Simultaneous optical and incoherent scatter observations of two low-latitude auroras

Plmut. Spce &I.. Vol. 24. pp. 425 to 442. Pcraamon Pms. 1976. PrInted in Northern Ireland

Sr~ULTANEOUS OPTICAL AND INCOHERENT SCATTER OBSERVATIONS OF TWO LOW-LATITUDE AURORAS Blue Hill Observatory,

J. P. NOXON* Harvard University, Cambridge,

J. V. EVANS Lincoln Laboratory, MIT, Lexington, (deceives

MA, U.S.A.

MA, U.S.A.

6 October 1975)

Abstract-During the Iast sunspot maximum, major auroras occurred over Boston, MA (L = 3.1) on 23/24 March 1969 and 8/9 March 1970, during which optical and incoherent scatter radar measurements were made simultaneously from the Blue Hill Observatory and Millstone Field Station, respectively. The paper presents the results of these measurements and attempts a self-consistent interpretation of them. It is found that a major increase (5-10 times) in the abundance of molecular species (0, and Np) at P-region heights must have occurred; this was not accompanied by any appreciable increase in the major neutral species, atomic oxygen. From the radar data, the energy input to the F-region can be separated into direct heating by secondary electrons, downward heat flow from the magnetosphere and a low-energy non-ionizing electron flux. The total observed intensity of the pr~minant optical emission from atomic oxygen at 6300 A can then be predicted satisfactorily by summing the con~butio~ to this emission from impact excitation by ambient electrons, from the low-energy pr~ipitating electrons and from dissociative r~mbination. 1. INTRODUCTION On 23-24 March 1969, and again on 8-9 March 1970, major auroras occurred over Boston, MA

(L = 3.1) providing the first opportunity for simultaneous study using both incoherent scatter radar The former were and optical measurements. accomplished at the Millstone Hill Field Station and the latter at the Blue Hill Observatory 50 km to the south. Possibly owing to the relatively low geomagnetic latitude of Boston these auroras were found to possess unusual features, in that large amounts of energy were deposited at high altitudes in the ®ion resulting in the dominance of OI(6300 A) radiation. The purpose of this paper is to present the results of the observations made during these two low latitude auroras by the two techniques and show how, in comb~ation, their analysis permits a comprehensive description of the energy balance in both the ionised and neutral components above about 200 km. The major points which emerge are (1) the observed intensity of the predominant optical emission, from atomic oxygen at 6300 A, can be predicted from an analysis of the radar measurements; (2) major changes in the neutral composition in the thermosphere seem to have occurred; and (3) the energy input to the F-region can be separated into several components, namely, * Present address Aeronomy Enviromnemal 80302, U.S.A.

Research

Laboratory,

Laboratories,

NOAA, Boulder, CO

425

direct heating by both ionizing and a low-energy non-ionizing electron flux together with heat conducted downward from the magnetosphere via the ambient plasma. Section 2 of the paper provides a brief description of the data collection and reduction procedures employed at the two observatories. The observational results are presented in Section 3, and the measurements during the aurorafly active period of 8-9 March 1970, are discussed in Section 4, where the evidence is presented for the existence of significant changes in the neutral composition. The production of 6300 A emission on this night is examined in Section 5, while Section 6 discusses the observations gathered immediately following the cessation of aurora1 activity. The results for 23-24 March 1969, are briefly discussed in Section 7 while the last section gives a summary and the conclusions. 2. DATA COLLECTION

AND REDUCTION

2.1 Photometric The optical measurements were performed with the filter photometer described by Noxon and Johanson (1970); they were limited to observations of the absolute intensity of the two oxygen lines at 5577 A and 6300 A at various zenith angles in the N-S meridian plane. Under relatively quiet conditions the red line was sampled every 2.5 min and the green line once every 12.5 min. With brighter and more active aurora the sampling rate was usually increased five or tenfold.

426

J. F. NOXON

and J. V. EVANS

Absolute calibration was accomplished by means of a NBS standard lamp and careful measurement of the filter tr~smission as a function of wavelength; the accuracy is about ilO% in absolute intensity. We have not applied any correction for atmospheric extinction. Most of the measurements we discuss were obtained in the zenith; from numerous solar observations we have concluded that the extinction at 6300 A on a ciear night is less than 15 OX,in this direction. To intersect a point over Millstone at 400 km it is only necessary to look about 9’ away from the zenith at Blue Hill. Visual observations indicated that the degree of homogeneity even in bright active forms was such that no significant error could be introduced by assuming zenith observations at Blue Hill to correspond to what would have been seen above Millstone.

Milktone Hill 23-24 Mor 1969

Measurements

i\ :\ \ \\ \._r

21 II

2.2 Radar The measurements of electron density electron and ion tern~~t~ were made at the Millstone Hill Observatory (71.S”W, 42.6%) @vans and Loewenthal, 1964; Evans, 1965). The manner in which the radar is operated to make F-region measurements has been changed from time to time in an effort to improve the accuracy and time resolution. The m~s~ements made during the two events (March 1969, March 1970) were carried out in a manner that explored the density variation over the height interval approx. I%-900km and the temperature variation over the range 22%8OOkm, the overall time resolution being 30 min. These data were collected and analyzed in the matmer described by Evans et af. (1970). For the measurements of electron density carried out at Millstone Hill the radar is employed only to obtain the shape of the profile and an absolute scale is assigned by measuring foF2 separately on an During very disturbed geoma~eti~ ionosonde. conditions this method runs into serious difficulties as then (at Millstone) foF2 becomes extremely low and is usually unobservable at night owing to the presence of D-region absorption and/or blanketing sporadic E. In the more intense events the behavior of foF2 at Millstone is unlikely to be the same as at nearby stations (Ottawa or Wallops) and in any case these stations also appear to have difficulty in obtaining useful records. In such instances, we are obliged to interpolate between values of foF2 measured on the local ionosonde before and after the event using the observed variation of the incoherent scatter echo power (allowing of course for

I IO

I IO

’

20

’ 21

22

’ 23

’ 24

’ 01

’

I

02

0s

’ 00

OS

EST i?Ii3.

1.

