Ionospheric constraints for mesospheric nitric oxide based on empirical models of the D-region

Adv. SpaceRes. Vol. 25, No. 1, pp. 4H6,2000 0 1999 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-l 177/00 $20.00 + 0.00 PII: SO273-1177(99)00895-9

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IONOSPHERIC CONSTRAINTS FOR MESOSPHERIC N ITR.IC OXIDE BASED ON EMPIRICAL MODELS OF THE D-REGION

M. Friedrich],

and K.M. Torkar2

/Department of Communications and Wave Propagation. Technical University Gruz. I@ldgasse A-8010 Graz, Austria, 2Space Research Institute, Austrian Academy qfSciences, Irtfleldgasse 1L A-801 0 Graz. Austria.

12,

ABSTRACT The formation of the daytime D-region crucially depends on the density of nitric oxide (NO). If other ionisation processes are assumed to be known or expected to be negligible, NO densities can be inferred, provided the effective recombination coefficient is known. A non-aurora1 D-region model based on rocket-borne wave propagation data only, together with the results during undisturbed conditions from a similar analysis from high latitudes provides the electron density basis from which NO densities are inferred. Two sets of NO density profiles are provided, one with the highest conceivable densities and another with the likely typical values. 01999

COSPAR.

Published

by Elsevier Science Ltd.

INTRODUCTION During daytime the dominating source of free electrons in the D-region is the ionisation of nitric oxide (NO) by solar Lyman-a. Figure 1 shows typical ionisation processes at mid-latitude and a solar zenith angle x of 75’. The NO is the mean of the values measured by the HALOE instrument aboard the UARS satellite (Siskind et al., 1998) and the solar fluxes are representative for low solar activity. One can see that in this example the concentration of NO is decisive in the height region from 73 to 98 km. For larger x the height where galactic cosmic rays dominate is higher, for smaller x ionisation of NO by Lyman-a dominates to lower altitudes, but X-rays and ionisation of 02( IA,) by EUV
_5

-2 $ cd

10-3

10-2

I

NC,=

d2

ff‘,,

10-l

100

10'

ion pair production,

102

103

104

CIII-’ s-’

(1) Fig. 1. Typical ionisation processes for a solar zenith angle of 75”, a latitude of 35”, low solar activity (Flu7 = 75 Jy) and equinox.

43

44


M. Friedrich and I<. M. Torkal

ion pair production

The ion pair production solved for [NO]:

[NoI=

rate, cm-3 s-t;c~~f...

effective recombination

cm3

coefficient,

rate Q is the sum of that due to NO and a QrCJ,Y,. The equation

s-1)

can

tllus

be

rewritten and

N‘,2ati,,- Qrc,,

(2)

aa

,w*s,-o

In Equation (2) Qresl, Glefj.and o Qj,yman_u (ionisation cross section times the local Lyman-a flux) can be determined from model calculation; we will here use the ion-chemical scheme by Torkar and Friedrich (1983). With the knowledge of realistic electron densities one can now in principle establish [NO] from electron concentrations N,, a procedure applied previously, however, using less representative data (Taubenheim. 1977). III the present context we employ the empirical D-region model by Friedrich and Torkar ( 1998) which is based solely on undisturbed electron densities derived by rocket-borne radio wave propagation methods or probes 011 the same payload calibrated by such methods. The basic features of the model are (I) the assumption of a sinusoidal variation with seasons symmetric to solstice, 6) a linear dependence on latitude and solar activity and C) that all altitudes are treated independently of each other. A little over 100 profiles from undisturbed from non-aurora1 latitudes are entered and the height range best covered by data is between 70 and I IO km. Figure 3 shows NO densities derived using Equation (2) the empirical electron densities applicable for 35” and equinox and a solar zenith angle of 75”. L

I._LI1 106

“““‘I

“““”

“““1

“I”‘1

1 108

107 NO

density,

109 cm-3

10’0 NO

density,

cm-3

Fig. 2. Derived NO densities using Equation (2) for conditions of low (left panel) and high solar activity (right panel) employing electron densities from an empirical model for equinox conditions and a latitude of 35”. The curve termed maximum density is obtained assuming all ionisation processes other than C~LINO to be zero. Below 75 km the electron density model is less reliable (dashed part). The effective recombination rate is based on the above ion-chemical model and the situation for two levels of solar activity is shown. Each of the panels contains two curves, one showing the probable results using Equation (2) the other assuming QrCurto be zero; the latter densities therefore constitute the maximum conceivable [NO] according to the ionospheric data. For these particular conditions the “probable” results sometimes lead to negative values in the height region between 80 and 8.5 km which is either due to an excessive QYCJ,st (excessive 02(tAg) ‘?) or too small an effective recombination rate aCeffThe transition from the region of cluster ion domination to that of molecular ones with their much lower recombination rate is also in this particular, apparently critical, region. For daytime conditions this transition has been experimentally determined to be located between 76 and 84 km for high and low neutral temperatures (240 to I IO K), respectively (Friedrich and Torkar, 1988a); the transition heights according to that analysis are indicated in this and the following figures. Otherwise the figure shows the expected behaviour, i.e. larger [NO] at times of high solar activity particularly at the minimum and below (cf:

