A comparison of ionization densities determined from spacecraft and incoherent scatter radar data

Journal of Atmospheric and Terrestrial Physics,Vol. 58, No. 15, pp. 1735-1740, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0021-9169(95)00200-6 00214169/96 $15.00 + 0.o0

~ ) Pergamon

A comparison of ionization densities determined from spacecraft and incoherent scatter radar data N. J. Miller, ~ J. M. Grebowsky, 1 W. R. Hoegy I and K. K. Mahajan 2 ~Laboratory for Atmospheres, NASA Goddard Space Flight Center, Greenbelt, MD 20771, U.S.A.; 2National Physical Laboratory, New Delhi, India 110012 (Received 27 October 1995; accepted in revised form 3 November 1995)

Abstract--By

comparing direct measurements taken from onboard Atmosphere Explorer spacecraft (AE), in eccentric orbit, with incoherent scatter radar (ISR) measurements taken from the ground, we illustrate both the merits and the difficulties involved in such comparisons. Five altitude profiles of ionization determined from AE, in near coincidence with ground stations making ISR measurements, compared favorably with the ISR data so long as the AE measurements were properly analyzed for the effects of variations in latitude and solar zenith angle along the spacecraft orbit. Copyright © 1996 Elsevier Science Ltd

INTRODUCTION Spacecraft measurements have been confirmed via comparisons among data taken by onboard instruments that measured similar parameters, via comparisons among data taken from separate spacecraft orbiting at the same time, and via comparisons with ground based measurements when there was nearcoincidence between the locus of the ground station measurements and the nearby transit of the spacecraft (Donley et al., 1969; Benson, 1973; GonzMez et al., 1992; Hoegy and Benson, 1988; Benson et al., 1977). Reduced payloads of future spacecraft probably preclude planned redundancies in measurements by onboard instruments and reduced numbers of missions decrease opportunities for intraspacecraft data comparisons, so the most frequent data comparisons will be between space based and ground based measurements. One such comparison is that between thermal ionization measured from spacecraft and from the ground. We surveyed direct measurements taken from Atmosphere Explorer spacecraft (AEC and AEE), in eccentric orbit, and found five instances of near-coincidence with ground based incoherent scatter radar (ISR) remote measurements of dayside ionization. A comparison of the data sets illustrates the merits and difficulties inherent in such comparisons and confirms a general consistency among the related spacecraft measurements. In the case of AEE, the comparison also confirms that ion composition measurements by the onboard ion mass spectrometer yield about half the value of ion densities

determined by other methods. No such difficulties appeared in the ion composition measurements from AEC. Comparisons between ground based ISR and spacecraft measurements of ionization parameters have been performed for electron and ion temperatures (T~ and Ti) (Evans, 1965; Hanson et al., 1969; Carlson and Sayers, 1970; Taylor and Wrenn, 1970; McClure and Troy, 1971; McClure et al., 1973; Wrenn et al., 1973). Benson et al. (1977) conducted the most comprehensive comparison between ground based ISR data and To and T~ measured from a spacecraft by comparing temperatures determined from ISR remote measurements above a ground station, to AEC direct measurements taken at the nearest approach to the ground station and at nearly the same time as the ISR measurements. However, data comparisons similar to those cited for temperatures have not been made for ionization density or Ne, although there have been comparisons between ionization profiles determined by spacecraft borne topside sounders and ground based ISR (Jackson, 1969; Muldrew and Vickery, 1982). In those cases all of the measurements were remote. In our comparison between AE direct measurements of dayside ionization density and ground based ISR data, the AE data used are limited to that taken within a region ~ 6 ° in latitude by 20 ° in longitude, approximately centered over ground station radar locations. By using all AE data taken within that region we obtain an ionization altitude profile that

1735

N. J. Miller et al.

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can be compared with remote ISR measurements taken at about the same time. OBSERVATIONS

During 1974, AEC was in an eccentric orbit inclined at 67 °. Figure 1 shows a set of ionization measurements taken from AEC after noon in 1974 near St. Santin (45°N, 2°E). The available AEC total ion/N~ measurements, taken by the onboard Bennett ion

