectively# and the NO densities "SR and SS# shown in Fig[ 2 are plotted in Figs 3 and 4\ respectively[ These zenith angles were chosen because\ at smaller angles\ X!rays may dominate the ion production down to say 74 km\ whereas at very large solar zenith angles\ such as those used for the HALOE measurements\ galactic cosmic rays "GCR# can be the dominating source of ionisation up to perhaps 79 km[ For the present comparison\ the above empirical electron density model was established using AM and PM data separately in Fig[ 3 "x 34>#\ whereas both AM and PM electron density data are entered into the model to establish the empirical curve in Fig[ 4[ The resulting pro_les are also plotted in Figs[ 3Ð4[ The appar! ent asymmetry in the D!region electron densities is clearly seen in the radio wave absorption\ which is based on more measurements than the above electron densities data base[ Figure 5 shows the signal strength vs[ solar zenith angle of a 1[72 MHz signal over a 499 km path in central Europe "Coburg\ Germany\ to Graz\ Austria\ between 0863 and 0865\ i[e[\ at a time of generally low solar activity[ Two features are apparent]
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Fig[ 3[ Equatorial electron densities calculated using the mean NO densities of Fig[ 2 for a solar zenith angle of 34> "dashed lines# and the corresponding empirical values from the empirical model by Friedrich and Torkar "0887# " full lines#[ Also indicated on the right is the variance in the densities of the empirical D!region model[
"a# in Winter the signal during the day is above the annual average "indicated by the dotted line# and "b# in Summer the hysteresis is much larger[ "The signal strength variation at night is due to the height of the re~ection level in the E!region and not to absorption in the D!region#[ The results in Fig[ 3 reveal an asymmetry in the empiri! cal electron densities similar to the result of the cal! culation using the HALOE SR and SS data[ Below about 71 km\ the modelled densities are consistently too low\ implying erroneous results from the HALOE instrument\ although less so with the higher sunset values[ However\ the higher sunset pro_le Fig[ 4 increases the factor between calculated and empirical electron densities in the 74 to 89 km region[ In the E!region\ no conclusions
pertaining to the NO densities can be drawn because\ there\ the ionisation by X!rays clearly dominates over that due to NO[ Taubenheim "0866# employed this pro! cedure in reverse\ i[e[\ he deduced ðNOŁ from inferred electron densities pro_les assuming "a# that the e}ective recombination rate could be assumed and "b# that ionisation processes other than by NO and Lyman!a "qrest# were known[ ðNOŁ
Ne1 c−qrest \ sFLyman−a
where s is the ionisation cross section of NO and FLyman!a is the local ~ux of Lyman!a radiation[
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Fig[ 4[ As Fig[ 3\ but for a zenith angle of 64>[
With the empirical D!region model now available\ one should be able to apply this procedure more successfully[ However\ the low empirical values between 74 and 89 km would lead to ne`ative NO densities because\ there\ the ion production from O1 "0Dg# alone provides Ne larger than that observed[ The fact that the computed Ne exceeds the empirical values can have several expla! nations]
0[ The computed Ne is correct\ but the empirical electron densities are too low[ This height region has the largest variance "about a factor of 0[8 larger or smaller than the curves shown in Figs[ 3Ð4#[ 1[ The empirical Ne values are basically correct\ but the computed Ne values are too high[ For this hypothesis\ there are two explanations] "a# The computed ion!pair production\ by processes
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Fig[ 5[ Strength of a 1[72 MHz signal over a 499 km path in central Europe as a function of solar zenith angle[ Dotted line represents the annual average "after Torkar et al[\ 0867#[