1969,

VARIATION DEDUCED

OF fOF;? FROM

THE

WITH ITME FOR INCOHZRENT

23124 MAaCE%

SCATIXR

RADAR

MEASUREMENTSOFTOlXLPOWER.

variations in T,/T,). Figure 1 shows a plot of foF2 obtained in this way. Besides the problem of obtaining j&F2 during these auroral events the incoherent scatter measurements are beset by the difficulty of making useful density and temperature measurements with the very low densities and correspondingly weak echoes that exist. In addition, there have been instances where aurora1 echoes seen in the sidelobes of the antenna have prevented useful measurements at certain ranges. It suffices to say that the uncertainties associated with the measurements made in aurora1 events tend to be considerably larger than normal and may reach as much as f25 % in the case of the temperature measurements. We have attempted to reduce the influence of these random errors of meas~ement on the results by constructing smooth profiles of I?,, T, and Ti vs height and using these to construct contour diagrams of these quantities vs height and time. Thus, the largest uncertainty remaining is that associated with the absolute density scale, i.e. the accuracy of the curve drawn through the points in Fig. 1, It seems possible that this curve might be in error by as much as +lO% and hence introduce errors as large as &20% in the results for N,. It is important to emphasize that the values shown for T, and T, were derived on the assumption that the mean ionic mass was 16. Normally this is the case, but we note below that, during the active

Scatter observations of low-latitude auroras

427

following 21:OO EST (02:OO UT). A major change in the character of the aurora took place just prior to 21:OO in coincidence with a large oscillation in the Earth’s field. Until this time, the aurora was of moderate intensity and the dominance of 5577 A over 6300 A indicated that the luminosity was 3. OBSERVATIONAL RESULTS predominately the result of electrons with energies of several keV capable of penetrating to E-region 3.1 23124 March 1969 altitudes. At such altitudes, the red Iine is suppressed The zenith intensity of both red and green lines owing to the effects of collisional quenching by N, throughout the night, together with the N-S hori- (see Rees and Walker, 1967; Rees, 1969). At zontal component of the Earth’s field as recorded at 21:00 EST a nearly one hundred-fold-increase in the Blue Hill Observatory are presented in Fig. 2. 6300 A brightness occurred together with a reversal in the red-green intensity ratio, which persisted for the remainder of the night. This dominance of the red line evidently reflected a softening of the incident electron flux and a major increase in the relative amount of energy being deposited at high altitude. (The dominance of 6300 8, over 5577 A just after sunset is primarily due to twilight enhancement of the red line, Noxon and Johanson, 1972; Schaeffer and Noxon, 1975.) At its brightest the red Iine exceeded 200 kR and a brilliant red corona was observed in the magnetic zenith at Blue Hill. As Fig. 3 shows, the aurora1 display covered the entire sky, extending to at least 70’ zenith distance in the south. Figure 3 aho shows that the two lines often varied quite differendy with time in~cating a EST considerable degree of independence on the part of Fro. 2. ZENITH INTENSITY OF TIIII63OOA AND 5577A their excitation mechanisms. [OI] LINRS THROUGH TWE MARCH 23124, 1969 AURORA The results for electron density N,, electron temAND THE N-S HORIZONTAL COMPONENT OP THE EARTH'S perature T, and ion temperature Xi observed at FIELD AS RECGRDED AT BLUE HILL. Millstone Hill are presented in Figs. 4(a-c), respecThe gaps in the optical measurements correspond to tively. Near 21:00, when the optical character of periods where measurements were made elsewhere aurora changed, the electron temperature above 200 km underwent a dramatic increase; no comparthan in the zenith. Figure 3 shows, on an expanded scale, the most active period during the hour able change appears either in Ti or N,, although a phase of the auroras, changes in the neutral composition of the thermosphere led to the ionic mass being greater than 16 below about 300 km. Consequently the T, and Ti values shown are sometimes too low in this altitude range.

21:oo

21:30

22:oo EST

FIG.

3. THE MOST IEF~ENSEPI~II OF THE 23f24 MARCHAURORA; VATIONIN THB ~G~TIC~~D~P~AREINDICA~D.

TRE ZENITH ANGLES OF OBSER-

20

02

0

22

04

c6

EST (a) WSJ

,000

I

../I

MILLSTONE 23-24 Mar _

HILL

1999

EST 09

Fm.

4. CON~PURS vs HEIGHT AND TIME OF THE MEASURED 23/24 MARCH AURORA AT MILLSTONE HILL; (a)log,,o~ (C)ION TEMPERATURE.

428

F-REGION PARAMETERS DURING THE N,(b) ELECTRON TEMPERATURE,

Scatter observ&ons of low-latitude auroras

429

MILLSTONE HILL 23-24 Mar 1969 T

2100

rapid drop in the altitude of maximum IV, did take place. The main quaIitative impression to be noted is the tendency for I(63OO) and T, to vary in the same senac through the night and Te and N, to exhibit an inverse relationship. 3.2 819 March 1970 Figures 5 and 6(a-c) present the optical, magnetic

_-_---__

I

I

and ionospheric observations during this aurora. Although of Iesser magnitude than in March 1969, this event lends itself better to detaiied analysis, in part, because it was less active and, in part, bemuse the quality of the incoherent scatter measurements was higher, owing to instrumental improvements at Millstone Hill. Moreover, the aurora1 activity ceased after local midnight, as is evident in the disappearance of ionization in the E-region; this has proved to be of great benefit to the analysis as we discuss later in detail. At the end of the night, the effects of photoelectrons from the magnetic conjugate point become apparent as an increase in both T, and the red line intensity. Hence on a single night, we can study three separate regimes in which there is energy input from aurora and conjugate photoelectrons, as well as an intervening quiet period. 4. EVIDENCE FOR CHANGES PHERIC

COMPOSITION-8/9

IN ATMOSMARCH

ES7

FIG. 5. ZENITHRWFMIIY OF THE AND

5577A 01

AURORA

AND &RTH’S

THE

LINES

DURING

N-S

HO~W~~

l’KF.LD AS RECORDED

6300 A @LID CURVE) 819 &fARCH 1970

THE

COh4PONENT AT BLUR

OF THE

HILL.

The missing portions correspond to measurements elsewhere than in the zenith.

4.1 General The in an energy during

819 March 1970 observations have been used attempt to form a coherent picture of the balance in the thermosphere above 200 km and after the aurora1 activity. We begin by

I

20

MILLSTONE

HILL

22

\

04

02

24

EST (4 I

IAILLSTONE

I

%ca Conjugate T Sunr!sa Sunset I

I 02

I 24

22

HILL

-

LOS9I Sunriu

04

(T-7 CM

FIG. 6. CONTOURS 819 MARCH

vs HEIGHT

AND

TIME OF THE

MEASURED

F-REGION

PARAMETERS

DURING

AURORA AT MILLSTONE HILL; (a) logIoN, (b) ELECTRON TEMPERATURE (C)ION TEMPERATURE. 430

W

431

Scatter observations of low-latitude auroras MILLSTONE HILL s-9Mw1970

TOO 20

I

I

22

24

I a2

L__,

a4

EST

FIG.

interpreting radar observations (this section) and then attempt to see if the conclusions which follow are consistent with the observed optical emission, principally of 6300 A (Section 5). The same approach is applied to 23124 March 1969, but in a less detailed manner for the reasons given above (Section 7). For both auroras, we find that an internally consistent interpretation demands a much higher ratio of molecular (N, and 0,) to atomic (0) species than is to be found in current models for the thermosphere under quiet conditions. The enhanced mixing ratio is nevertheless consistent with recent composition determinations in the high latitude aurora1 regions during geomagnetic storms. In Section 8, we summarize the general behavior of the F-region during these two low latitude auroras in the light of our conclusions concerning composition and energy balance. There are three separate lines of evidence implicit in the backscatter measurements on 819 March which all lead to the conclusion that the molecular to atomic mixing ratio must have been enhanced; these involve the gradient of ion temperature with altitude, the rate at which the electron density N, decays at 300 km near 22: 30 EST, and the energy balance in the electron gas.

