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Temporal and spatial variations observed in the ionospheric composition of Venus: Implications for empirical modelling
Adv. Space Res. Vol. 1, pp.3]5l. © COSPAR, 1981. Printed in Great Britain.
02731177/81/03010037$05.OO/O
TEMPORAL AND SPATIAL VARIATIONS OBSERVED IN THE IONOSPHERIC COMPOSITION OF VENUS: IMPLICATION FOR EMPIRICAL MODELLING H. A. Taylor Jr., S. J. Bauer, R. E. Daniell, H. C. Brinton, H. G. Mayr and R. E. Hartle NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA
ABSTRACT In Situ measurements of the thermal ion composition of the ionosphere of Venus have been obtained for a period of two Venus years from the Bennett rf ion mass Spectrometer on the Pioneer Venus Orbiter. Ion measurements within an altitude interval of 160 to 300 kilometers, corresponding to an overall latitude interval of about _40 to 34°N, are assembled from the interval December 1978 to March 1980. This time interval corresponds to two revolutions of Venus about the Sun, designated as two “diurnal cycles”. The distributions of several ion species in this data base have been sorted to identify temporal and spatial variations, and to determine the feasibility of an analytical representation of the experimental re; suits. Th~first results from the sorting of se~era~ promine~tions including 0 02 , and H and several minor ions including CO 2 , C , and H2 reveal significant diurnal variations, with superimposed modulation associated with solar activity and solar wind variations. The diurnal variation consists of strong day to night contrast in the ion concentrations, with differences of one to two orde~s o~magn$— tude,~dependingupon ion mass and altitude. The concentrations of 02 , 0 , CO2 and C peak throughout the dayside decreasing sharply at the terminators to night— side levels, lower by one to two order~of mag~ituderelative to the dayside. The diurnal variations of the light ions H and H7 peak during the night, exhibiting asymmetric nightside bulges favoring the pre—aawn sector, near 0400 solar hour angle. Superimposed upon the diurnal distributions are modulation signatures which correlate well with modulation in the F10 ~ index, indicating a strong influence of solar variability on the ion production ah~distribution. The influence of solar wind perturbations upon the ion distributions are also indicated, by a significant increase in the scatter of the observations with increasing altitude as higher altitudes, approaching 300 kilometers, are sampled. Together, these temporal and spatial variations make the task of modelling the ionosphere of Venus both very interesting and challenging. INTRODUCTION The first in situ exploration of the composition of the ionosphere of Venus began on December 5, 1978, with the insertion of the Pioneer Venus Orbiter (PVO) into orbit about the planet. Since encounter, the PVO has orbited the planet at a rate approximately equal to one earth day (Period 24.03 hours). The Bennett rf ion mass spectrometer (OINS) instrument on the PVO continues to provide measurements of the thermal ion composition on a daily basis. The purpose of this paper
37
38
1-LA. Taylor et al.
is to describe some of the first results of an examination of the global characteristics of a selected portion of the ion composition and its variation with respect to temporal and spatial parameters. The OIMS provides information on the bowshock, the ionosheath, the ionopause, and the detailed composition of the main body of the ionosphere. The instrument measures directly the composition and concentration of thermal ions, and in addition contributes data on superthermal plasma and ion drifts. Details of the instrument operation are given elsewhere (Taylor, et al., 1980 a [1]). Earlier papers have provided results on the overall characteristics and aeronomic implications of the ion composition, including day to night variability (Taylor, et al, 1979 a [2]; Taylor, et al., 1980 b [3]; Bauer, et al., 1979 [4]; Nagy, et al., 1980 [5]), and evidence and implications of strong interaction between the solar wind and the ion composition (Taylor, et al, 1979 b [6]; 1980 b [3]; 1980 c [7]; Hartle, et al., 1980 [8]). In general, these results have shown the Venus ionosphere to be both noticeably repeatable (e.g., at lower altitudes on the dayside), and highly responsive to variations in the solar wind (e.g. at higher altitudes, and throughout the nightside). Owing to this variability, in order to identify underlying aeronomic characteristics of the ion composition and its structure, it is necessary to examine a large body of data, and to sort out the observations as a function of temporal and spatial parameters. In this paper, we describe early results obtained from the sorting process, which will subsequently be used to provide inputs required for determining the feasibility of representing the composition of the Venus ionosphere by an