other than Lyman!a and NO\ is too large[ The most important such process is O1 "0Dg# being ion! ised by solar radiation in the region from 092 to 009[4 nm[ The ~ux measured by Hall and Hin! teregger "0869# is probably still the most recent in this band and the absorption by O1 and CO1 was calculated for 64 discrete lines by Paulsen et al[ "0861#[ The densities of O1 "0Dg# used here are the ones obtained from the ozone experiment aboard the satellite SME "Thomas et al[\ 0873#[ "b# The e}ective electron recombination rate c of the theoretical model is too small[ Larger c "by orders of magnitude# occur at heights where the dom! inating ions are of the water cluster type[ The transition height "equal number of cluster and molecular ions# was found by Friedrich and Tor! kar "0877# to be a function of temperature and varies between 67 and 89 km " for 124 to 099 K\ respectively#[ Cluster ions do\ however\ disappear rather abruptly at the height where atomic oxygen exists with appreciable densities[ The ðOŁ values employed in the present context are idealised pro! _les based on rocket!borne data with a ledge at 71 km[ Only densities below that height are varied with the solar zenith angle beyond 74>[ The recent empirical model by Llewellyn and McDade "0885# is representative for diurnal averages and there! fore displays hardly any ledge[ However\ in the height region where too large electron densities are calculated "75 km#\ the Llewellyn and McDade model yields even larger O!densities "by a factor of three#\ hence the calculated cluster transition would be even lower "smaller recombination rate# and would increase the discrepancy[ Furthermore\ the empirical c "Friedrich and Torkar\ 0877# are somewhat lower than the results of the theoretical modelling\ but suggest the same transition height[
It appears from the above that electron densities in this height region "74 to 89 km# are the least predictable in the whole altitude range considered "49 to 049 km#\ but the data on the solar spectral range ionising O1 "0Dg# are also older than the more recent data on that minor spec! ies[ Let us assume the daytime electron densities given by the empirical D!region model to be representative and ignore all ionisation processes other than by Lyman!a and NO "qrest 9#[ By using recombination rates from the ion!chemical computation\ we can infer upper limits of NO densities for the three conditions for which we have representative results from HALOE "equator\ Fig[ 2 as well as 27> and 46>#[ Figure 6 shows the NO densities required to produce Ne at the equator\ as predicted by the empirical model in the absence of non!NO ionisation processes[ Naturally\ the NO thus required is di}erent for di}erent solar zenith angles\ x\ because for small x\ X!rays or Lyman!b radiation would contribute more to the ionisation and have to be replaced by excessively large NO densities[ On the other hand\ at low altitudes\ the ionisation by galactic cosmic rays can dominate for large x[ The low density envelope of the NO!densities thus computed for various x constitutes the upper limit of NO that can exist in order to reproduce the Ne of the empirical D!region model[ One can see that the HALOE NO!den! sities exceed the upper limit based on ionospheric con! siderations in the region between 79 and 84 km\ both for SR and SS[ Corresponding exercises were also carried out for the equinoxes at 27> and 46> "Figs 7 and 8#[ It is evident that at 27>\ the HALOE results exceed those conceivable on ionospheric grounds in only a limited height region "SS only#[ At 46>\ the electron density model largely hinges on rocket measurements from South Uist\ Scotland\ at a geomagnetic latitude of 59>\ whereas the same geographic latitude in the American sector is higher by about 09> in geomagnetic latitude[ This excess
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Fig[ 6[ Upper limits of NO densities assuming ion production solely due to NO and Lyman!a in order to reproduce the empirical electron densities[ The calculations were carried out for the equator and for di}erent solar zenith angles using electron loss rates due an ion!chemical scheme[ Also shown are the corresponding HALOE results "cf[ Figure 2#[ Note that between 79 and 89 km the NO densities "both SR and SS# should be considerably lower in order to be compatible with the ionospheric data[
density is signi_cant because the recombination rate\ c\ prevailing at these heights is the well established one of ¦ O¦ 1 and NO \ i[e[\ largely above the region of cluster ions with their much larger recombination rates[ During the day\ the transition from cluster to molecular ions was
found to be between 60 and 76 km\ depending on the temperature "Friedrich and Torkar\ 0877#[ The highest transition heights only occur when the mesosphere is extremely cold "high latitude Summer#^ here\ however\ only equinox conditions are considered[ The test shown
