Electron density and energetic particle precipitation observed during the eclipse of 26 February 1979

Journalof Atmosph ericand Terrestrial Physics, Vo!. 45. No.7, pp. 427-436. 1983. Printed in Great Britain.

oo21- 9169{83$3.oo + .00 Pergamon Pres. LI d.

Electron density and energetic particle precipitation observed during the eclipse of 26 February 1979 L. G. SMITH, M . K. McINERNEY and H . D. Voss* Aeronomy Laboratory, Dep artment of Electrical Engineering, 1406 West Green Street, Un iversity of Illinois at Urbana-Champaign, Urbana, IL 61801, U.S.A. Abstract-Electron density profiles and energetic particle fluxes have been determined from two rockets launched, respectively,at the beginningand end oftotalit yduring the solar eclipseof26 Febru ary 1979.These, and one othe r rocket at the same time ofday on 24 February 1979, were launched from a temporary site near Red Lake, Ontario. The electron density profile from 24 Febru ary 1979 shows the electron density to be normal (at I x 105 em - 3) abo ve 110 km, to rocket apogee (130.5 km), Below 110 km the electron density is enhanced, by an order ofmagnitude in theD-region,compared withdata from WallopsIsland at thesamesolar zenith angle (63°). The enhancement is qualitatively explained by the large flux of field-aligned energetic particles (mainly electrons) observed on the same rocket. During totality, on 26 February 1979,the electron density above 110km to rocket apogee (132.6and 132,3km) is reduced by a factor of about three, as seen in other eclipses.Below 110 km, however, the electron density is much greater than observed during previous eclipses, Again this is attributed to the additional ionization due to energetic particles. The part icle flux measured on the 26 Februa ry was an order of magnitudelessthan that on the 24February but showed greater variability,particularl yat the higher energies(100 keY).Afeature of theparticle fluxis th at, forthe two rock ets th a t were separated horizontally by 38km while above the absorb ing region, the variations are uncorrela ted.

I.

INTRODUCTION

The first rocket measurements of the ionospheric Dand E-regions during an eclipse were made at Fort Churchill, Canada, on 20 July 196 3 (SMITH et al., 1965). Subsequently, rocket launches wer e made a t Cassino, Brazil, during the eclipse of 12 November 1966 (for example, SMITH et al., 1968 ; M ECHTLY et al., 1969 ; WEEKS and SMITH, 1971) and at Wallops Island, Virginia, during the eclipse of 7 March 1970 (fo r example, SMITH, 1972 ; ACCARDO et al.; 1972 ; MECHTLY et al., 1972). A va riety ofexperiment al studies ha ve been performed in each eclipse operation including studies of: D-region recombination coefficient s ; ionizing flux of solar Lyrnan-« (both direct a nd scattered) and of solar X-rays ; and electron temperature. It is now clear that the E-region remains in photochemical equilibrium even during totality, because ofthe r esidual flux of solar X-rays. The progress that has been made in recent years in understa nding th e complex chemical and physical processes of the D-region has revived inter est in the ecl ipse phenomena in this region. A further stim ulus is the a va ila bility of cryo-pumped mass sp ectrometers capable of determining the ion composition without breaking u p the cluster ion s of the D- and lo wer Ere gions. This p aper presents electron density and energetic

"Present address : Lockheed Palo Alto Research Labo rat ory, 3170 Po rter Dri ve, Palo Alto, CA 94304, U.S.A.