empirical model. EMPIRICAL DATA BASE The data which comprise the empirical model data base were accumulated in the period December 14, 1978 to March 8, 1980 during the first two revolutions of Venus about the sun. The sidereal period for one revolution of the planet about the sun is about 224 earth days, during which, due to its retrograde rotation, a fixed point on Venus rotates relative to the sun approximately twice. In the present work planetary variations (e.g., longitude effects) are not considered, and a single Venus revolution about the sun is designated as the “diurnal” cycle. In terms of time at the earth, the first and second diurnal cycles at Venus correspond to the periods December 5, 1978 to July 26, 1979 and July 27, 1979 to March 8, 1980. Of the 448 orbits which occurred during the two diurnal cycles, data from a total of 310 orbits were inserted into the data base for the empirical model. The remaining orbits were omitted for a variety of reasons, including gaps in coverage, incorrect instrument mode, and inappropriate spacecraft data format for tie OINS. During the second diurnal cycle, a significant gap in the data coverage in the dusk region of the planet (19—22 hrs SHA) results from a period of superior conjunction when spacecraft operations and data recovery were precluded for an interval of 37 days (orbits). In the same cycle, a smaller gap (15.30 — 16.30 SHA) results from incorrect instrument mode configuration. The PVO orbit is nearly polar, with an inclination at insertion of 105°relative to the equator. Periapsis was initially located at a latitude of about 17°N,with the in~oundleg approaching the planet across the north polar region and with the outbound leg crossing the equator and exiting from the planet across the south polar region. Periapsis altitude drifts upward, and is periodically lowered by on— board propulsion so that the height of periapsis averages to about 160 km. Because of excursions in periapsis height, the latitude of selected orbit heights on the Inbound and outbound passes varies by a few degrees, as will be discussed in the results. In selecting the data for the model, the majority of all available orbits were included, even though on limited occasions data Is obtained for either only the inbound or outbound passes. This Imbalance, however, applies to only about 10% of the orbits within the total, so that no significant bias is introduced.
Ionospheric Composition of Venus
39
Th~ion~entered ~n the data base are J4+, H
2+, He+, O~, C+, N+, 0+, 0l8+~CO+(N2+) NO , 0~ , and CO2 , including the prominent and secondary species measured throughout the ionosphere. Measurements at the mass positions 17, 24, 40, and 56 AMU are not included in the sorted results, on the basis of either infrequency of observations or current lack of evidence sufficient to firmly establish the presence of the particular i~n. ~n t~epresent study, the so~ting~rocedure+is applied only to the major ions 0 , 02 , H , and the minor ions, C , CO9 , and H9 as a preliminary test of the sorting and analysis procedure. Subsequent work wili extend to the other ions, including the lowest concentration constitutents, for which sorting parameters may require further adjustment and fine tuning. To construct the data base, inbound (15° to 34°N)and outbound (—3°S to 15°N) observations were assembled within the altitude range of periapsis to 300 km. These data were in turn sorted according to latitude, altitude, and diurnal cycle, as a function of solar hour angle (SHA), using intervals for these parameters found to be most appropriate from examination of preliminary sorting results. OBSERVATIONS FROM SORTING DATA BASE Diurnal Variations. In general, the results of the measurements of diurnal variations of the ion composition reflect a prominent, stable distribution of ions throughout the dayside relative to a reduced, highly irregular nightside distribution, consistent with earlier observations based upon more limited data (Taylor, et al., 1980 b [3]). The contrast in day to night variability+is exemplified in the distribution of the prominent upper ionosp~ereconstitutent 0 , shown in Figure 1. In this figure, the diurnal variation of 0 observed at 220 km (+ 2 km) is constructed from observations from both inbound and outbound passes, irrespective of latitude, and the results from both diurnal revolutions are combined. ALT: 220km(±2km)
PV
OIMS
REV: Iond2
LAT:
—
21—30°N l~— 9°N
10
I::! ~
~
—
7
~
= ~ ;i: 10 Z
7~4~7~ —
~Io
3.~’
7*
~‘
~
~r
7
,~ ‘7
(j
7
—;7
~ lot 100
—
I
12
jig
24
SOLAR4HOUR ANGLE Fig. 1 The diurnal variation of 0 observed for the period of two planetary revolutions of Venus about the Sun, occurring between December 1978 and March 1980. Data from latitude intervals of 21° to 30°Nand 10° to 9°N are combined over the altitude interval 220 km + 2 Ion. Relative lack of observations in dusk—midnight sector results from data gap during superior conjunction.