6 (c) 4.2 Ion temperature results The electron and ion temperatures for 819 March were presented in Figs. 6(b and c). Figure 7 shows

0

FIG. 8

1000

OK

7. SOLID LINES SHOW T,

MARCH 1970 OBTAINED SUREMENTSA.WJMING

2000

3000

AND Ti AT 21: 00 EST ON FROM BACKSCATTER MEA-

mi = 16 (FIGs.~~,~).

The

dashed curves show T, and Ts below 400 km after correction as explained in the text; the curve labelled T, is the neutral temperature profile given by the Jacchia f 1971) model for 1200 K exosDheric temoerature.

J. F. NOXONand J. V. Evm

432

as solid lines the measured Tl and T, at 21 :OOEST which has been chosen as a time of relatively steady conditions during the aurora. It can be seen that there appears to be a significant vertical gradient in T, between 200 and 400km in comparison to the curve for the neutral temperature T,, which illustrates the neutral atmospheric temperature profile expected for an exospheric temperature of 1200 K (Jacchia 1971). The true neutral temperature profile at this time is not known, but the following energy considerations show that the shape (i.e. gradient of T, with height) could not have departed greatly from the cmve shown, provided the neutral composition is similar to that given by Jacchia for 1200 K. Associated with the gradient in T;, at any altitude is a downward heat flux through the neutral gas given by Banks and Kockarts (1973) for X, - 1000 K.

F, z 7 x lOlo 2

eV/cm*/sec,

(1)

where the gradient in T, is expressed in K/km. Thus, e.g. when T, -1OOOK a gradient of 12K per 100 km is required to conduct a flux of F, = l@O eV/cms/sec, and hence at 300 km the gradient in T,, could not exceed this value unless the heat supplied to the neutral atmosphere above 300 km exceeded lOlo eV/cm2/sec. We shall later show that the neutral heating could not have significantly exceeded this value and a large gradient in T, should not have existed. We also need to consider how rapidly the temperature proiile responds to changes in heat input. This can be investigated by estimating the relaxation time of the neutral atmosphere following the cessation of heating. For T, - 1000 K the Jacchia model gives a total neutral content above 300 km of -lOle particles/cm2. The thermal energy content (for 1000 K) is then -5 x 1014eV/cm2, and if a neutral temperature gradient at 300 km of, say, 1 K/km were to exist then in less than one hour the energy conducted downward through the neutrals would be approx~ately equal to the entire thermal energy content above 300 km. It follows that the neutral atmospheric temperature profile at and above 300km must adjust itself to heating at higher altitude quite rapidly. At 200 km, where the neutral density is an order of magnitude greater, the time constant is correspondingly longer. It can be concluded, however, that near 300 km and higher the neutral atmospheric temperature gradient must reflect the total neutral heating at higher altitudes over a time scale of less than one hour.

In the normal ionosphere T$ - I;, at altitudes below 400km owing to the rapidity with which collisions between ions and neutrals transfer energy between them. Ion heating can arise from electronion collisions (if T, > Ts), from the presence of an electric field perpendicular to the magnetic field, and from downward heat conduction through the ion gas. In the present circumstances, we can ignore the second and third sources of heat; electric field heating of the ions would produce a profile in which Ti decreased with increasing altitude above about 200km (Rees and Walker, 1968) while the ion heating by conduction can be shown to be completely negligible, using the formulas given by Takayanagi and Itikawa (1970). If the heating of ions by coulomb collisions with electrons is equated to the cooling by collisions with neutrals then for an electron-ion temperature difference of -1OOOK and the Jacchia (1971) neutral model at 1000 K the difference in ion and neutral temperature at and below 300 km must be less than -10 K. Figure 7 thus presents a paradox in that T$ and T, cannot differ appreciably, at least up to 300 km and yet, if Ti - T, in this altitude range, we are faced with an unacceptably large gradient in T, near 300 km ( >l/km). This problem presents itself throughout both auroras and required a reexamination of the validity of the Ti measurements, at least below 400 km. The backscatter radar technique does not directly yield 3;, but rather the ratio (T+/m*), where mi is the mean ionic mass (Evans, 1969). The values of TI in Figs. 4(c), 6(c) and 7 were obtained under the assumption that above 200 km the predominant positive ion is O+, i.e. mt = 16 AMU. Were mf to have been larger than 16, higher values of Ti would have heen obtained. If, in fact, Ti - T, near 450 km, as we show in Fig. 7, then to remove the gradient difficulty it must be assumed that below 400 km mi becomes progressively larger than 16; at 300 and 250 km, respectively, mt would have to be -19 and -23 if the “corrected” Ts curve is to match the dashed curve corresponding to T-. In the P-region the important positive ions are O+, Os+ and NO+, with the latter two produced largely by reaction of O+ with 0, and N2, and being destroyed by dissociative recombination with electrons, In the steady state we have

D+l KM = 6, P,+1 i?bl

(2)

KdO+I[N,l = &[NO+lD%l,

(3)

Kl

Scatter observations of low-latitude auroras

433

hence (4)

NO+/O+

= -Ka -lN11

K, [NJ and /

200 .-

[O,+l -t- [NO+1

WI

100i

’

7

’

i

(6)

Accordingly, if f < 1 the mean ionic mass will be close to 16, if it is 05, e.g. mt zz 20. In the F-region &] - lO[O,] and since K, - 10 K,, and K, - K4, [Os+] and [NO+] are of similar magnitude. Thus, using ICY- 10-11 cms/sec and Ks - lO_’ cms/sec, equation (6) may be rewritten approximately as 2K1 EW f z%x N zz 2 x lo-‘[os]/~~~]. 3

(7)

6

It follows that when N, - 105 cm-s and [O,] z 10ecm-s then f c 1. Using the Jacchia (1971) model atmosphere, values of [O,] as large as IO@crnw3 are found only well below 200 km even at T, - 1500 K. It is based on considerations such as these, as well as direct ion composition measurements, that the assumption mi = 16 is made in interpreting the incoherent scatter results above 200 km. If, on the other hand, we require f r 1 at 250 km then [Os] must be 5 x 10s/cms at this level; this is roughly 10 times greater than given in the Jacchia model at T, = 1200 K. If, at 300 km, we require that mt - 19 then f =0.25 and [O,] g 1.3 x lOs, again about 10 times the T, = 1200 K Jacchia value. Our concern here is to emphasize primarily the qualitative aspect of the Ti problem with respect to the molecular abundance and to point out that resolution of the problem (below - 350 km) demands an enhanced abundance for the molecular species, at least during the aurora1 period. Accordingly, if we assume that at 21:00 EST the molecular abundances were increased in the F-region to -10 times their normal value, but that the exospheric temperature was indeed -1200 K, then Ti - T, below -350 km. At higher altitudes mi must be -16 and any remaining difference between Ti and T, must be real. The difference to be expected between the two may be computed by equating the ion heating from the electrons with cooling to the

1 1000

1 2000

OK

FIG.