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Fig. 1. Ionization density and Te at St. Santin measured by ISR at the ground (solid line) and by CEP and BIMS from AEC (data points) on 22 January, 1974. Only CEP measured T, from AEC. No RPA data were available. Information in the legend box refers to AEC. Universal times of measurements are associated with data by arrows in the figure,where time is in seconds. Solid arrowheads point to ISR data and open arrowheads point to AEC data. Note that the ISR scan occurred 48 minutes before the spacecraft passage. Also the times associated with the ISR data represent a longer period than the AEC data because the St. Santin radar was the continuous wave type that made measurements at different heights by moving the antenna. The nearest approach of AEC to the ground station coordinates was approximately 0.1° in latitude and 2.2° in longitude.

mass spectrometer (BIMS) (Brinton et al., 1973) and the Langmuir probe (CEP) (Brace et al., 1973), were consistent with each other and with measurements taken by the ISR based at St. Santin. The lower portion of Fig. 1 displays CEP To data which is shown in close agreement with the ISR measurements. The CEP data point (identified by arrow) at the nearest approach to St. Santin are in exact agreement with ISR data but there is a divergence between CEP and ISR Te measurements at lower altitudes. A similar low altitude divergence develops between the BIMS and CEP measurements (upper figure) because the ionization densities determined from CEP data are interpreted from the collected ion current assuming effects of molecular ions are negligible, a questionable assumption below 200 km where molecular ions become dominant. The following two data sets (shown in Fig. 2), taken after noon near St. Santin, do not agree as well as the data in Fig. 1. Although the AEC data agree among themselves to within 20%, the AEC data diverge from the ISR data near NmF2. The AEC data include measurements by the retarding potential analyzer (RPA) (Hanson et al., 1973) that had no measurements for the period shown in Fig. 1. The close agreement among AEC data attests to its accuracy, and the close agreement between the ISR profiles, taken an hour apart, illustrates the stability of the dayside ionosphere near midday. To values, associated with the data in Fig. 2, are compared in Fig. 3. The differences between CEP and ISR To data are dramatic. Figure 4, containing the latitudes at which T¢ was determined by CEP, provides an explanation for some of the differences between AEC and ISR data in Figs 2 and 3. As AEC descended equatorward on the January orbit, the measured Te decreased, an altitude variation to be expected at altitudes below 200 km. Altitude effects, rather than latitude effects, dominated the Te variations. On the April orbit, Te increased as AEC descended, contrary to the expected altitude trend in Te. In this case, poleward orbiting of AE into a warmer plasma environment caused latitude effects, rather than altitude effects, to dominate the Te variations. Apparently, for the January case, the latitudinal trend in Te was not strong enough to make a significant difference between the AEC and ISR determinations of T~ and Ne.

The latitudinal trend in Te displayed within the specific data sets in Fig. 4 is consistent with the general trend of poleward increases in Te contained in the model calculations of Brace and Theis (1981). In that study AEC circular orbit data were used to develop empirical models of Te vs invariant latitude and uni-

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versal time for equinox at altitudes 300 and 400 km. During 1976, A E E was in eccentric orbit inclined at 20 °. Figure 5 displays A E E measurements of ionization density taken near Arecibo (18°N, 292°E) during 1976. The A E E measurements do not agree among themselves, though the CEP and R P A data are close. Discrepancies among A E E ionization density measurements have already been identified by other researchers (Oppenheimer et al., 1981; Breig et al., 1985; Gonzfilez et al., 1992) who found BIMS values for ionization densities to be lower than required to be consistent with calculations and with R P A measurements. Breig et al. (1985) assumed that the mix of ion species should affect the interpretation of the BIMS measurements whereas Gonz~dez et al.

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CEP ELECTRONTEMPERATURE,K Fig. 4. Latitudes at which the AEC measurements in Figs 13 were taken. Altitudes associated with some data points are included near the data. Note the agreement among the data points shown for April, even though they were taken on consecutive days. AEC was descending during each set of measurements but, in January, AEC orbited equatorward whereas, in April, AEC orbited poleward.

