The D-region background at high latitudes

0 1999 COSPAR.

Pergamon www.elsevier.nl/locate/asr

PI I: SO273

Adv. Space Res. Vol. 25, No. I, pp. l5-33.2000 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 027% 1177/00 $20.00 + 0.00

I I 77(99)00892-3

THE D-REGION BACKGROUND AT HIGH LATITUDES

M. Friedrichl,

and Sheila Kirkwood?

1Department of Communications and Wave Propagation, Technical University Graz, Ingeldgasse 12, A-8010 Graz, Austria 2Swedish Institute of Space Physics, POB 812, S-981 28 Kiruna, Sweden

ABSTRACT At high latitudes the ion-pair production is most of the time dominated by fluxes of energetic particles. The resulting electron densities exceed the ones expected due to solar control alone; the D-region part of these excess densities is responsible for the absorption measured by a riometer. Hence riometer absorption is a good parameter to describe the disturbed ionosphere. An approach is presented in which a huge number of lower ionosphere electron densities from EISCAT and in-situ measurements from sounding rockets are analysed jointly. The much smaller number of rocket borne measurements constitutes an important extension to lower altitudes and densities. The undisturbed ionosphere deduced from these data sets is compared to a model of the non-aurora1 D-region extrapolated to high latitudes. Typical profiles for disturbed conditions are given separately for day and night as a 01999 COSPAR. Published by Elsevier Science Ltd. function of riometer absorption.

INTRODUCTION

AND DATA BASE

Measurements of plasma densities in the E- and particularly the D-region are severely hampered by the presence of a relatively large background neutral density. From the ground the best measurements are by incoherent scatter of VHF or UHF radio waves which require very large installations (e.g. Arecibo, Puerto Rico, EISCAT, Scandinavia, or Srandre StrPrmfjord, Greenland), whereas methods successfully employed higher up, such as the ionosonde, are not useable. The height region is too low to be investigated in-situ by satellites and one must therefore rely on sounding rockets. Many instruments (probes) which provide reliable data at higher altitudes, however, do not yield useful information aboard sounding rockets traversing the D-region. One of the reasons is that the fast flying rocket creates a disturbance of the plasma to be measured and another that the payload may acquire an electrical potential distorting the measurement of charged species with a probe. In the aurora1 zone the European Incoherent wer facility (EISCAT) is the one that provides the longest time series of data (Folkestad et al., 1983); of the many operating modes the Common Programme always yields electron densities (Baron and Person, 1985). From the time period 1984 to 1993 over 30,000 profiles were gathered in a volume sampled by the radar approximately located over Tromsra, Norway, at a geographic latitude of 70” (66” geomagnetic). The echoes to pulses of different lengths are sampled in bins of different width (altitude range). Three electron density curves are stored from each of the basic 5 minute measurement intervals. An algorithm is applied which rejects individual profiles with a negative gradient below 90 km and ignores values in excess of 2x1012 m-3. Densities below 2x109 m-3 may be the result of an averaging process which includes “negative” electron densities; since these values would naturally be ignored before averaging, the result would be biased towards averages larger than the true value. Consequently 2x 109 m-3 is taken as the threshold value of the EISCAT data for the present study.

15

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M. Friedrich and S. Kirkwood