9

8. SOLID LINES SHOW T, AND T‘ AT 01:OO EST ON

MARCH

1970 OBTAINED FROM BACKSCATTER MEASUREMENTSANJMING ml = 16 (FIGS. 6b,c).

The curve labelled T,, is the neutral temperature profile given by the Jacchia (1971) model for a 900 K exospheric temperature. neutrals, assuming in the cooling calculation that the neutral composition is that given by the Jacchia (1971) model at 12OOK, but with [Os] and @Gs] raised by 10. We obtain at 400, 500 at 6OOkm (Ti - T,) = 13,46 and 120 K, respectively, which compare well with the differences shown in Fig. 7. Thus, it may be concluded that the measured values for Ti at 21: 00 EST are entirely consistent with ion heating by the ambient electrons alone, together with a neutral exospheric temperature of -1200 K provided a major enhancement in molecular species is accepted Figure 8 shows the temperature variation observed after cessation of the aurora1 precipitation. Here the ion temperature (deduced assuming rn+ - 16) exhibits no unusual behavior befow 300km and is consistent with heating by electrons and cooling to the neutral atmosphere at an exospheric temperature of 900 K. Even a ten-fold increase in molecular abundance in a 900 K model will not significantiy change the Ti profile. Hence we cannot exclude the possibility of an enhanced molecular abundance over the Jacchia model at 900 K on the basis of Ti alone, but it is not now required. In later discussion, it is shown that other conside~tions do rule this out and that the “anomalous” enhancement had disappeared. 4.3 Rapid decay in N, In this section, we discuss the rapid decrease in electron density iV, at 300 km near 22:30EST. This is illustrated in Fig. 9 which shows N, as a function of time at several altitudes as well as the integrated electron content above 200km. Since

J. F. NOXON and J. V.

434

IFIG.

’ I800

1 2000

I 2200

I 0000

I 0200

I MOO

9. THE ELJXTRON DENSITY N, AT FOUR ALTITUDES THE TOTAL ELXX-RON CONTENT As A FUNCTION OF TIME ON 819 MARCH 1970.

the measurements are of limited time resolution it is possible only to set an upper limit on the time constant for decay in N,, namely, ~3 x 1O-3 se&. In the absence of any production of fresh ionization, and assuming the loss of ionization to be equally controlled by reaction of O+ with 0, and N, (equations 2 and 3) the 0s density must have been at least 108 cn?, in order to establish the observed rapid rate of decay. This value is similar to the one that was inferred in the previous section dealing with Tc and is an order of magnitude greater than expected at 300 km for an exospheric temperature -1200 K. It must be emphasized that this estimate of the increased molecular density is a lower limit, since we have assumed no ion production, and the time constant could be shorter. In Section 5 where we consider the production of 6300 A it is shown that the ionization rate at 300 km could not have excedeed 10/cm3/sec in the OO:OO01:OO EST period during which N, is seen to increase. Since N, is a maximum near 300 km this increase can only be due to local ionization; the observed increase from 4 x 104 to 1.5 x lo5 in 2 hr corresponds to a rate of -13/cma/sec for ion production implying that the loss rate had now become very small. An upper limit on the loss rate by reaction of O+ with 0, would then be -10e4 se@ implying an upper limit on [O,] of 3 X 106/cms. Figure 8 indicates that the exospheric temperature was -900 K at 01 :OOEST and the O2 density at 300 km expected from the Jacchia (1971) model is 2 x 106/cm3. It thus follows that by 01:OO EST the anomously high molecular density must have disappeared.

EVANS

It is possible to account for the high abundance of molecular ions as well as the rapid loss in electron density by invoking an increase in the vibrational temperature of N, instead of increased molecular constituents. This explanation does not appear tenable for the following reasons. In order to produce a relative vibrational population for Y > 0 in N, sufficient to raise K, by even a factor of two the vibrational “temperature” must exceed 1500 K (McFarland et al., 1973). To enhance the rate coefficient by the required factor of ten Tvib must be -3000 K. An enhanced Tvib must primarily be the result of electron excitation and so will depend upon T, and a balance between excitation and vibrational quenching by collisions of N,* with atomic oxygen. In order to estimate the importance of this effect, we assume at 300 km that T, - 2500 K, N, - 105 cm-, and [Ol - lo9 cme3; the rate coefficient for vibrational excitation of N, by electrons is lo-l1 ems/ set (Dalgarno et al., 1968). The rate coefficient for cooling of N, * is lO_” cms/sec for a kinetic temperature near 1200 K, (McNeal et al., 1974). Thus we have [N,][N,]

x lo-l1

= [Na*][O]

Ns*/Ns

c 10-s

x lo-”

(8) (9)

corresponding to Tvib - 800 K. To produce Tvib - 1500 K(N,*/N, - 0.1) would require T, 3OOOK,and even with very much higher T, (up to 5000 K) the value of K, cannot be raised by more than a factor of 3 in the face of cooling of N,* by collision with atomic oxygen. It follows that the entire burden of explaining both the Ti problem and the rapid disappearance of electrons must be placed upon an enhanced molecular abundance. Figure 1 shows a similar rapid decrease in foF2 on 23124 March near 21:00 EST; this too is probably associated with an enhanced loss rate resulting from the high molecular abundance. 4.4 Energy balance in the electron gas The third argument which implies an enhanced molecular abundance follows from a consideration of the thermal balance in the ambient electrons. In fact, it was the realization that a fundamental problem existed in the thermal balance which first led us to suspect the presence of the enhancement. The problem is illustrated in Fig. 10 where the downward heat flux conducted through the ambient electron gas is shown together with the cooling of the electron gas integrated up to each altitude from below. The downward heat flux is given by F, = K, dT,/dh where Kc-l = Kaiml + K&l (Takayanagi