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Fig[ 7[ Upper limits of ðNOŁ on ionospheric grounds for a zonal mean at a latitude of 27> and equinox[ Also indicated are the averaged NO densities from HALOE for the same conditions[ The satellite data appear to be generally compatible with the ionospheric data[
in Fig[ 8 is somewhat distorted because we compare upper ðNOŁ!limits based on non!auroral electron densities with NO densities partly gathered in the auroral zone where charged particle precipitation constitutes a very important source of NO[
3[ Conclusions We have shown that the ionospheric data are generally consistent with HALOE NO pro_les that show a mini! mum in the 69 to 79 km region[ There are signi_cant
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Fig[ 8[ As Fig[ 7\ but for 46> latitude[ The ionospheric data on which the conceivable upper limits of ðNOŁ are based are from geomagnetic latitudes ³50>\ whereas the NO data are partly from higher geomagnetic latitudes[ This may explain the NO densities exceeding the ionospheric limits in the upper D!region[
di}erences "up to a factor of 3# in the details of the magnitude and height of the minimum[ Given the di.! culties in obtaining reliable NO measurements in this region\ one should be cautious about over!interpreting these di}erences[ Whereas the unexpected large di}erence between sun! rise and sunset values is at least indirectly supported by
ionospheric measurements\ the magnitude and the height of the NO minimum is not re~ected in ionospheric data and may\ indeed\ be a problem of low signal!to!noise ratio[ The disagreement in the NO densities to reproduce the observed electron densities is never larger than a factor of four[ The exercise reported here points to the following problems]
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, The gross features of the comparison between HALOE NO and the ionospheric results indicate the need for reduced NO between 74 and 84 km[ , The ionisation contribution of O1 "0Dg# should be addressed to see if smaller values are conceivable[ , The possibility of a diurnal variation in theoretical models of NO should be investigated[ According to conventional wisdom\ the lifetime of an NO molecule in the mesosphere and thermosphere is of the order of days^ hence\ there should be no diurnal variation "e[g[ Solomon et al[\ 0871#[ However\ some asymmetry with respect to noon has long been observed in mid!latitude ionospheric absorption data[ Las³tovic³ka "0866# published values larger in the afternoon for the same solar zenith angles "e[g[\ 06 vs[ 6 dB#[ He also found that this asymmetry was most pronounced in Summer "when the Sun rises fast#\ whereas it was hardly noticeable in the Winter data[ From the absorption ratio between morning and afternoon\ one can conclude a cor! responding ratio in electron density or approximately that ratio squared in ionisation "which is roughly pro! portional to the NO!density#[ A mechanism which may\ at least in part\ explain D!region asymmetries is via a change of the electron loss rate due to diurnally asym! metric neutral temperatures[ The recent measurements by the CRISTA experiment aboard the Shuttle Pallet Satellite\ ~own in November 0883 "Preusse et al[\ 0886#\ revealed dayÐnight temperature di}erences of up to 29 K in the mesosphere\ centred at the equator[ Inserting these temperatures in the ion!chemical model yields electron density di}erences of up to 14)[ It is interesting to note that the HALOE NO data shows diurnal asymmetries in the same sense as the ionospheric data which have previously been reported without an explanation[ Unfor! tunately\ the large uncertainties inherent in quantifying the NO minimum at 69 km preclude further discussions at this point[ Opposite diurnal asymmetries "less in the afternoon# were found by Monro et al[ "0865# in the E! region in incoherent scatter data[ This observation also seems to agree with the present NO!data[ Sounding rock! ets instrumented to measure NO\ O1 "0Dg# and electron densities by the same payload should be launched on the same day at the same solar zenith angle in the morning and the evening[ One should thus gain an insight into the causes of the unquestionable diurnal asymmetries in radiowave absorption[ The preferred condition for such an experiment is a scenario with fast sunrise and sunset\ such as in the tropics or mid!latitudes in Summer[