40
1-l.A. Taylor et
al.
N~ar the dawn and dusk terminators, the day to night decrease in concentrati~nof 0 is s~en to be approximately an order of magnitude, 4dropping3from about 10 ions/cm during the day to an upper limit of about 10 ions/cm during the night. Through the night, the degree of variability in concentration is intense, exhibiting a range of two orders of magnitude, w~ile the dayside variation is only about a factor of two or three. In the case of 0 , the altitude of 220 kin (-I- 2 kin) is selected for best data coverage, and least evidence of scatter; a similar tuning of the sorting altitude range is applied to each ion distribution which fo~lows. 4he diurnal variations in the concentrations of two other dominant ions÷ 0 , and H are given in Figures 2 and 3, respectively. The distri~utionof302 a~200 km pe~ks on the dayside at a concentration of about 5 x 10 ions/cm . In the case of 02 , the diurnal variation exhibited at 200 km (±2 kin) is of the order of 100:1, again with considerably more variablity exhibited during night relative to day.
ALT: LAT: P.V.
OIMS
REV: I and2
—
200km(±2km) 20°— 30°N 1°— 9°N
10
~
10
~ThT~
0
~
12 SOLAR HCUP ANGLE
iS
24
Fig. 2 The diurnal variation of 02+ for the same conditions as Figure 1, except altitude window is 200 km + 2 kin, and latitude intervals are 20° to 3O°Nand 1°to 9°N. The light ion H+, sorted at 2~Okin (±2 km) has a diurnal distribution essentially anticorrelated with that of 0 , with strong concentrations during the night relative to the day. The extreme in the diurnal variation of a factor of approximately 100 to 1 occurs between the pre—dawn region and the n~onquadrant and reflects a pre—da~nbulge ~n the nightside distribution, wher~H reaches a maximum of about 5 x 10 ions/cm The ratio of the dawn to dusk H concentration observed at 220 km is approximately 5:1 in favor of the pre—dawn region, although the average density level in both of the nightside regions is difficult to characterize due to the considerable variability encountered ther~. As in the case of each of the other ions examined, the variability in the H concentration is typically greater during the night, than the day.
Ionospheric
PV 1
OIMS
Composition of Venus
REV: Iand2
41
ALT:
220km(~2km)
—9
N
-~
H
-
Ii
io~_J’
1~I~~~%,
~
~
~510
I
o~~1
____
____
10
I
0
6
12
18
24
SOLAR HOUR ANGLE
Fig. 3 The diurnal variation of H+, for the same conditions as Figure 1. As a test of the resolution capabili~y of th~sorting procedure for examining minor ion species, the measurements of CO2 and H2 are sorted, and the resulting diurnal variations are illustrated in Figures 4 and 5, respectively. In the cage of 112+, sorted at 220 km (±2 Ion) the dawnside bulge in the distribution of H is also exhibited in the distribution of this trace ion, as shown in Figure 4. Allowing for the large variability superimposed on+the distribution, a ratio of at least lO:~is ascribed to the concentra~ionof H2 , between dawn and dusk. The peak in 112 concentration of about 30/cm occurs near a solar hou~ angle of 04 hours in approximately the same positio~as the pre—dawn peak in the H distribution. Near noon, the concentration gf 112 frequently drops below the instrument sensitivity limit of about 1 ion/cm The diurnal variation of the minor ion CO9+, shown in Figure 5, is relatively stable,3exhibiting a rather uniform daysi8e distribution averaging about 5 x 10 ions/cm , and dropping by two orders of magnitude or more at the terminators. For this ion, which has a relatively low scale height, the altitude range of 180 (±2 kin) is used to obtain a reasonably smooth distribution. The dayside CO2 distribution exhibits evidence of some modulation, which is also evident to a varying degree in the other ions. This modulation is discussed in a later section. Variations Associated with Latitude and Altitude. In order to identify one example of the degree of ion concentration+variations associated with latitude, we compare the combined diurnal profiles of 0 at a fixed altitude of 220 km (+ 2 kin), for the two separate ~atitude bands traversed on inbound and outbound passes. In Figure 6, profiles of 0 are separated into inbound (latitude 21° to 30°N)and outbound (latitude = 1° — 9°N) groups to illustrate latitude related effects. Within each latitude band, small latitude variations from orbit to orbit (of at most 3 — 4°) are not yet considered in the sorting.