Probably the least questionable method for determining electron densities in the D-region is based on semi in-situ methods, i.e. transmitting a radio wave from the ground to the flying rocket payload. The received signal is measured in amplitude and polarisation, both of which are functions of the electron content along the ray. This method completely bypasses problems associated with aerodynamics and payload charging. If such a wave propagation measurement is combined with an in-situ instrument (probe), the latter can be calibrated and provides the fine structure not obtainable by the radio method (Thrane, 1974; Jacobsen and Friedrich, 1979). A total of 99 such measurements are available from aurora1 latitudes since 1957 (Ft. Churchill, Canada) to the most recent flights in 1998 from ESRANGE, Sweden. Both data sets (or earlier versions thereof) have been processed separately for the purpose of establishing high latitude D-region models (e.g. Friedrich and Torkar, I983a; 1995; Kirkwood and Collis, 1991; Kirkwood, 1993); the present paper is a first effort to combine the best of the two sets of data and to demonstrate the potential of this large body of measurements. To gain insight into the vast amount of available electron density profiles in a first step they were grouped in solar zenith angle ranges. Figure I shows all profiles in the solar zenith angle range from 70 to 80”. Note that below 2x109 m-3 and 70 km only two profiles exist, both from rockets. Other features are a few extremely low values in the E-region (erroneously determined ?) and an isolated bulge between 100 and I IO km. The latter may be a sporadic E-layer descending during the five minutes of observation time of that particular EISCAT profile. One can furthermore see that the shape of the profiles changes from shallow peaks near 120 km to more pronounced ones as low as 90 km. The quiet ionosphere for this zenith angle can be deduced from the envelope down to about 95 km; below that height other methods have to be employed to establish densities believed to be representative for quiet conditions. Figure 2 displays the corresponding situation for the twilight zenith angle of 90 < x < 100”. Again the rocket data clearly extend to much lower densities and the quiet E-region can no longer be determined from the envelope ofthe EISCAT data because obviously values below the instrumental threshold may exist. The extremely low densities in the E-region are even considerably below the lowest densities found at low latitudes at even larger solar zenith angles (cf: IRl or Friedrich and Torkar, 1998).

N=lO2JN=ZksRd,diW-107 lso-

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Fig. 1 Electron density profiles for a solar zenith angle range between 70 and 80”. N and Nr are the number of EISCAT and rocket profiles, respectively. Note that only two of the rocket profiles extend to lower densities and altitudes.

Fig. 2 As Fig. I, but for a larger solar zenith angle (90 to IOOO).

II-Region High Latitudes

loo

17

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Fig. 3 Electron densities vs. solar zenith angle at 100 km (left panel) and for a neutral density level of 8x1018 m-3 (right panel). Shaded values are from the equinoxes, dark points from around summer solstice. ASSUMED

VARIATIONS

OF AURORAL

ELECTRON DENSlTIES

Electron densities in the lower ionosphere undergo a diurnal variation essentially due to solar ionisation; at solar zenith angles beyond 98” also the chemistry governing the recombination changes markedly because of an abrupt disappearance of atomic oxygen below a ledge at about 83 km. This effect is to be expected from atmospheric models which compute [0] (Isaksen, 1973), but was also clearly seen in ionospheric absorption data (Stauning, 1996). At aurora1 latitudes the largest variability of electron densities, however, is due to the irregularly incident fluxes of ionising energetic charged particles. In order to study the behaviour of the D-region in a more general way all available data are plotted at constant neutral number density levels vs. solar zenith angle. Density rather than altitude is chosen since it is assumed that the ionosphere will behave comparably for the same background densities. The relation between altitude and neutral density is based on a combination of CIRA-86 (Rees et al., 1990) and the empirical model for Andoya by Liibken and von Zahn (1990) which on occasions markedly differs relative to CIRA. In order to test whether the use of neutral density levels indeed removes some of the seasonal variation, electron densities at 100 km are plotted vs. solar zenith angle (left panel of Figure 3) and also for a neutral density representative for that altitude (8.2~1018 m-3, right panel). The shaded points are values taken at the equinoxes +6 weeks, whereas the dark point are from summer solstice +6 weeks. One can see that low density envelopes in the solar zenith angle range where there is overlap are somewhat closer when plotting the data for constant a neutral density, most evident between 70 and SO”, although there still remains a residual seasonal difference which might alternatively be the result of poor statistics. The left panel of Figure 4 shows the situation for the neutral density level of I .2x 101s m-3 (appr. 110 km at equinox) together with the variation of the non-aurora1 empirical model by Friedrich and Torkar (1998) adjusted for Number Den&v = I.2 18 (m-3). # = 30891

Number Density= &Eel9 (m-3), # = 18868

.