45

Mesospheric Nitric Oxide

Siskind et al., 1998). Above the minimum fluxes, whereas tainty. SEASONAL

below the minimum

depends on Qresh ie. a good knowledge

the derivation

NO clearly dominates

AND LATITUDINAL

the ion pair production,

of the X-ray

but a@jf presents an uncer-

VARIATION

The procedure outlined above was carried out for the middle of each month and for a latitude of 45O. Low solar ac-

tivity (F10.7 = 75 Jy [Jansky = lo- 22 W m-2 Hz-l]) was chosen because the empirical electron density model is better covered by data taken under these conditions. In Figure 3 lines of constant [NO] (probable values) are shown between 68 and 105 km. The corresponding seasonal variation of the empirical electron densities shows a much larger variation in the mesosphere than [NO]. In the present derivation we employ a,,, based on an ion chemistry scheme which uses the neutral atmosphere from the CIRA-86 model as an input (Rees et al., 1990). Alternatively, we have tested the use of an empirical effective recombination coefficient (Friedrich and Torkar, 1988b) which is defined for neutral density levels; since these heights also undergo a seasonal variation. The results are qualitatively very similar. In the lower thermosphere (above the minimum) [NO] is larger in winter (as expected), but in the mesosphere the seasonal variation contradicts the results of theoretical models (Siskind et ul., 1997). This behaviour is due to the variation of the computed aef$ whereas Ne shows the expected variation, i.e. A corresponding larger in winter. analysis to display the latitudinal variation is shown in Figure 4. Again the expected enhancement towards the darker (winter) hemisphere is not as pronounced as expected from theoretical models of [NO] or the empirical variation of N,. k

/

JAN

I

FEB

I

I

MAR APR

I

I

MAY JUN

I

I

I

JUL

AUG

SEP

I

I

I

OCT NOV DEC

For the aurora1 zone a gion model is employed _ Kirkwood, 1999) which Fig. 3. Seasonal variation of derived [NO] for low solar activity at a absorption as the main latitude of 45” (probable values). Values below 75 km are less certain. rameter. Inserting 0 dB time of year

-60

-45

-30

-15 0 latitude,

15 deg

30

45

60

-60

-45

-30

-15 0 latitude,

15 deg

Fig. 4. Latitudinal variation of derived VO] for low solar activity and summer in the northern probable densities, right: maximum values. Below 75 km values are less certain.

separate D-re(Friedrich and uses riometer describing paone can estab-

I

I

30

45

hemisphere;

60

left:

46

M. Friedrich and K. M. Torkar

lish a quiet profile essentially by extrapolation

from

conditions when particle production dominates to those 100

where particle production is absent; such an electron density profile

is, however, still “disturbed” because

[NO] will remain enhanced for some time following particle events. The really quiet high latitude profile is the one determined from simply taking the envelope of all electron density profiles taken at the same solar zenith angles. The “true” quiet profile which is significantly below the one assumed

to be free from particle

production, can only be determined for altitudes above 90 km because of the threshold of the EISCAT

radar,

the main source of high latitude data. In Figure 5 [NO] is shown derived from the two quiet D-region profiles,

70

one is the “true”, the other the extrapolated quiet curve;

t 106

also shown is the result obtained from the non-aurora1 to7

lo8 NO density,

109 cm-3

10’0

model, extrapolated to high latitudes. Two in-situ measurements from the amoral zone by Witt et ~1. (1976)

Fig. 5. Amoral zone Ir\rO] profiles from D-region mod- taken under quiet conditions

(C32-2) and S 18-2 derived during a particle event (Arnold, 1980) are also depicted. els and in situ measurements. The profile based on the extrapolated quiet electron density profile is indeed close to the one derived from the rocket flight S 18-2. CONCLUSIONS NO density measurements in the mesosphere

are rare and even the recently available

data from the HALOE ex-

periment aboard the satellite UARS are not unambiguous. Otherwise only theoretical calculations provide a clue to the behaviour of this important trace constituent. At present, the very preliminary derivation of [NO] does not constitute a measurement per se, but should rather be seen as a check of the plausibility

of both empirical

and theo-

retical models. REFERENCES Arnold, F., The Middle Atmosphere

Ionized Component, ESA SP-152, pp. 476-479 ( 1980). Friedrich, M., and Sheila Kirkwood, The D-Region Background at High Latitudes, this volume, (1999). Friedrich, M., and K.M. Torkar, Empirical Transition Heights of Cluster Ions, Adv. Spice Rex 8 (4). pp. 235-238 (1988a). Friedrich, M., and K.M. Torkar, Empirical Electron Recombination Coefficients in the D- and E-Region. .I. atmos. terr. Phys. 50 (8), pp. 749-76 I (1988b). Friedrich, M., and K.M. Torkar, Comparison Between an Empirical and a Theoretical Model ofthe D-Region, Adv. Space Res. 21 (6), pp. 895-904 (1998). Rees, D., J.J. Barnett, and Karin Labitzke, COSPAR International Reference Atmosphere. Adv. Space Rex 10 (12), (1990). Siskind, D.E., J.T. Bacmeister, M.E. Summers, and J.M. Russell, Two-Dimensional Model Calculations of Nitric Oxide Transport in the Middle Atmosphere and Comparison with Halogen Occultation Experiment Data, J. geophys. Rex, 102 (D3), pp. 3,527-3,545, ( 1997). Siskind, D.E., C.A. Barth, and J.M. Russell III, A Climatology ofNitric Oxide in the Mesosphere and Thermosphere, Adv. Space Res., 21 (lo), pp. I ,353- 1,362. (1998). Taubenheim, J., The Distribution of Nitric Oxide and its Variation Near the Mesopause Derived from Ionospheric Observations, Spuce Res., 17, pp. 27 I-278 (1977). Torkar, K.M., and M. Friedrich, Tests of an Ion-Chemical Model of the D- and Lower E-Region, J. atmos. terr. Phys., 45 (6), pp. 369-385 (1983). Witt, G., J.E. Dye, and N. Wilhelm, Rocket-Borne Measurements of Scattered Sunlight in the Mesosphere, ./. atmos. terr. Phys. 38 (3), pp. 223-238 (1976).