particle measurements from two rockets launched during the eclipse of 26 February 19 79 and from one other launched two da ys previously. The pre-eclipse launch was intended to characterize the normal ionosphere and a lso to exercise the temporary launch site (50.g c N, 93SW) near Red Lake, Ontario. The rocket (Nike Tomahawk 18.1020) was launched at 1652 UT on 24 February 1979 and reached a maximum altit ude of 130.5 km . The first rocket into the eclipse was N ike Tomahawk 18.102 1, la unched at 1652 UT. Nike Tomahawk 18.1022, was launched 130 s later, at 1654 + 10 s UT. The trajectories o f these two rockets are shown in Fig.L The eclipse circumstances at the position of the rocket are indicated along each trajectory. For times when the rocket was outside totality the value given is the percentage of the solar disc that was visible. Where the rocket is in totality the time since second contact is given. The first ro ck et entered totality a t T + 60 s, a t an alt itude of 61 km on ascent and exited at T + 300 s, an a ltitude of 68.5 km on descent. The second rocke t remained in totalit y from launch until it ex ited a t T + 106 s, at an altitude of 105 km on ascent. At the time w hen the rockets were at the same altitude (16: 56: 08 UT, 112.5 km) they were separated by onl y 38 km. They maintained this horizontal separation throughout most of the flight since their horizontal velocities were nearly equal. This situation makes it possible to examine the count rates for temporal and sp atial vari atio ns.

427

1. G. SMITH, M. K. MCINERNEY and H. D. Voss

428

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~40 20

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55

54

55 56 UNIVERSAL TIME

57

58

59

Fig. 1. Eclipse circumstances at the position of the rocket. Marked along each rocket trajectory is the percentage of the solar disc that is visible and, inside totality, the time (s)since second contact.

A complementary view of the trajectories is shown in Fig. 2. This is a polar plot of the sun-moon distance against the angular position relative to the sun's north point, and gives a picture ofthe rocket's position viewed along the axis of the shadow. It is a remarkable coincidence that the relative motion of the shadow and the first eclipse rocket (18.1021) was such as to keep the rocket in totality for an extended period. 2. INSTRUMENTATION

The general arrangement of the Nike Tomahawk payloads is shown in Fig. 3. The forward section contains the University of Bern mass spectrometer, the TRADAT tone-ranging instrumentation (for trajec-

320"

330. 340· 350" o06

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tory determination) and a telemetry system. The ion mass spectrometer experiment is discussed in EBERHARDT et al. (1980) and in Kopp et al. (1980). The University of Illinois experiments are in the section to the rear ofthe mass spectrometer. They used a second, independent telemetry system. Further to the rear are the recovery system and the firing-and-despin module. Figure 4 shows the arrangement of the Illinois experiments in the payload. Electron density measurements are made by a combination ofa Langmuir probe (SMITH, 1967)and a propagation experiment (MECHTLY et al., 1967). The probe gives the electron density with good height resolution but without sufficient absolute accuracy. Accordingly it is calibrated by the

50"

40'

50'

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26 FEBRUARY 1979 40

70"

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80'

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260' 250

100'

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Fig. 2. The eclipse circumstances here show the rocket position viewed along the axis of the shadow. The rocket altitude (km) is marked along each curve.

Electron density and energetic particle precipitation

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Fig. 3. The general arrangement of the three payloads for the eclipse operation.

propagation experiment. Together they have previously given the electron density with an accuracy of about 5% and a height resolution of about 10 m. The booms, shown folded inside the payload, carry the two probes (connected in parallel to minimizewake effects)and are deployed at 65 km. The probes are held at 4 V until about 120 km on ascent when a sweep is introduced at 2 s intervals to measure electron temperature (ZIMMERMAN and SMITH, 1980). The radio propagation experiment uses differential absorption and Faraday rotation. This method is implemented experimentally by a feedback configuration (KNOEBEL and SKAPERDAS, 1966). The ground station generates CW (clockwise, looking up) and CCW (counter-clockwise) polarized waves at frequen-