I

60

80 100 Zenith angle (deg)

120

I I 140 40

60

.

80 100 Zenith angle (deg)

120

140

Fig. 4 Electron density vs. solar zenith angle for two neutral density levels approximately representative for I10 and 90 km, respectively. Also shown for comparison is the corresponding variation from a non-aurora1 model.

18

M. Friedrich and S. Kirkwood

summer and low solar activity. One can clearly see the variation of the quiet electron densities according to a cosx behaviour characteristic of solar control and - that for solar zenith angles beyond 90” - the expected envelope will be below the instrumental detection limit of EISCAT, although the non-aurora1 model suggests night-time values just above the EISCAT limit. Another interesting feature is that the largest electron densities at this altitude occur at night; this is explicable by the fact that the most favourable time for particle precipitation at EISCAT is geomagnetic midnight. The right panel shows the situation for an altitude of approximately 90 km. Obviously for all zenith angles electron densities below the EISCAT threshold are conceivable and at night some much lower values from rocket measurements exist approximately of the same order as expected from the non-aurora1 model. Due to the scarcity of the rocket data the method of using the envelope to determine quiet electron densities can not be employed. In Figure 5 various possibilities of defining quiet day-time electron density profiles are depicted (summer solstice, low solar activity, x = 50“). Above 90 km the envelope of the data in plots such as in Figure 4 are used (“true quiet”), the line labelled “low latitude model” is the result of the non-aurora1 model (mis-)applied to 65” latitude. The result of electron densities measured under disturbed conditions suitably extrapolated to 0 dB absorption is also indicated (“extrapolated quiet”, for the procedure see below and Friedrich and Torkar, 1995). There are several noteworthy features, such as: a) The extrapolation of the non-aurora1 model beyond the highest latitude which that model is based on (60”) to the aurora1 zone (68”) overestimates somewhat the densities that may actually occur (cf: the right panel of Figure 4) and b) the extrapolation from disturbed to undisturbed conditions leads to even larger densities (see later). This latter discrepancy can plausibly be explained by the fact that during disturbed condition (i.e. additional ionisation by charged particles), also the density of neutral nitric oxide (NO) is enhanced (Siskind, 1994). In the absence of particle ionisation, but during times of frequent particle events, the mesospheric [NO] remains enhanced and the solar controlled ionisation is also larger. The fact that the “true” and the “extrapolated quiet” curves converge above 100 km lends credence to the hypothesis that the difference between the two is due to enhanced WO] because in the quiet E-region X-rays and not Lyman-o and NO constitute the dominating ionisation process. In order to explain these electron densities about one order of magnitude larger than predicted by the non-aurora1 model (around 85 km), [NO] two orders of magnitude larger than implicitly assumed for non-aurora1 latitudes must be invoked as have indeed been derived in aurora1 events (e.g. Witt et al., 1976; Arnold, 1980). For the establishment of the “true” quiet night ionosphere EISCAT data can contribute nothing: Beyond zenith angles of 100’ electron densities at all altitudes can be below the instrumental threshold (2x109 m-3). Below 92 km the envelope method can only be applied to the insufficiently small number of rocket data (cf: Figure 2 which includes the lowest rocket profile in the whole dataset). Other methods to establish the quiet d-region have therefore to be investigated.

Fig. 5 Quiet day-time electron densities for x = 50” using different approaches.