Scatter observations of low-lati~de auroras

435

Throughout the aurora1 period on 819 March (i.e. up to 0O:OOEST) the problem of F, exceeding the integrated cooling by factors of 2-3 is always encountered below 500 km. The problem can only be avoided by altering the neutral composition so as to increase the electron cooling rate. If the concentration of N, and 0, were raised by factors of 5 and 10 then the cooling rate would be raised as shown in Fig. 10. This increase also serves to lower the electron conductivity term, and as discussed above requires a reinterpretation of the radar data since 2ao the radar only determines Tilmi and TJT,. Figure 7 showed how a lo-fold increase in N, and 0, alters the values of T, deduced. Comparing Figs. 7 and 10 it is clear that in order to resolve the energy balance Fro. 10. THE DOWNWARD HEAT FLUX, F, THROUGH THE problem for the electron gas the molecular concenAMBIENT ELECTRON GAS (SOLID LINES) AND THE INTEGRATED tration must be raised by a factor of between 5 and COOLING RATE OF THE ELECTRON GAS COMPUTED AS EXPLAINED IN THE TEXT AT 21: 6fJ EST ON 8/9 h,fAReH. 10. The same result would have been achieved by a The different curves correspond to multiplying the similar increase in atomic oxygen density since with atmospheric molecular densities by the factor shown. the u~od~ed Jacchia (1971) atmosphere the cooling rate for electrons is roughly the same for and Itikawa, 1968). The conductivity is composed Ns and 0. The previous sections show, however, of a term appropriate to a fully ionized plasma and that it is an increase in the molecular species that one which takes account of the electron-neutral seems required; increasing [O] will help neither the collisions. The electrons are cooled both by ions and Ti nor the N, decay problem. neutrals and the relevant rates are given by the same authors. The curves marked xl are computed at 21:OO EST using the unmodified Jacchia model at 4.5 Other evidence for aurorally associated enhancement in molecular density 12:OOK and the measured T, (solid line) given in Fig. 7. A thermal balance problem is evident below Early efforts to interpret the radar and optical 450 km where the flux I7, exceeds the cooling curve. observations faifed because we were not prepared to F, is the heat flux flowing down through the electron accept the drastic changes in neutral composition gas and in the steady state this heat must be transimplied by the radar measurements. However, in ferred out of the electron gas below this altitude. the past several years it has become evident that such It can only be transferred by electron cooling to changes do indeed occur. Direct observations of the other species and the integrated cooling curve gives Ns density above auroras at high latitude have this transfer rate. The situation evidently is one in shown the existence of regions of upwelling of the which energy conservation is violated since the atmosphere (Taeusch et al., 1971; Reber and Hedin, cooling curve must always be at least equal to Fe at 1974; Prijlss and von Zahn, 1974) and recent every altitude. If it exceeds F, then the differenee theoretical studies (VoIland and Mayr, 1971 and, simply measures the energy given locally to the partic~arly, Hays et al., 1973) have shown how this ambient electrons at lower altitude by processes such phenomenon can be explained. The presently as ionization yielding secondary electrons which in accepted interpretation is that the neutral atmosturn can heat the ambient electrons. We also see phere is heated in the lower thermosphere at such a from Fig. 10 that the integratedcooling of ambient rate that a strong upward wind is created which electrons to neutrals is
J. F. NOXON and J. V. EVANS

436

increase in Du,ll[O] requires au energy deposition Figure 6(a) shows that of ~2 x lo5 erg/cma. between 2000 and 0000 N, is sustained at -3 x lo”cm* in the lOO-200km region. The recombination rate will be ~200 cm4/see since the rate is determined by dissociative r~ombiMtion (KS N 2 x lo-’ cm3 set-l) and this must equal the ionization rate. The formation and recombination of one ion pair leads ultimately to a deposition of -30 eV in the neutral gas; integrating over 100 km we find an average energy deposition of 4.1 erg/cm%ec. Since the aurora had been in progress for -10 h.r (3 x ltisec), we infer a total deposition less than 1Operg crne2 ; this is certainly not sufllcient. However, Fig. 5 shows that prior to the beginning of the radar measurements the aurora1 intensity was nearly 10 times greater than after 2000 thus suggesting that the required heating might well have taken place. While Hays et al. considered the effects of heating by electric fields orthogonal to the magnetic field, the magnitude of the field required (-100 mV/m) would lead to significant ion heating (see Rees and Walker, 1968) and Tf profiles very different from those observed. Also, as we have remarked earlier, the evidence based on energy balance for the ions favors principally ion heating by ambient ekzctrons. Consequently, in view of the strength of the aurora1 intensity early in the night, as well as the behavior of Ti, we consider that the compositional changes we infer were primarily the result of particle heating in the lower thermosphere. This heating may not have been just that over Boston. The theoretical work mentioned above suggests that modified composition effects will occur outside the region of immediate heating as a result of lateral flow set up by the heating. Since the aurora was even stronger to the North, such nonlocal effects may have been present. 5. OPTICAL 5.1

EMISSION--$/P

MARCH

Production of low energy electrons

In order to account for the observed 63OOA emission, it is necessary to estimate the amount of excitation by low energy ekzctrons. Evidence for the existence of low energy electrons is contained in Fig. 10 which demonstrates that there existed a significant locui heating of the ambient electrons in the 3005OOkm region. Figure 10 shows that if a S-fold increase in molecular abundance is adopted then the heat flux F, and cooling curves match fairly well below 4QOkm. At higher altitudes, however, Fe decreases and a maximum in the flux in the 300400 km region is a feature evident in all of the

600

500

400

300

200

5 i

100

5

0

2

I

eV/cm3*sec

x

-IO IO

-3

.E 600

III1

Q

I

400 -

I

III

I

XI0

300 -

100

1

0

500

1000

eV/cm~*Sec ~1~. 11. TOP: ~IFFSRENCEBETWEEN~,ANDINTBGRATSD LXKKING TAKEN FROM F%G. 10. BOTTOM: DERIVATIVE OFTOP CURVE PORX10,THISGIVESTHE HEATING RATE OFAMBIENTELE(;TRONSFORTHSx~oCASE.

computed F, curyes in both auroras. The difference (F,-cooling) at any altitude must be equal to the heat deposited locally in the electron gas below that altitude and the derivative of this difference with respect to altitude gives the local heating rate for the electron gas. Figure 11 shows the difference between the heat flow and integrated cooling at 21:00 EST for both 5 and lo-fold increase in the molecular abundance. Also shown is the derivative of the difference, i.e. the rate at which the electrons are locally heated. This local heating is probably caused by low energy electrons, which may be either soft primaries or secondary electrons produced by ionization. Additional heating may be caused by electric fields, but we shall show that low energy electrons are sufhcient. In considering the amount of energy given to the ambient electrons by each low energy electron, it is not necessary to distinguish between soft primaries and locally produced secondaries since the trausfer