429

ciesdiffering by 500 Hz and then transmits these to the rocket. At the rocket a receiver detects the two signals and telemetersthe output to a ground telemetry station. Here the receiver output is transmitted (via telephone lines) back to the ground station where it is used to control the transmitted signal levels. This feedback system provides simultaneous measurement of differential absorption and Faraday rotation. The frequencies used on each payload are 2.225 and 5.040 MHz. The antennas and receivers for the propagation experiment are located in the section labeled Illinois Receivingin Fig. 3. Also shown in Fig. 4 are the two solid-state particle detectors used in the energetic particle experiment (Voss and SMITH, 1974). These are mounted on the booms. A geometrical factor of 0.05 cm 2 sr is used, providing a look angle ofabout ± 10°.With the booms deployed the axis ofthe detectors is 25° from the rocket spin axis. Data regarding the particles are grouped by three energy thresholds. These thresholds vary slightly from payload to payload, but are nominally: > 40, > 70 and > 120 keY. Although included in the payload as a precaution in the unlikely event of a high particle flux during the eclipse, the energetic particle experiments have become very important in subsequent analysis. Instrumentation is also carried for the measurement ofsolar X-rays (0.2-0.8nm) and of Lyman-a (121.6nm). The Lyman-« experiment measures both direct and scattered solar radiation and, by the use of the two ion chambers, gives the solar aspect angle during totality. The Lyman-a experiment was successful (BLISS and SMITH, 1980) but no useful X-ray data were obtained because of the high flux of energetic particles. Finally, also included in each payload is a solar sensor. This provides confirmation of the position of the rocket with respect to the umbra which is established primarily bycalculation from the trajectory data. 3. OBSERVATIONS

3.1. Probe experiment Figure 5 shows the probe current profile for the launch two days prior to the eclipse: Nike Tomahawk 18.1020 on 24 February 1979.This profile is presented to indicate certain events common to all three probe profiles.Events to be noted are: boom deployment at 65 km; the initiation of the sweep at 115km, which, when removed, results in the interrupted trace; and the decrease of current by a factor of 1.5 at 75 km on descent. This decrease occurs when the payload separates from the Tomahawk motor at the start ofthe recovery sequence.

L. G. SMITH, M. K. McINERNEY and H. D. Voss

430

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Fig. 4. Arrangement of the University of Illinois experiments in the payload.

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PROBE CURRENT (A)

Fig. 5. Profile of probe current from Nike Tomahawk 18.1020, launched at 1052 LST (1652 UT) on 24 February 1979.

3.2. Propagation experiment

All three flights indicate a high electron density. In fact, because of the very high absorption of the extraordinary wave the differential absorptionexperimentcanonlybe used until 67,73 and 78km(on ascent) for 18.1020,18.1021 and 18.1022,respectively. The high density degrades the performance of the Faraday rotation experiment but favors the differential absorption experiment. A plot of differential absorption for 18.1020is given in Fig. 6. Differential absorption of the 5 MHz frequency is given in decibels (extraordinary relative to ordinary) and the time from launch is in seconds. The altitude in kilometers is also indicated in the figure. Similar plots are found for both 2 and 5 MHz frequencies on all three rockets. The high absorption level (indicating a high electron density) at low altitudes is evident in this plot. Also notable is the negative differential absorption between 53and 60 s (52-60 km).During this period there ismore absorption of the ordinary wave than of the extraordinary wave. This phenomenon has also been observed at the magnetic equator (SOMAYAJULU et al., 1971) but has not yet been explained.

431

Electron density and energetic particle precipitation ALTITUDE (krn) 742

;n

6

6

5

44

46

48

50

52

54

56

58

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62

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NIKE TOMAHAWK 18.1020 FREQUENCY: 5.040 MHz

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Fig. 6.Differential absorption at 5.040MHz from Nike Tomahawk 18.1020,launched at 1052 LST (1652 UT) on 24 February 1979.