The absorption of an extraterrestrial radio source (noise) suffered by traversing the whole of the ionosphere can be measured by a riometer (relative ionospheric opacity meter). The chosen frequency is a compromise between sensitivity and the maximum conceivable plasma frequency which is of the order 20 MHz in order to ensure that the signal will never be totally reflected. Frequencies between 27 and 52 MHz are commonly used to determine additional absorption, i.e. a decrease of signal strength below a regular diurnal variation. The latter function is a composition of a variation

D-Region High Latitudes

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(12

0

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08

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1

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Fig. 6 Electron densities vs. integral absorption for an altitude of about 90 km at night. Dots are EISCAT data, crosses rocket measurements. .__

19

due to the movement of the main radio source in the milky way (sidereal time) and the solar controlled absorption which is tied to the local solar time (Hargreaves, 1969; Cane, 1978). Radio wave absorption for the frequencies considered here is to a very good approximation proportional to the product of electron density and collision frequency; the latter in turn is proportional to neutral pressure (Sen and Wyller, 1960). We now surmise that the riometer absorption in dB (plus a small, initially assumed absorption of the quiet ionosphere) at any height is proportional to the electron density. Figure 6 shows electron densities vs. integral absorption (riometer + a quiet value of 0.006 dB based on a very low density profile) at about 90 km (neutral density 6.8~1019 m-3) of night conditions. The riometer data were either measured at Ramtjordmoen for the EISCAT profiles, or at the respective rocket ranges. All riometers used in this analysis use vertically oriented aerials and the values were converted by an inverse square law to what one should expect for 27.6 MHz. Dots are

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Fig. 7 Night time electron density profiles derived from intersecting RMS lines such as depicted in Fig. 6 at variriometer absorption levels between 0 and 2.5 dB in increments of 0.5 dB. The 0 dB curve is broken off below 95 km. Also indicated on the right of the figure are the number of EISCAT (E) and rocket profiles (R) that entered this particular analysis. Note that below 85 km the main contribution comes from the rocket data. ous

20

M. Friedrich

and S. Kirkwood

EISCAT data, crosses from sounding rockets. Because of the difficulty of correctly determining the riometer’s quiet day curve, absorption values below 0.2 dB are at present not used for the EISCAT data. The line through all data may seem a somewhat daring fit, but its significance will be tested later. The intersections of this line with the riometer absorptions at 0 dB (i.e. 0.006 dB integral absorption) and any other value of integral absorption yields the electron density profiles for these riometer absorption conditions as depicted in Figures 7 and 8. For the convenience of the reader the electron density profiles are here plotted conventionally vs. altitude applying the atmospheric model for equinox. The profiles shown in Figure 7 for night conditions (x > 98”), which are believed to be representative for riometer absorptions between 0 and 2.5 dB, exhibit features similar to many of the individual profiles in Figures 1 and 2. Note that the “extrapolated” quiet curve is significantly larger in the E-region (8x 1010 m-3) than the “true” quiet which apparently is below the EISCAT threshold of 2x I09 m-3. In order to test whether they are of the correct order of magnitude, tiometer absorption was simulated using collision frequencies representative for equinox. The extended magneto-ionic theory (Sen and Wyller, 1960) was employed and the collision frequency set proportional to pressure by a factor of 6.41 xl05 m2 s-1 N-1 as derived by Friedrich and Torkar (1983b) from ionospheric data in agreement with laboratory values collected by Aggarwal and Setty (1980). The resulting integral absorptions (i.e. including 0.006 dB quiet absorption) are 0.02 1 dB for 0 dB riometer absorption, as well as 0.36 and 0.72 dB for 0.5 and I .O dB, respectively. The integral absorption representative for 0 dB riometer absorption should of course be the 0.006 dB and eventual agreement can be expected by repeating the analysis with a somewhat higher guess for the quiet absorption; however, since not all riometer data are yet available in their finally processed form, this iterative process has not been carried out yet. The discrepancy between simulation and associated riometer measurement for larger values will not be influenced by another choice of the quiet absorption value and the fact remains that the simulation is below the measurement. It has to be borne in mind that the simulation was carried out for vertical incidence, whereas the radio source will in general be under a zenith angle larger than zero; an angle of 45” removes the discrepancy between simulation and observation. The situation for daytime is a little more complex, because the quiet absorption varies with solar zenith angle. AS a first guess electron densities from the non-aurora1 model were used to compute simulated riometer absorption. It is assumed that the solar dependence of the quiet integral absorption has the form Lquief = Lo (ChxY where Lo is the

7------

Fig. 8 As Fig. 7, but for daytime conditions. For each riometer absorption value between 0 and 2.5 dB two curves are shown in 0.5 dB increments, one for a solar zenith angle of 60” (full line) and the other for 90” (dashed line). Note that blow 75 km the main contribution comes from the rocket data.