437

Scatter observations of low-latitude auroras of energy from the fast to the ambient electrons does not become appreciable until their energy has been reduced to -1OeV (see Dalgarno and Lejeune, 1971). In the last stages of theorization the electron’s energy is distributed between the atomic oxygen, the molecular nitrogen and the ambient electrons. Knowing the relative abundance of these three constituents it is possible to compute bow much energy each fast electron gives to the ambient electrons. Using the results of Dalgamo and Lejeune (1971) as well as references in Takayanagi and Itikawa (1970), this energy has been estimated as a function of altitude at 21:OO EST, Figure 12 shows the result of dividing the local heating rates of Fig. 11 by the energy lost to ambient electrons and hence may be taken as the local production rate of low energy electrons necessary to heat the ambient electrons by the amount observed. At the lower altitudes these must nearly all be secondaries, since a primary energetic enough to penetrate there will generate a large number of secondaries. Above -300 km, however, we may expect to encounter a very much greater contribution from soft primaries whose initial energy is only a few hundred eV. The secondary electron production rate can be estimateddirectlyfrom the ionization rate which may be assumed to be balanced by loss. The latter has been calculated from the observed electron density and assumed neutral composition and the resulting production rate for secondaries is shown in Fig. 12. Between 200 and 400 km there is a fair agreement between the secondary production rate and the

number of soft electrons required to give rise to the observed heating rate for the ambient electron gas (Fig. 11) when the latter is computed for the lo-fold enhanced molecular abundance model. For the 5-fold enhancement in molecular concentration, the derived local heating is small below 400 km and is much less than the heating value expected from the secondaries alone. Thus, it may be concluded that a major portion of the local heating is accomplished by secondaries, though the precision of the overall computation is not sufhcient to decide whether a very soft non-ionizing flux of incident electrons is required above 400 km. 5.2 Production of 6300 A emission The illustrative discussion of the energy balance at 21 :OOEST may now be applied to the question of red line production. In particular, it is important to see whether the observed intensity is consistent with the interpretation so far presented and, if so, what further inferences may be drawn with respect to the effects of the aurora upon the neutral atmosphere as well as the F-region plasma itself. The production of excited OCD) atoms arises from three d~t~~ishable sources: dissociative recombination of O,+ (as in the normal nightglow), low energy electron excitation (soft primaties and secondaries), and excitation by those electrons in the high energy tail of the M~we~i~ distribution of ambient electrons (e.g. “hot” electrons). These three processes have been discussed in great detail in the past decade by many authors, such as Rees and his co-workers. In the normal nighttime ionosphere it is well established that the nightglow emission of the red line is the result of recombination of O$ ions formed in the ion-atom interchange reaction of O+ with 0,. The two-step process is then o+ + 0s + 0 + 0,’ 0,”

+ e -

O(‘D)

+ OCP).

(10) (11)

At high enough altitudes where O+ > O,+ the ratedetermining step is (10) and in the normal nighttime ionosphere this is the case above 200 km. Since OCD) is quenched by collisions with N, at a rate exceeding radiation below 300 km, under normal circumstances one finds that little error is introduced by assuming (10) to be rate determining at all 500 0 altitudes where emission of the red line is sign&ant Number/cmqsec (see discussion by Noxon and Johanson, 1970). In Fro. 12. SQUARES: NUMBEROF THERhfALIZING LOW the present circumstances, however, it is necessary ENERGY ELECTRONS PER cm*sec-l DERIVED FROM FIG. to consider whether the apparent enhancement of 11. DOTS: PRODUCTION RATE OF SECONDARY ELECTRONS molecular species will nullify this approximation. COhfPUTEDASDEScRIBEDYNTHETEXT.

J. F.

438

NOXON

In fact, it does not for though the enhancement raises the altitude above which O+ dominates this is compensated by the enhanced quenching rate by Nz. The production of red line emission from dissociative recombination can thus be calculated assuming (10) to be rate determining. It follows that I(6300)

WJW

= K1a0.76

J. V. EVANS

and

dh (12) s 1 + KQIN,I/A (see Noxon and Johanson, 1970), where KS is the quenching coefficient, A is the radiation probability for OCD) and E is the efficiency for production of O(lD) in (11) which we take to be unity. To determine the rate of excitation by non-thermal (low energy) electrons, appeal must be made to the flux estimate derived above from thermal balance considerations. Employing this number the production rate for O(rD) in the same volume can be computed by drawing upon the work of Dalgamo and Lejeune (1971) who give the number of O(lD> atoms produced per thermalizing electron as a function of the ratio Nd[O]. The effects of quenching by N, must then be allowed for, in order to arrive at the emission intensity of the red line. To compute the rate of impact excitation by hot electrons, the corrected T, curves shown in Fig. 7 can be used together with the formula given in Takayanagi anditikawa (1970), again allowing for the effects of quenching. In both this and the computation for excitation by low energy electrons, the [0] abundance has been taken to be that given by Jacchia (1971) for an exospheric temperature of 1200 K, i.e. the lo-fold enhancement in molecular

species has been assumed, but no enhancement in lo]. The results of these three computations for red line emission are shown in Fig. 13. The dominant source is found to be the low energy electrons. The total emission is estimated as 600 R which compares reasonably well with the observed value of 500 R. It is clear that the [0] density must indeed be close to that given by Jacchia; with any appreciable enhancement, or diminution, the predicted red line intensity would no longer agree with observation. A joint consideration of incoherent scatter and optical measurements thus requires a major increase in the thermospheric molecular abundance but no significant change in [O]. 6. POST AURORAL

PERIOD-8/9

MARCH

6.1 Near midnight The previous sections show that an internally consistent interpretation of all the observations at a typical period during the aurora can be achieved, provided an enhanced molecular abundance is accepted. It has also been shown in 4.2 that after 0O:OO the observations no longer require enhancement of the molecular concentration. We now explore the post-aurora1 period commencing at 0l:OO EST. Figure 14 presents results for the heat flux F, and the integrated cooling for an unmodified Jacchia model atmosphere at 900K exospheric temperature based on the results of Fig. 8. It is clear that there is no anomaly with respect to energy balance in the electron gas and that no molecular enhancement is needed. An excess of cooling over downward heat flux above 250 km remains, suggesting that low energy electrons must still be present in

600 600

z

300

I-

I

I

I

I

I

-

200

OL

I

0

6300

FIG.13. THE 21:oo EST KLh

1

IO

HOT

8

Photons /cm3

EMISSION

COhiFUTED

FOR

20

ELECTRONS

RECOMBINATION

RATE (THERMAL

(Rec.)

ELECTRONS.

30

. set

AND

FIG.

14.

AMBIENT

63OOA

OF

AT

EXCITATION) WW

ENERGY

GRATED PUTED PERIOD

THE

DOWNWARD

ELECTRON COOLING

RATE

AS EXPLAINED AT

HEAT

GAS IN

(SOLID OF THE

FLUX,

F,

LINES) ELECTRON

THROUGH

AND GAS

THE

THF! INTBAS

COM-

THB TEXT FOR THE POST AURORAL

0l:OO EST 9

MARCH

ENHANCEMENT

WHBRR

IS ASSUMED.