The electron density was calculated from the experiment (contamination of the extraordinary mode differential absorption rate. Computed electron density by the ordinary mode) and is not real. values for Nike Tomahawk 18.1020are shown in Fig. 7 Also notable is the dip in electron density below 60 for the two frequencies. Also shown are computed km. Recall that this is the altitude range over which electron density values for 18.1022in Fig. 8. Electron negative differentialabsorption occurred (Fig. 6). Since density values are derived from the differential negative differential absorption is not consistent with absorption data by a method using the Sen-Wyller magnetoionic theory, the electron densities are not equations of generalized magnetoionic theory reliable at these low altitudes. (GINTHER and SMITH, 1975). An electron collision frequency model vm = 6.3 X 105p (SMITH et al., 1978) 3.3. Electron density profiles was assumed. The atmospheric pressure p (inN m- 2) is The probe current calibrations (Nil) and the time taken from COSPAR International Reference periods over which they were used are given in Table 1. Atmosphere (CIRA) 1972. For each flight the first value is used until payload The decrease in electron density at the highest separation and the second value after separation (see altitudes (evident in Fig. 8) is an artifact of the Fig. 5). Although the calibration factor is established 100

100

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Fig. 7. Electron density calculated from the differential absorption rate measured at 2.225 MHz and 5.040 MHz, from Nike Tomahawk 18.1020, launched at 1052 LST (1652 UT) on 24 February 1979.

50 100

10'

10' ELECTRON

10' DENSITY

10'

10'

(em')

Fig. 8. Electron density calculated from the differential absorption rate measured at 2.225and 5.040 MHz, from Nike Tomahawk 18.1022,launched at 1054: 10 LST (1654: 10 UT) on 26 February 1979.

L. G. SMITH, M. K. McINERNEY and H. D. Voss

432

Table 1. Probe current calibrations usedto determineelectron density profiles for Nike Tomahawks 18.1020, 18.1021 and

160

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Fig. 10. Electron density profile from Nike Tomahawk 18.1021, launched at 1654: 10 UT on 26 February 1979, duringtheeclipse. Ascent:solidcurve; descent:dashedcurve. (18.1021) but the difference between the ascending and descending electron density profiles is not as great. 3.4. Energetic particles

The two solid-state particle detectors used in the energetic particle experiment are identical except for the thickness of the aluminum surface. A detector with a

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over a limited height range in these flights the values are in agreement with the factors established in similar flights at Wallops Island and are expected to be accurate to 10%. The electron density profiles for Nike Tomahawks 18.1020, 18.1021 and 18.1022 are shown in Figs 9-11, respectively. It can be noted in the profile for Nike Tomahawk 18.1020, the pre-eclipse launch, that the electron density is essentially constant at 10 5 em - 3 from 85 km upwards. For Nike Tomahawk 18.1021 the most prominent feature of the electron density profile is the difference in electron density between ascent and descent. Comparison with the particle data (Section 3.4) shows this difference is related to different ionization rates. For Nike Tomahawk 18.1022 the electron densities are comparable to the previous launch in totality

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Fig. 9. Electron density profile from Nike Tomahawk 18.1020, launched at 1652 UT on 24 February 1979. Ascent: solid curve; descent: dashed curve.

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Fig. 11. Electron density profile from Nike Tomahawk 18.1022, launched at 1654: 10 UT on 26 February 1979, duringthe eclipse. Ascent:solidcurve; descent:dashedcurve.

433

Electron density and energetic particle precipitation

40 flg em -2 aluminum surface is used during rocket ascent. For descent a detector with a 100 flg cm- 2 aluminum surface is used. The ratio of the fluxes in the two detectors allows particle identification (FRIES et al., 1979). Figure 13 shows the data for Nike Tomahawk 18.1021.Apogee occurred at approximately 180 s after launch. At that time the experiment was switched to the

detector with the thicker metal layer. If many particles heavier than electrons (e.g.H, He, O) were present there would have been a significant change in the count rate, which was not observed. Count rates for energetic particles vs time are given in Figs 12-14 for 18.1020, 18.1021 and 18.1022, respectively, The particles consist mainly of electrons. All three flights show high count rates, indicating an

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TIME AFTER LAUNCH (s)

Fig. 12. Count rates for energetic particles from Nike Tomahawk 18.1020, launched at 1652'UT on 24 February 1979. NIKE TOMAHAWK /8.1021

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Fig. 13. Count rates for energetic particles from Nike Tomahawk 18.1021, launched at 1652 UT on 26 February 1979,during the eclipse. NIKE 1400 T-

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Fig. 14. Count rates for energetic particles from Nike Tomahawk 18.1022,launched at 1654: 10 UT on 26 February 1979,during the eclipse.