42
H.A. Taylor et a7~.
ALr: P.V~ CIMS __________
=
I8Okm(t2km)
~
REV: I and2 __________
19°— 30°N 0°— 9°N
____
+
CO2
clot
41
~10 (I)
z o
to
F--
z
2 ~
F
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4: 0 U
-
~
F
F
F
~
F
FF~W
FF
~ F~ F~F
r
100
ThT~m~rnfl~ 6 12 -
18
SOLAR HOUR ANGLE
24
Fig. 4 The diurnal variation of CO + for the same conditions as Figure 1, except altitude window is ~8O kin + 2 Ion, and latitude intervals are 19° to 30°Nand 0°to 9°N.
P.V
OIMS
REV: I and2
ALT:
220km(±2km)
LAT:
21°— 1°—30°N 9°N
—
+
-H °~ U N
Z 2 ~j10 ~
a
:0 (1)
-~
2
~
a
z I-.
—~
°~ F---
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~
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-~
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a
~
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(2
al
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—r
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~
2
~
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aj
~
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~
~\~2
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2 2,1 22 2 2
2 2 22è2~
22 2
Ii~
~°
~2
2 °~-~i~mff~iflThT 18
24
SOLAR HOUR ANGLE
Fig. 5
The diurnal variation
of 112+ for the same conditions
as Figure 1.
Ionospheric Composition of Venus
43
Considering first the expected consequence of the variation in solar zenith angle (SZA) as a function of latitude, we note that across the dayside, where the rela— + tively stable ion distributions permit comparison, the average concentrations of 0 in the 21°— 30° latitude interval are estimated to be lower by about 10 — 20% relative to the corresponding ion concentrations at 4 — 9°latitude. This relationship is associated with a difference of about 20° SZA near the sub—solar point, between these latitude regions.
P.V. OIMS
ALT: 220 kmfi2km) LAT AS SNOWN
REV: I and2 0
0
21 —30 LAT
~ INBOUND
~ ~
‘~‘
(0~ ~
,~,
~
o~
,
~
lot -
(.1) -
z 0
10
~
to:
OUTBOUND
i°—90 LAT
~ ~
ID’
100
-
I
I
I
I
I
l~2 I
I
I
1
118
I
I 24
SOLAR HOUR ANGLE
Fig. 6 A comparison of the diurnal variation in 0+ observed at the separate latitude intervals: 21° to 30°Nand 1°to 9°N. The altitude range is 220 km + 2 km. The diurnal period corresponds to the first Venus—Sun revolution, in the interval December 1978 to July 1979. The solar zenith angle at noon is about 27° for the upper distribution and about 6°for the lower distribution. A second aspect of latitude related variations is the possibility of spatial and temporal effects in the coupling of the solar wind with the ionosphere. Further inspection of Figure 6 reveals that, aside from the foregoing SZA effect, the structure of the mid and low latitude 0 profiles is generally quite similar, indicating that in this sample, the degree of interaction between the solar wind and the ionosphere is relatively similar in the two latitude regions. It may also be
44
H.A. Taylor et al.
assumed from the observed similarities that there are no pronounced planetary latitudinal effects, such as might result from anomalies in the distribution of an intrinsic magnetic field, were such to exist. Although not illustrated, the distributions of some of the other ions also indicate a lack of pronounced latitudinal variations. However, more sorting and analysis of the data will be required to support this initial observation. Although sorting and analysis of the data base is not complete for all ions, a feature typical of all species examined is the significant increase in variability in concentration encountered with increasing height. The height dependence÷of the variability is illustrated in Figure 7, where the diurnal distribution of 0 is illustrated for the height ranges 220, 260 and 298 kilometers (±2 km). As the altitude of sampling is increased, the variability increases dramatically across the dayside until at the 300 km level the dayside variability becomes equivalent to that observed at night. Similar trends are observed for other ions not shown. ALT: AS
P.V.