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of cluster ions[ Advances in Space Research 7 "3#\ 124Ð 127[ Friedrich\ M[\ Torkar\ K[M[\ 0887[ Comparison between an empirical and a theoretical model of the D!region[ Advances in Space Research 10 "5#\ 784Ð893[ Gordley\ L[L[\ Russell III\ J[M[\ Mickley\ L[J[\ 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[\ 0885[ Validation of nitric oxide and nitrogen dioxide measurements made by the halogen occultation experiment for UARS platform[ Journal of Geo! physical Research 090 "D5#\ 09130Ð09155[ Hall\ L[A[\ Hinteregger\ H[E[\ 0869[ Solar radiation in the extreme ultraviolet and its variation with solar rotation[ Jour! nal of Geophysical Research 64\ 5848Ð5854[ Heaps\ M[G[\ 0867[ A parametrization of cosmic ray ionization[ Planetary and Space Science 15 "5#\ 402Ð406[ Hu}man\ R[E[\ Paulsen\ D[E[\ Larabee\ J[C[\ 0860[ Decrease in D!region O1 "0Dg# photoionization rates resulting from CO1 absorption[ Journal of Geophysical Research 65 "3#\ 0917Ð 0927[ Jacobsen\ T[A[\ Friedrich\ M[\ 0868[ Electron density measure! ments in the lower D!region[ Journal of Atmospheric and Terrestrial Physics 30 "01#\ 0084Ð0199[ Las³tovic³ka\ J[\ 0866[ Seasonal variation in the asymmetry of diurnal variation of absorption in the lower ionosphere[ Jour! nal of Atmospheric and Terrestrial Physics 28 "7#\ 780Ð 783[ Llewellyn\ E[J[\ McDade\ I[C[\ 0885[ A reference model for atomic oxygen in the terrestrial atmosphere[ Advances in Space Research 07 "8:09#\ 198Ð115[ Mechtley\ E[A[\ 0863[ Accuracy of rocket measurements of the lower ionosphere electron concentration[ Radio Science 8 "2#\ 262Ð267[ Monro\ P[E[\ Nisbet\ J[S[\ Stick\ T[L[\ 0865[ E}ects of tidal oscillations in the neutral atmosphere on electron densities in the E!region[ Journal of Atmospheric and Terrestrial Physics 27\ 412Ð418[ Paulsen\ D[E[\ Hu}man\ R[E[\ Larrabee\ J[C[\ 0861[ Improved photoionization rates of O1 "0Dg# in the D!region[ Radio Sci! ence 6 "0#\ 40Ð44[ Preusse\ P[\ Riese\ A[A[\ Oberheide\ J[\ Bittner\ M[\ Grossmann\ K[U[\ O}ermann\ D[\ 0886[ Evidence for zonally trapped diurnal tide in CRISTA temperatures[ Advances in Space Research 08\ 468Ð471[ Russell III\ J[M[\ Gordley\ L[L[\ Park\ J[H[\ Drayson\ S[R[\ Hesketh\ D[H[\ Cicerone\ R[J[\ Tuck\ A[F[\ Frederick\ J[E[\ Harries\ J[E[\ Crutzen\ P[J[\ 0882[ The halogen occultation experiment[ Journal of Geophysical Research 87 "D5#\ 09666Ð 09686[ Siskind\ D[E[\ 0883[ On the radiative coupling between meso! spheric and thermospheric nitric oxide[ Journal of Geo! physical Research 88 "D00#\ 11646Ð11655[ Siskind\ D[E[\ Bacmeister\ J[T[\ Summers\ M[E[\ Russell III\ J[M[\ 0886[ Two!dimensional model calculations of nitric oxide transport in the middle atmosphere and comparison with halogen occultation experiment data[ Journal of Geo! physical Research 091 "D2#\ 2416Ð2434[ Siskind\ D[E[\ Barth\ C[A[\ Russell III\ J[M[\ 0887[ A clima! tology of nitric oxide in the mesosphere and thermosphere[ Advances in Space Research 10 "09#\ 0242Ð0251[
M[ Friedrich et al[:J[ Atmos[ Solar!Terrestrial Physics 59 "0887# 0334Ð0346 Solomon\ S[\ Crutzen\ P[J[\ Roble\ R[G[\ 0871[ Photochemical coupling between the thermosphere and the lower atmosphere 0[ Odd nitrogen from 49 to 019 km[ Journal of Geophysical Research 76\ 6195Ð6119[ Taubenheim\ J[\ 0866[ The distribution of nitric oxide and its variation near the mesopause derived from ionospheric obser! vations[ Space Research 06\ 160Ð167[ Thomas\ R[J[\ Barth\ C[A[\ Rusch\ D[W[\ Sanders\ R[W[\ 0873[ Solar mesospheric explorer near!infrared spectrometer] Measurements of 0[16 mm radiances and the inference of meso! spheric ozone[ Journal of Geophysical Research 78 "D5#\ 8458Ð8479[ Thrane\ E[ V[ "0863# Ionospheric pro_les up to 059 km*A
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review of techniques and pro_les[ In COSPAR Methods of Measurements and Results of Lower Ionospheric Structure\ ed[ K[ Rawer\ pp[ 2Ð10[ Akademie Verlag\ Berlin[ Torkar\ K[M[\ Friedrich\ M[\ 0872[ Tests of an ion!chemical model of the D! and lower E!region[ Journal of Atmospheric and Terrestrial Physics 34 "5#\ 258Ð274[ Torkar\ K[M[\ Friedrich\ M[\ 0877[ Empirical electron recom! bination coe.cients in the D! and E!region[ Journal of Atmo! spheric and Terrestrial Physics 49 "7#\ 638Ð650[ Torkar\ K[M[\ Friedrich\ M[\ Wallner\ W[\ Rose\ G[\ Widdel\ H[U[\ 0867[ Preliminary results of absorption measurements of a central European A2 path[ Kleinheubacher Berichte 10\ 044Ð050[