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auroral event on both days. The eclipse data, however, show unusually large fluctuations in the count rate , indicating a pulsating aurora (SMITH et al., 1980). An interesting feature of Figs 13 and 14 is that the fluctuations are more pronounced at the higher energies. The count rates for 18.1021 and 18.1022 have been examined for correlations. Taking simultaneous values at the two rockets, separated horizontally by about 38 ken, the count rates are uncorrelated. Taking a time difference of 130 s (the time interval between the two launches) also shows the count rates to be uncorrelated. This indicates that, on scales of the order of 1kmand 1s, the flux is variable in both space and time.

and H. D. Voss

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4. DISCUSSION

ELECTRON DENSITY (em-'J

The electron density profile from the pre-eclipse day is shown again in Fig. 15. This time together with a previous full-sun profile, from Nike Apache 14.435 launched at Wallops Island for the March 1970eclipse. The electron density is normal above 110 km. Below 110 km the electron density is enhanced by an order of magnitude in the D-region. This enhancement may be qualitatively explained by the large flux of electrons observed on the same rocket. During totality, on 26 February, the electron density above 110 krn (Fig. 16) is reduced by a factor of about three as expected for the photochemical equilibrium model. Below 110 km, however, the electron density is much greater than that observed during previous eclipses(Nike Apache 14.436, Wallops Island , 1970,for example). Again this can be attributed to the additional ionization due to energetic particles. Figure 17 shows the particle fluxes measured on the

Fig. 16. Electron density profiles for three rockets launched during eclipse totality .

24 and 26 February vs altitude. The flux on the 26 February is much less than that measured on the 24 February at altitudes below 80 km. The particle flux on the 26 February does show a greater variability above 80 krn, particularly at the higher energies (> 120 ke'V), 14 0

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Fig. 15. Electron density profiles for two pre-eclipse rockets.

Fig. 17. Count rates for energetic particle ( > 40 keY) sensors for Nike Tomahawk 18.1020 (pre-eclipse) and Nike Tomahawk 18.1022(eclipse totality).

Electron density and energeticparticle precipitation

435

(18.1020) is significantly greater than those for the two eclipse rockets (18.1021 and 18.1022). This is consistent with the high count rate observed below 80 km on the 24 February (Fig. 17: higher energy particles penetrate

~ 10~ a:: >-

z

8u AVERAGE COUNT RATE FROM T+ 120. TO H169. 40

60

80

100

120

THRESHOLD ENERGY (keV)

Fig. 18. Averagecount rates plotted against threshold energy from T + 120 s to T + 169 s UT (110 km to apogee) for the three rockets of the 1979 eclipse operation.

The average count rate from 120 s after launch to 169 s (110-130 km) for the three rockets is shown in Fig. 18. It can be noted that the average count rates for a threshold energy of 40 ke V are nearly equal for all three flights. For threshold energies of 70 and 120 keY, however, the average count rate on the 24 February

deeper into the D-region). The high flux of energetic particles during the eclipse masked the eclipse effects in the D-region ; however, the lower E-region (above 110 km) did show a decrease in electron density. The occurrence of the particle event during the eclipse, which initially appeared an unfortunate coincidence has, it now appears, given particularly valuable data both for the particles as an ionization source and for the chemical processes and possibly even for the dynamical effects in the V-region. Acknowledgements-1. J. JOHNSON was payload engineer for the Aeronomy Laboratory, University of Illinois and D. F. DETWILER, JR.was payload manager for the Sounding Rocket Branch, NASA/Wallops Flight Center. The authors benefited from participation in eclipse workshops organized by W. W. BERNING and held in 1979 in Las Cruces, New Mexico, and in 1981 in Bedford, Massachusetts. This research has been supported by the National Aeronautics and Space Administration under Grant NGR 14-005-181.