OIMS
REV: I
SHOWN
LAT: -~s—o N
106
~ o~
298km (±2km)
I 3 10
~
~ ~
4
~
-~ ~
‘ 7
‘
-I
‘
102J
a
~,
~
I
v’:’
‘
~,‘
-
5:
i~
1~ ~
~
~
‘j’~ a
10
4
1 ,
F~
10~ ~
8
iø
Z 0
~
~
-
I
~:
‘
220km ~(±2km)
106
10
-~j
~
-1 r
,
~,,i,,
l~ ~
~
‘
~ ~0
777
7 ,
, ‘~ ‘,
-
2~4
-
260km ~,(±2km)
~TTI
‘
I
I
ITrFi
24
SOLAR F-FOUR ANGLE
Fig. 7 An indication of the increase in variability in 0+ concentration on the dayside, as a function of increasing altitude, for the levels 220, 260 and 298 km (±2 kin). Observations are from the —3° to 9°Nlatitude interval of first diurnal revolution (December 1978 to July 1979).
Ionospheric
Composition
45
of Venus
Temporal Variations. In comparing data from the first and second diurnal cycles, temporal variations are observed which significantly modify at least the dayside portions of the distr$buti~nsof+certai~ions, as shown in Figures 8 and 9. The dayside profiles of 0 , 02 , CO 2 and C shown in Figure 8 exhibit a distinct modulation with relative maxima near 7, 10, and 13 hours SHA. This modulation appears to be a continuation of a perturbation sequence which first became noticeable near noon in the first diurnal cycle, shown in Figure 9. This modulation, which features concentration differences as large as a factor of 2 — 3, is evident in the distributions in each of the latitude intervals sampled, although the amplitude of the modulation appears to vary as a function of both mass and latitude. Although
P.V. OIMS
REV: 2
-~
220km (±2km)
P
I’ —7°N
0
10~ I0~
~
+
-
200krn(±2km)
a
10
3
F
b~,1~f
I
~÷
~.
~f’~ ~22Okm’±2km) ~I°—7°N 1
5
o2
I -
!b0
180km (±2km)a ~
~
~
~ ~
~to2
II—-
IO7 I
-.-~‘.
•L
2Hz
300 I
~
.. S..-
-too 200
~~22
M
SOLAR HOUR ANGLE
Fig.~8 ~odu4~tion of ~he dayside portions of the diurnal profiles of 0 , 0~ , C , and CO 9 for the second diurnal revolution, corresponding to July 1979 to March 1980. Vertical lines indicate relative peaks and troughs in concentrations+and ~ssocia~ed variations in the solar F10 ~ index. Observations ~f 0 , 02 and C are from the latitude interval 10°to 7°N. The CO2 results are from 14° to 22°N. Altitude levels vary according to mass, as noted. The data is plotted beginning with orbit 235 (July 29, 1979) at 1700 SHA, with orbit number and date increasing with SHA to orbit 440 (February 17, 1980), at 1500 SHA. The data gap between about 1500 SHA and 1700 SHA results from incorrect instrument configuration; the gap between about 1900 SHA and 2300 SHA results from superior conjunction. The F10 7 data taken at earth has been transformed to the location of Veniis, as described in text. The F107 magnitudes shown are those observed at earth.