21

/>-Region High Latitudes

subsolar absorption and Chx the Chapman function of the solar zenith angle (Smith and Smith, 1972). The initial values are obtained from the non-aurora1 model as Lo = 0.11 dB and n = 0.45. A correlation similar to the one in Figure 7 was carried out with the daytime profiles (x < 98”), but now the quiet absorption was varied for each individual case depending on the solar zenith angle at the time of the measurement. Figure 8 shows daytime electron densities believed to be representative for riometer absorption between 0 and 2.5 dB. There are two curves for each absorption, one for x = 60”, and the other for 90”. Naturally the difference between the pairs of profiles becomes smaller for larger absorption, i.e. when particle production by far a.04 exceeds solar controlled ionisation. A final O.Of 0.4 test that the derived statistical electron density rion~clcr i~h~orptl~~~ + quwl .tht~~~11~~~1~.,111 profiles are at least representative for the variconditions in Figure 9 Fig. 9 Calculated integral absorption vs. riometer absorption plus ous geophysical riometer absorption (plus the appropriate the appropriate quiet absorption. quiet absorption) is plotted vs. calculated integral absorption using some of the statistical electron density profiles. One can see that the relative departures from the ideal straight line are most pronounced for small absorption, a discrepancy that can certainly in a final analysis be reduced by a better choice of the quiet absorption. CONCLUSIONS

AND OUTLOOK

The huge EISCAT electron density data base could successfully be augmented by about 100 rocket borne measurements because the latter provide values at lower altitudes and densities. The statistical models, separate for day and night, yield simulated absorption values basically consistent with observations. Remaining differences may a)

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Fig. 10 Electron densities vs. solar zenith angle for all solar activity levels (shaded points) and for F 10.7 > 200 Jy (black dots) at about I 10 km.