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N. J. Miller et al. A R E C I B O RADAR 18.3°N, 292.2°E Five minute ISR altitude scan begins at 4 5 4 6 8 s UT, 34 minutes after A E E overflight i i i i i I ~ iF i i i i i i i

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IONIZATIONDENSITY,cm3 Fig. 5. Ionization densities at Arecibo measured by ISR from the ground (curves) and by BIMS, CEP, and RPA from AEE (data points) on 13 April and 13 May, 1976. Information in the legend box refers to AEE. The Arecibo radar is the pulsed type that can scan a range of altitudes simultaneously. Consequently, the ISR scans 120-290 km in 5 minutes or less. Solid arrowheads point to ISR data and have a local time listed because the equatorial orbit of AEE results in a changing local time along the orbit; open arrowheads point to AEE data, and have a universal time listed in seconds for the associated data point. The nearest AEE measurements to the ground station were within 0.4 ° latitude, 0.8 ° longitude, 1.1 hours LT (upper plot) and 0.2 ° latitude, 0.4 ° longitude, 0.1 hours LT (lower plot). The dashed curves represent ISR measurements made 24 minutes after (top) and 28 minutes before (bottom) the measurements represented by the solid curves.

determinations are low, the densities follow the same trend as the o t h e r A E E measurements, all of which deviate from the ISR m e a s u r e m e n t s in some regions. In this case the m e a s u r e m e n t s in Fig. 5 were taken during m o r n i n g (top) a n d evening ( b o t t o m ) w h e n the p r o d u c t i o n o f ionization by solar radiation is very sensitive to solar zenith angle. The differences between the I S R altitude profiles o f ionization determined 30 minutes a p a r t d e m o n s t r a t e the increased variability in the m o r n i n g a n d evening ionosphere relative to the m i d d a y ionosphere t h a t was characterized in the plots o f St. Santin ISR data. In the m o r n i n g measurements, A E E data display a significant disagreement with ISR data at the u p p e r altitudes, a n d in the evening measurements, A E E data display a n a b r u p t decrease at the lowest altitudes. W h e n the A E E data are plotted against solar zenith angle as in Fig. 6, it is clear t h a t on orbital segments where A E E has m o v e d into areas with the highest solar zenith angles, Ne m e a s u r e m e n t s are low comp a r e d to I S R data. Those locations are near the term i n a t o r s where insolation sharply decreased with only small m o v e m e n t s away from the g r o u n d station. This example emphasizes the i m p o r t a n c e o f m a k i n g such data c o m p a r i s o n s near m i d d a y w h e n the spacecraft m e a s u r e m e n t s are t a k e n well away from the terminator.

CONCLUSIONS The data presented d e m o n s t r a t e t h a t m e a s u r e m e n t s of ionization densities t a k e n f r o m spacecraft in eccentric orbits can be confirmed by m e a s u r e m e n t s from

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(1992) assumed that a n overall scaling factor of 2.15 would correct the data. The data in Fig. 5 confirm t h a t ionization densities determined from B I M S m e a s u r e m e n t s t a k e n from A E E are ~ 0 . 5 of the ionization densities determined from CEP, R P A , or ISR and, above 200 km, B I M S m e a s u r e m e n t s are 0.5 R P A m e a s u r e m e n t s with C E P m e a s u r e m e n t s approximately halfway between those by B I M S a n d RPA. T h o u g h the absolute values for the B I M S ionization

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A comparison of ionization densities determined from spacecraft and incoherent scatter radar data g r o u n d stations over a range o f altitudes w h e n p r o p e r a c c o u n t is t a k e n o f the changes in latitude a n d solar zenith angle along the spacecraft orbit. The m o s t reliable c o m p a r i s o n s are those m a d e near m i d d a y at midlatitudes where the ionosphere is m o s t stable a n d where the p r o d u c t i o n rate o f ionization by solar radiation is least affected by changes in solar zenith angle along the spacecraft orbit. I n t r a c o m p a r i s o n of the A E C d a t a showed ~Lhat BIMS, CEP, a n d R P A closely agreed in their m e a s u r e d ionization density values. However, o n A E E , ionization densities determined

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f r o m B I M S m e a s u r e m e n t s were ~ 50% of the C E P a n d R P A m e a s u r e m e n t s below 200 km. A b o v e 200 k m B I M S m e a s u r e m e n t s were ~ 5 0 % o f R P A m e a s u r e m e n t s a n d C E P m e a s u r e m e n t s were ~ 7 5 % of R P A measurements. Acknowledgements--We are grateful to Arecibo observatory and the CEDAR program for providing incoherent scatter data. We thank a referee for detailed comments which gave us better insight into how the incoherent scatter facilities operate. Also KKM is grateful to R. Kohli for his assistance with this study.

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