46
H.A. Taylor et al.
the phasing of planetary longitude relative to solar hour angle has not been taken into account, it seems clear from the period of the observed modulation in the ion concentrations that the source of the modulation is a temporal and not a planetary inherent effect. Although the modulation observed in the ion distributions during both the first and second diurnal revolutions is complicated by short term orbit—to—orbit variations and small latitude variations resulting from orbit periapsis movement, it is possible to estimate the period of the modulation across the dayside. In Figures 8 and 9, the indicated approximate positions for successive peaks and troughs in the concentrations shows that the source responsible for the variation is varying with a period of about 27—28 earth days, approximately the rotation period of the Sun. In order to relate the observed diurnal modulation to possible solar effects, we have also included a plot of the F 10 ~ solar radiation parameter in Figures 8 and 9. To compare the F107 data (takenat earth) with the OIMS data (taken at Venus)
P.V. OIMS
REV: I
~~~~2Okrn(±2km)
,0
a1O
T
+
~
ID~
200km (±2km)
02
3°—9°N
P
220 km (±2km) 10~
o2
CO2
(±2km) =
~
‘~,I5°—23°N
F107 •
III
-
I~ ~
I~Il
2 0~
~,o2 300
2Nz ~
T
~
. ~.
•
...
200
~ ~
111112111111181111
-
IO22__~_
100
M
24
SOLAR HOUR ANGLE
Fig. 9
Modulation of diurnal variations of 0~, 02+~C+ and CO
2~, for the first diurnal revolution, corresponding to December 1978 to July 1979. The data is plotted beginning with orbit 10 (December 15, 1978) at 1700 SHA, and orbit number and date increase with SHA to orbit 234 (July 27, 1979) completing the cycle at 1700 SHA. Since the F10 ~ fluxes on the first and last orbits of the cycle are different, there is a discontinuity~in the ~lO ~ at 1700 SHA, which is also possibly indicated in the 0 and CO2 • prcfiles.
Ionospheric Composition of Venus
47
the relative positions of the earth and Venus must be taken into account. This was accomplished by assuming that the solar activity was localized in solar longtitude so that a particular active region seen at earth on a given day would have been seen a number of days earlier (or later) at Venus. Thus, to determine the flux corresponding to an OIMS observation at a particular hour angle, the angular separation of Venus and earth at the time of the 01145 observation is determined. This angle, combined with the relative angular velocities of the earth, Venus, and solar surface, determines how much earlier (or later) the same face of the sun would have been seen at earth. (No attempt has been made to account for the temporal variation of solar activity between its observation at earth and its impact on the Venus atmosphere.) In this manner, we have been able to plot the daily earth based F 10 ~ flux data at 15 minute SHA intervals on Figures 9 and 10, revealing the excellent correlation between the 27 day modulations in solar activity and the ion concentrations. ALT 2ookmI~2km)
P.V. OIMS
REV: 2
LAT4—26N
in F
H I
io~ 2 2
~ I~7~’ ~
~ ‘, A~ II,
— —
+
j1,I1
II
~till
‘I
t~
I
,
Io
~
_~IoI
100
...,
-
300 ‘10.7
0
I
I
1
1
I
.../• ......... 11 Il........... 12 1 II Ill SOLAR HOUR ANGLE
•~—.I
18 I
•...— III
....
w
I~.24200 100
Fig. 10 The diurnal variation of H+ for the second Venus/Sun revolution, with the corresponding variation of F 10 ~ index. Plot constructed as in Figure 8.
Relative to the dayside, it is more difficult to identify possible modulation ef— fect~ in the variable nightside ion distributions. As an example, the distribution of H for the second diurnal revolution is plotted with the corresponding F10 7 distribution in Figure ~O. It may be seen that there is a subtle hint of an anti— correlation between n(H ) and F10 7 at several intervals across the dayside and a possible indication of a positiveS correlation near 0600 SHA where the abrupt upsurge occurrs in F10 ~• These observations are obviou~1yprecarious, however, in view of the consideragle variability involved in the H distribution.