REFERENCES

ACCARDO C. A.. SMITH L. G. and PINTAL G. A. EBERHARDT P., HERRMAN U. and Kopp E. KNOEBEL H. W. and SKAPERDAS D. O. Kopi' E, ANDRE 1., EBERHARDT P. and HERRMANN U. MUCHTLY E. A., BawHILL S. A., SMITH 1. G. and KNOEBEL H. W. MECHTLY E. A., RAG M. M., SKAI'ERDAS D. O. and SMITH 1. G. MECHTLY E. A.. SECHRIST C. F. and SMITH 1. G. SMlTH1.G.

1972 1980 1966 1980 1967

J. atmos. terr. Phys. 34,613. EOS Trans. AGU 61, 311. Rev. scient. Instrllm. 37, 1395. EOS Trans. AGU 61,311. J. geophys. Res. 72, 5239.

1969

Radio Sci. 4,517.

1972 1967

J. atmos. terr. Phys. 34, 641.

SMITH 1. G. SMITH 1. G., ACCARDO C. A., WEEKS 1. H. and McKINNON P. J. SMITH 1. G., WEEKS 1. H. and McKINNON P. J. SMITH 1. G., WALTON E. K. and MECHTLY E. A. SMITH M. J., BRYANT D. A. and EDWARDS T. SOMAYAJULU Y. V., AVADHANULU M. B.,ZALI'URI K. S. and GARG S. C. WEEKS L. H. and SMITH L. G.

1972 1965

J. atmos. terr. Pnys. 34,601.

J. atmos. terr. Phys. 27, 803.

1968 1978 1980 1971

J. atmos. terr. Phys. 30, 1301. J. atmos. terr. Phys. 40, 1185. J. atmos. terr. Phys. 42, 167. Space Res. XI, 1131.

1971

Solar Phys. 20, 59.

BLISS H. M. and SMITH 1. G.

1980

FRIES K. 1., SMITH 1. G. and Voss H. D.

1979

GINTHER J. C. and SMITH L. G.

1975

Rocket observations of solar radiation during the eclipse of 26 February 1979, Aeron, Report No. 93, Aeron, Lab., Dept. E1ec. Engng, University of Il1inois, Urbana-Champaign. A rocket-borne energy spectrometer using multiple solid-state detectors for particle identification, Aeron. Report No. 91, Aeron, Lab., Dept. Elec. Engng, University of Illinois, Urbana-Champaign. Studies ofthe differentialabsorption rocket experiment, Aeron.Report No. 64, Aeron.Lab., Dept. Elec, Engng, University of Illinois, Urbana-Champaign.

Electron Density and Temperattlre Measurements in the Ionosphere (Edited by K. MAEDA), COSPAR

Technique Manual Series.

Reference is also made to the following unpublished material:

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L . G . SMITII, M . K. McINERNEY and H. D. Voss

VossH. D. and SMITH L. G.

ZIMMERMAN

R. K.

JR.

and

SMITH

1974

L. G.

1980

Design and calibration of a rocket-borne electron spectrometer for investigation of particle ionization in the n ighttime rnidJatitude E-region, Aeron. Report No. 62, Aeron. Lab ., Dept. Elec . Engng, University of Illino is, Urbana-Champaign. Rocket measurements of electron temperature in the Eregion, Aeron . Report No. 92, Aeron. Lab., Dept. Elec, Engng, University of Illinoi s Urbana-Champaign.