NO MOLECULAR

Scatter

observations of low-latitude auroras

439

magnetically conjugate photoelectrons in both T, and the red line have been discussed at length in recent years (see Carlson, 1972). Here it is intended only to see whether the si~ific~t increase in red line intensity after 01:OO is consistent with the low energy flux which we can infer from the backscatter measurements. Figure 15 shows the electron heating rate at 04:OO EST. As was found to be the case at Ol:OO, T;: exhibits no low altitude anomaly with respect to T, and the same holds for energy balance in the electron gas. The ambient electron heating rate, however, maximizes near 500 km compared with ~350 km at 01 :oO EST. Even though the total ambient electron heating is comparable with that at 0l:OO EST the lower electron density then 3 eV/cm set prevailing leads to a total 63OOA intensity of -130 R due to low energy electrons, since a greater FIG. 15. AMBIENT ELECTRONLOCAL HEATING AT 01 :m EST (SOLID)AND 04: 00 EST (DASHED) ON 9 MARCH 1970. fraction of the energy goes to exciting O(lD). By contrast, the dissociative recombination contribution or&r to heat the electron gas locally. If we compute is now only 20 R. the ionization rate (-lo/cm-%ec at 300 km) we find Thus, it can be concluded that at 04:OO the red it s&cient to account for electron heating only up to line is predominately due to excitation by low energy -300 km; at higher altitudes the bulk of the heating electrons, but it is not clear that they are associated must be sustained by a soft, non-ionizing flux of with conjugate sunrise. To some extent the increase approx. 5 x l@fcma@c, which is stopped between in T, after 01:OO EST could follow from a steady 300 and 4OOkm. Fii 15 shows the ambient flux of precipitating soft electrons coupled with the electron heating rate at this time. The production of decrease in N, since the cooling of ambient electrons 6300 A emission by fast electrons, computed from at high altitudes is primarily by positive ions and the heating rate as described above, is N40R. proceeds at a rate proportional to Nz. In an earlier Ionic recombination yields another 50 R for a total paper dealing with the effects of a conjugate electron of 90 R, which may be compared with the 70 R flux in enhancing the red line intensity Noxon and observed. Owing to the low values of T,, impact Johanson (1970) suggested that the enhancement excitation of the red line by hot electrons is negligible. before dawn on 8 March might be due primarily to An enhanced molecular ~n~entration is not conjugate phoroelectrons and be much more prorequired at 0l:OO EST; morever, it would be inconnounced than was usually found to be the case at sistent with the 63OOA intensity. The reason for Blue Hill, owing to the unusually low ambient this is if the abundance were increased by a factor of electron density which leads to a greater efficiency 10 as late as 01 :OOEST then the requisite ionization on the part of the photoelectrons in producing rate implies a secondary electron excitation rate of O(lD). The analysis given here weakens that 300 R for 6300 A plus another 150 R from ionic suggestion since the soft electron heating rate for the recombination. The resulting total is far in excess of ambient electrons does not increase significantly the observed intensity. over the conjugate sunrise period, There is no way While the period after 0O:OO has been termed to separate the total heating rate into aurora1 and “post aurora” this usage is mainly justified by the conjugate contributions, decline of the green line intensity to a normal 7. RESULTS FOR 23/24 MARCH 1969 nightglow level of ~100 R and the disappearance of ionization below 200 km; a soft electron flux still 7.1 General appears necessary to heat ambient electrons above As Figs. l-4 show this was a far more intense 350 km. aurora than the one a year later, but the intensity changes were so rapid during the strongest phase 6.2 Cmj..gate sunrise that no detailed steady-state analysis seems warFigure 6(b) shows that T, starts to rise after 01:OO ranted to the degree carried through for 819 March 1970. We have therefore merely chosen three times when the solar zenith angle at the point magnetically quasi-stable conditions and have conjugate to Boston reaches lo?. The effects of of apparent

440

J. F. NOXON and J. V. EVANS

FIG. 16. T. AND Td ON 23124 MARCH 1969 AT THREE SELECTED TIMES SHOWING BOTH UNCORRECTED (SOLID) FOR mi = 16 AND CORRECTED VALUES. AT 0O:OOEST

A TEN-FOLD BNHANCIZMENTIN MOLKWLAR ABUNDANCE ISASSUMED,BUTNOTATTHEOTHERTIMES.

attempted to apply to these the type of analysis outlined above. To this end, plots of temperature, heat flux F, and cooling have been constructed for these three times and are shown in Figs. 16 and 17.

concentration required at 400 km is actually somewhat greater than at 21:45 EST. About 1.2 kR emission of the red line from low energy electron excitation can then be expected. This appears more than enough to account for the observed value when the [O] density is that given in the 1500 K Jacchia model Thermal electron excitation is negligible. 7.4 01:30 EST At this time the exospheric temperature is estimated as 1800 K and only a slight compositional anomaly exists. Less than a factor of two increase in the molecular abundance suffices to remove the problem that the heat flux Fe cannot be balanced by cooling. A contribution of 1.3 kR is estimated from thermal excitation and -3.5 kR from low energy electrons which agrees well with the total observed 5 kR. 8. SUMMARY AND CONCLUSIONS 8.1 B/9 March

FIG. 17. THE HEAT FLUX F, AND INTEGRATED COOLING RATESFORTHE THRBETIMESSHOWNIN FIG. 16; ATo&oo A

MAJOR

INCREASE

IN MOLECULAR REQUIRED.

ABUNDANCE

SEEMS

7.2 21~45 EST At this time the exospheric temperature appears to be close to 2000 K and the molecular density is very large at high altitude even in an unmodified Jacchia model. As a result, there seems to be no need to invoke any enhancement of the molecular concentration as the heat flux F, never exceeds the integrated cooling, and the ion temperature deduced using the computed mean ionic mass yields seems sensible. With such a high molecular abundance (Ti - T,) < 10 K even at 500 km. The computed 6300 A intensity is 5 kR of which 3 kR is attributable to the soft electrons, 1.7 kR from impact excitation by ambient electrons and 0.3 kR from The observed intensity changes ionic recombination. from 10kR at 21:40 to 4kR at 21:50 and this appears to be in accord with the computed values.

1970

Figure 18 shows the final result for 63OOA computations throughout this night following the procedures discussed in Section 5 and 6. We shall not reproduce the details, but only remark that at each time chosen the exospheric temperature was estimated from the Ti; the required relative enhancement in molecular abundance was then determined from Ti and from the Fe and cooling calculations. In general, the molecular enhancement remained in the range 5-10 up to 0O:OOEST, but disappeared

8-9

March 1970

7.3 0O:OOEST This was the quietest and steadiest period of the aurora. From Figs. 16 and 17 it seems that the “anomaly” in composition has returned and that a lO-fold molecular enhancement over the 1500 K Jacchia model must be invoked. The molecular

FIG. 18. THE OBSERVED AND COMPUTED 63OOA ZENITH INTF,NsITYON 819 MARCH~ITHTHB LATTERDECO-. INTO THE THREE SOURCES AS DESCRIBED IN THB TEXT.