48
H.A. Taylor et al. DISCUSSION AND INTERPRETATION
The global features of the variation of the ion composition of the Venus ionosphere are characterized in two general categories: spatial (latitudinal, altitudinal) and temporal (Diurnal, short term) variations. In the case of the spatial variations, we note that neither the latitudinal nor the altitudinal variations in the ion distributions sampled appear to deviate significantly from general theoretical predictions made prior to the in situ exploration of the ionosphere. With respect to altitude effects, the increased variability of 0+ as a function of altitude on the dayside illustrates that as one moves upward above the region dominated by chemical equilibrium, the upper ionosphere becomes increasingly subject to interaction with the solar wind. As a consequence, the compressive and expansive forces generated by the solar wind interaction can result in significant scale height variations (Hartle, et al., 1980 [8]) which in turn produce an increasing scatter in the distributions of a given ion within a fixed altitude window. With decreasing altitude, the ion distributions should increasingly reflect the influence of the ion—neutral chemistry and variations in the neutral gas density. Although we have not yet performed a complete analysis of the height variations for each mass, it is likely that the present results include height variations associated with changing conditions in both solar radiation and solar wind parameters, as discussed later. Regarding latitude variations, again typified by the 0+ distributions (Figure 6) the results obtained from the first two diurnal revolutions are also not surprising in view of current theoretical concepts of ~he formation and maintenance of the dayside ionosphere. The concentration of 0 at 220 km, i.e., near its “peak”, which was identified earlier as an “F 2 ledge” (Bauer and Hartle, 1974 [9]), depends on both the photochemical production rate (q) and plasma transport processes. To a first order, the average daytime concentration near this peak should be related to cos ~ where X is the solar zenith angle (SZA) si~ce the photoionization process has this SZA dependence, w~ile the concentration n(O ) enters linearly in the balance equation, q = div [n(O )], where v is the plasma transport velocity. 0 (NoteThus, that for average the a photochemical level of equi1i~rium the 0 concentration layer, E or at F~iigher , N latitudes ~ i.e.,(21°cos — 30°) which reflects a 26° SZA should be correspondingly (o~10%) lower than that at 1° — 9°N with a 5° SZA, as found in the observations (Figure 6). Aside from the SZA effect, the general lack of any pronounced latitudinal variability in the ion profiles is attributed both to the relatively small latitude interval encompassed between the inbound and outbound measurements as well as the lack of important latitudinal variations in any intrinsic planetary magnetic field, if such exists.
x).
Considering the long term temporal variations, the diurnal variation exhibits important observational results relative to theoretical predictions. As indicated in earlier results from the OINS (Taylor, et al, 1979 a [2]; Taylor et al., 1980 b [3]) and as underscored by these more complete model results, the very significant concentration of ionization often observed at night is surprising in view of the long Venus night and absence of photoionization across the nightside. The variability of the nightside ionization is analogous to that observed in the auroral
regions of earth and as such, is supportive of arguments for both dynamic transport of dayside ionization as well as particle ionization as processes contributing to the maintenance of the nightside ionosphere. Owing to the highly structured ion profiles generally encountered at night, it is difficult to make definitive tests of competing transport and impact ionization hypotheses. Nevertheless, it has been shown (Taylor, et al, 1980 a [1]; Knudsen, et al, 1980 [10]) that ion flow velocities observed near the dusk terminator are of sufficient magnitude to provide the transport of ionization from the dayside necessary to maintain the observed nightside ionosphere. Such being the case, the variability in the solar
Ionospheric Composition of Venus
49
wind which drives the day—to—night transport is consistent with the high degree of variability observed in the nightside ionization. Such variability does not, how-ever, rule out the possible effects of impact ionization, and studies of these competing processes for maintaining the nightside ionosphere will continue to be pursued. Other significant ~eatures+of the diurnal variation include nightside bulges in th~ distri~utionsof H +and H The observed predawn density maxima in the species H and H 2 and also He (alt~oughnot shown) are in striking contrast to the re~ative— ly more symmetrical diurnal variations of the heavier ions. Assuming that H is in chemical e~ui1ibr4uinnear periapsis, Brinton et al., 1979 [11] have used measurements of H and 0 , combined with measurements from the PVO neutral mass spectrometer (ONMS) to determine the distribution of atomic hydrogen, which also peaks during early morning hours (4.00 LT). Moreover, the diurnal variation in neutral He (Niemann, et al., 1980 [12]) shows a similar behavior which is significantly different in character from the more symmetrical daytime