22

M. Friedrich and S. Kirkwood

be due to the fact that the radio source measured by a riometer is at an angle from zenith, and b) that some absorption events are confined to a small area and thus the volume sampled by EISCAT (or measured by the rocket) may be different from that seen by the riometer (Rosenberg et al., 1991; Stauning, 1996). The large amount of apparently “truly” quiet electron densities also allows to assess the solar activity dependence of the E-region, although this dependence should be even better investigated at non-aurora1 latitudes. Figure 10 shows electron densities vs. solar zenith angle for a neutral density approximately representative for 110 km. The shaded values are from all solar activity levels, the black dots from solar activities F,,, > 200 Jy. There is a clear enhancement of about a factor of 2 over the whole day-light portion of the zenith angle range. The present analysis is but a first attempt to merge two different datasets and not all EISCAT data were yet included primarily because of the preliminary state of the processing of the riometer data. Also it remains to be tested whether a correlation for pressure surfaces yields better results than using constant neutral densities. ACKNOWLEDGEMENTS The contribution to the numerical computations by M. Harrich in pursuance of his MSc. degree is greatly appreciated. The riometer data from Ramfjordmoen and Lavangsdalen were generously made available by P. Stauning of the Danish Meteorological Institute. REFERENCES Aggarwal, K., and C.S.G.K. Setty, Collision & Transport of Electrons in the Ionosphere, Indian J. Radio Space Phys., 9, pp. 105-I 11 (1980). Arnold, F., The Middle Atmosphere Ionized Component, ESA SP-152, pp. 479-496 (1980). Baron, M., and K. Person, EISCAT Technical Note 85-43 (1985). Cane, H.V., A 30 MHz Map of the Whole Sky, Aust. J. Phys., 31, pp. 561-565 (1978). Friedrich, M., and K.M. Torkar, High Latitude Plasma Densities and Their Relation to Riometer Absorption, J. utmos. ferr. Phys., 45 (2/3), pp. 127-I 35 (1983a). Friedrich, M., and K.M. Torkar, Collision Frequencies in the High-Latitude D-Region, J. utmos. terr. Phys., 45 (J), pp. 267-271 (1983a). Friedrich, M., and K.M. Torkar, Typical Behaviour of the High Latitude Lower Ionosphere, Adv. Space Res., 16 (l), pp. 73-81 (1995). Friedrich, M., and K.M. Torkar, Comparison Between an Empirical and a Theoretical Model of the D-Region, Adv. Space Res., 21 (a), pp. 895-904 (1998). Folkestad, K., T. Hagfors, and S. Westerlund, EISCAT: An Updated Description of Technical Characteristics and Operational Capabilities, Radio Sci., 16 (6), pp. 867-879 (1983). Hargreaves, J.K., Auroral Absorption of I-IF Radio Waves in the Ionosphere: A Review of Results from the First Decade of Riometry, Proc. IEEE, 57 (8), pp. 1348-1373 (1969). Isaksen, I.S.A., Diurnal Variation of Atmospheric Constituents in an Oxygen-Hydrogen-Nitrogen-Carbon Atmospheric Model, and the Role of Minor Neutral Constituents of the Lower Ionosphere, Geofys. Publ. 30, pp. t-63 (1973). Kirkwood, Sheila, Modelling of the Undisturbed High-Latitude E-Region, Adv. Space Res., 13 (3), pp. 102-104 (1993). Kirkwood, Sheila, and P.N. Collis, The High Latitude Lower Ionosphere Observed by EISCAT, Adv. Space Res., 11 (lo), pp. 109-112 (1991). Liibken, F.J., and U. von Zahn, Thermal Structure of the Mesopause Region at Polar Latitudes, J. geophys. Res. 96 (Dll), pp. 20,841-20,857 (1991). Jacobsen, T.A., and M. Friedrich, Electron Density Measurements in the Lower D-Region, J. utmos. ten-. Phys., 41 (12), pp. 1195-1200 (1979). Rees, D., J.J. Barnett, and Karin Labitzke, COSPAR International Reference Atmosphere, Adv. Space Res. 10 (12), (1990).

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High Latitudes

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Rosenberg, T.J., D.L. Detrick, D. Venkatesan, and G. van Biel, A Comparative Study of Imaging and Broad-Beam Riometer Measurements: The Effect of Spatial Structure on the Frequency Dependence of Aurora1 Absorption, J. geophys. Res. 96 (AlO), pp. 17,793-17,803(199I). Sen, H.K., and A.A. Wyller, On the Generalization of the Appleton-Hartree Magnetoionic Formulas, J. geophys. Res. 65 (12), pp. 3,931-3,935 (1960). Siskind, D.E., On the Radiative Coupling Between Mesospheric and Thermospheric Nitric Oxide, J. geophys. Res. 99 (Dll), pp. 22,757-22,766 (1994). Smith III, F.L., and C. Smith, Numerical Evaluation of Chapman’s Grazing Incidence Integral ch(X,x), J. geophys. Res,. 77 (19), pp. 3,592-3,597 (1972). Stauning, P., High-Latitude D- and E-Region Investigations Using Imaging Riometer Observations, J. atmos. terr. Phys. 58 (6), pp. 765-783 (1996). Thrane, E.V., Ionospheric Profiles up to I60 km - A Review, in Methods of Measurements and Results of Lower Ionosphere Structure, edited by K. Rawer, pp. 3-2 1, Akademie, Berlin (1974). Witt, Cl., J.E. Dye, and N. Wilhelm, Rocket-Borne Measurements of Scattered Sunlight in the Mesosphere, J. atmos. terr. Phys. 38 (3), pp. 223-238 (1976).