441

Scatter observations of low-latitude auroras later as already noted. Next the flux of low energy electrons needed to account for energy balance in the ambient electron gas was computed and, from this, the production rate for O(lD) from the low energy electrons. The contributions from ionic recombination and from thermal excitation were computed as we have discussed earlier. Figure 18 shows that a satisfactory agreement between predicted and observed 63OOA intensity exists and indicates the contributions from the three excitation sources. During the aurora1 period all make an appreciable contribution; after 0O:OOEST the effect of thermal excitation is negligible. At -22:OOEST the ambient electrons below 600 km are heated at a rate close to 10rl/eV/cms/sec; of this about 90% is the result of a low energy electron flux and only 1 x 101”eV/cm2/sec is due to heat conducted down through 600 km in the ambient electron gas. By 01:OO EST both sources are less by a factor of 20. It is interesting to note that at 22:00 EST, the rate of energy deposition over all altitudes was BwlOra eV/cm2/sec as inferred from the 1 KR intensity of the green line or alternatively from the ionization rate required to sustain the observed electron density. It follows that about 10% of the total energy input of the aurora at this time went into heating the ambient electron gas above 300 km and that an even larger fraction (as much as 20°k was deposited in this altitude range if neutral heating is included. As has been noted earlier, the enhancement in molecular abundance during the aurora, prior to 00: 00 EST, follows as a requirement for a consistent interpretation of all the observations, but the evidence is also strong that the enhancement disappeared within an hour or two after the cessation of major aurora1 precipitation. Once the heating in the lower thermosphere has ceased, diffusive separation should restore the thermosphere to its normal composition fairly rapidly. Hays et al. (1973) did not specitlcally consider the relaxation phase, but one can infer from the calculated behavior at the onset of heating that the time constant associated with the change is of the order of an hour or two. 8.2 23124 March The most unusual feature in this aurora is the inferred behavior of the atomic to molecular density ratio in the thermosphere. The [0] density actually changes very little. At 400 km the Jacchia model gives 4.5 x 10s at 1500K and 7 x 108 at 2000 K while [Nz] has corresponding values of 4.5 x 10’ and 1.8 x lo*. Since we reouire a nearlv, 1

IO-fold enhancement in molecular abundance at 0O:OOEST, the absolute molecular density must be twice as large at 0O:OOEST than during two more active periods. It is by no means obvious to us why the molecular abundance should be higher in the quiet period than in the active. It may simply be that although the kinetic temperature has declined by 0O:OOEST diffusive equilibrium has not been reestablished and/or the effects of horizontal transport need to be considered. The composition we require at all three times is very similar and does not depart greatly from a 1900 K Jacchia model. With such a composition the Ti profile, the electron heat balance and the 6300 %Lintensity can all be satisfactorily accounted for within the measurement errors. Why an enhancement of the molecular concentration should occur near OO:OOEST, but not at the other times is not understood and we must conclude that the major source of uncertainty in understanding the behavior in any very active aurora1 event is introduced by our poor understanding of the dynamics of the neutral atmosphere at such times. Acknowledgement-We

are greatly indebted

to the late

A. E. Johinson and to WY A. -Reid who assisted in gathering the data reported at the Blue Hill Observatory and the Millstone Hill Field Station, respectively. Mr. Johanson also assisted in the reduction of the observations together with Mrs. A. Freeman. At the time these measurements were made the Millstone Hill Field Station was supported by the Department of the Army, and the present ionospheric studies are being supported by the National Science Foundation (Grant GA42230), the observations at Blue Hill were supported by a grant to Harvard University (GA28371) from the National Science Foundation.

REFERENCES Banks, P. M. and Kockarts, G. (1973). Aeronomy. Academic Press, NY. Carlson, H. C. (1972). Predawn airglow enhancement project: perspective. Annls Giophys. 28, 197.

Dalgarno, A., McElroy, M. B., Rees, M. H. and Walker, J. C. G. (1968). The Effect of Oxvaen Coolinr! on Ionospheric Temperatures. Planet. S’&e Sci. 16, r371. Dalgarno, A. and Lejeune, G. (1971). The absorption of electrons in atomic oxygen. Planet. Space Sci. 19, 1653. Evans, J. V. (1965). Ionospheric backscatter observations at Millstone Hill. Lincoln Laboratorv.4. Lexineton. Y MA, Tech. Rept. 374. Evans, J. V. (1969). Theory and practice of ionosphere study by Thomson scatter radar. Proc. IEEE 57, 496. Evans, J. V. and Loewenthal, M. (1964). Ionospheric backscatter observations. Planet. Space Sei. 12, 915.

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Evans, J. V., Julian, R. F. and Reid W. A. (1970). Incoherent scatter measurements of F-region density, temperatures and vertical velocity at Millstone Hill. Lincoln Laboratory, Lexington, MA, Tech. Rept. 477. Hays, P. B., Jones R. A. and Rees, M. H. (1973). Aurora1 heating and the compositicn of the neutral atmosphere. Planet. Soace Sci. 21,559. Jacchia,’ L. (1971) Smithsonian Astrophysical Obs. Spec. Rept. 332. McFarland, M., Albritton, D. L., Fehsenfeld, F. C.. Ferguson-, E.. E. and Schmeltekopf, A. L.. (1973); A flow-drift techniaue for ion mobilitv and ion molecule reaction r&e constant measurements. II. Positive ion reactions of N+, O+ and Na+ with Oa and N, from thermal to ~2 eV. J. Chem. Phvs. 59. . 6620. McNeal, R. J., Whitson, M. E. and Cook, G. R. (1974). Temperature dependence of the quenching of vibrationally excited nitrogen by atomic oxygen. J. geophys. Res. I9 , 1527. Noxon, J. F. and Johanson, A. E. (1970). Effect of magnetically conjugate photoelectrons on OI(6300 A). Planet. Space Sci. l&1367. Noxon, J.-F. and Johanson, A. E. (1972), Changes in thermospheric molecular oxygen abundance inferred from twilight 63OOA airglow. Planet. Space Sci. 20, 2125.

Priilss, G. W. and von Zahn, U. (1974). Esro 4 gas analyzer results 2. Direct measurements of changes in the neutral composition during an ionospheric storm. J. geophys. Res. 79, 2535. Reber, C. A. and Hedin, A. E. (1974). Heating of the thermosphere during magnetically quiet periods, J. Geophys. Res. 79,2457. Rees, M. H. and Walker, J. C. G. (1967). Aurora1 excitation of the forbidden lines of atomic oxygen, Planet. Space Sci. 15, 1097. Rees, M. H. (1969) Aurora1 electrons, Space Sci. Rev. 10,413. . Rees. M. H. and Walker. J. C. G. (1968). Ion and electron heating by aurora1 electric fields; Ann. Geophys. 24,193. Schaeffer. R. C. and Noxon, J. F. (1975). Further study of changes in thermospheric molecular oxygen abundance inferred from twilight 63OOA airglow, Planet. Space Sci. 23, 1413. Taeusch, D. R., Carignan, G. R. and Reber, C. A. (1971). Neutral composition variations above 400 km during a magnetic storm. J. geophys. Res. 76, 8318. Takayanagi, K. and Itikawa, Y. (1970). Elementary processes involving electrons in the ionosphere. 3pace Sci. Rev. 11,380. Volland, H. and Mayr, G. G. (1971). Response of the thermospheric density to aurora1 heating during geomagnetic disturbances. J. geophys. Res. 76, 3764.