maxima in CO2 and 0 around noon. On the basis of these results, it appears that at lower altitudes, the gross diurnal variations in the ionospheric composition reflect the behavior of the neutral atmosphere. This in turn,7can2be un~erstood if one postulates large eddy diffusion coefficients (K 3 x 10 cm sec ) and thermospheric superrotation rates wits periods between 5 and 10 days (Mayr, et al., 1980 [13]). The presence of the H2 bulge is also consistent with theoretical predictions for the distribution of neutral H2 on the nightside (Kumar et al., 1978 [14]) and is the subject of a companion paper in these proceedings (Kumar et al., 1980 [15]). Superimposed on the diurnal variation, the short—term 27 day modulation is interpreted as the result of variable heating and ionization of the neutral atmosphere, coupled perhaps with a variable interaction between the solar wind and the upper ionosphere. Evidence for the importance of the solar radiation as the perturbation source is provided by the fact that the phasing of the modulation is consistent with fluctuations of similar time scales in the F10 ~ flux. The observed correlation between F1 and the ion concentration is again a direct consequence of the important varia~iiityof the neutral (ionizable) constituents with changes in solar EUV flux. First, the ETJV radiation is ~ primary souce of ionization. For chemical equilibrium, the ion concentration N (X ) F~0~ .exp (K/F10 7)~ leading to a more 1ieionizing radiation and the concentration of neutral constituents involved complex dependence on ratio the solar flux due to tfie~actthat tl in production and loss processes vary with solar activity. The solar radiation is also a source of energy for the neutral atmosphere, where the temperature, T , measured by the ONMS, shows a 27 day periodicity in phase with F 10 7 [Niemann, pr~vatecommunication]. Any such change in T affects the neutrat ~as composition, which is closely tied to the photochemistry o~the ion composition. Owing to the complexity of these coupled effects, we have not yet made a quantative analysis of the response of the ion concentrations to the solar F10 7 variations. We note also that planetary wave motions have been identified in the neutral den— city measurements (Niemann, et al., 1980 [12]; Keating, et al., 1979 [16]), although the period reported for these planetary waves is about 5 — 6 days, which is well separated from the 27day periodicity observed in the modulation of the ion distributions. Although this secondary source of short term perturbation may in some instances couple with the solar radiation effects to perhaps modify the patterns in the ion and neutral distributions, a more refined analysis of the ion data and careful correlation with corresponding neutral data will be required to identify such presumably subtle effects. As indicated earlier, the nighteide ion variability makes identification of pos— ible modulation effects of the 27 day solar variation difficult. While there are hints of relationships between the solar variability and fluctuations in the
50
H.A. Taylor at al.
nightside ion distributions, such as might be suggested from the H+ distribution in Figure 10, we have as yet no firm evidence. Identification of such relationships would tend to support the case for ion transport as the principal mechanism for maintaining the nightside ionosphere. With this objective, we will refine the sorting and analysis of the data base in an attempt to confirm any such relationship in the nightside data. Venus—Earth Relationships and Empirical Modelling. The foregoing results reveal important differences in the ionospheres of Venus and Earth, which have significant implications for empirical modelling. First, the fact that the effect of the solar variation is rather easily identified in the ion distributions apparently relates in part to the relative uniformity of the lower ionosphere. In the apparent absence of an intrinsic magnetic field at Venus, the ion distributions are not regulated on a large scale, in contrast to the Earth ionosphere, where the strong dipole magnetic field significantly influences the ion distributions. At Earth, magnetic effects are sufficiently pronounced to produce distortions of seasonal variations in the ion distributions, leading to “solar—geomagnetic season” effects (Taylor, 1972 [17]), and to distortions of altitude and latitude variations, e.g., in the equatorial anomaly (Taylor, et al., 1978 [18]). Such distortions are evidently not present at Venus, and the lower ionosphere apparently reflects the solar variations in a more straightforward way than at earth. A second implication of the Venus—Earth relationships is that the upper portion of the dayside ionosphere and the overall nightside ionosphere of Venus are relatively more closely linked to the solar wind than in the case of Earth. As a consequence, these regions exhibit a degree of variability which is a particular challange for both data sorting and modelling techniques. In summary, the presence of both long and short—term variability observed in the ion distributions is a factor influencing the objective of and approach toward developing an -empirical model of the Venus ionosphere. The repeatable day—to—night gradients in the ion distributions appear to be adaptable to parametric modelling. This also applies apparently to the observed dayside latitudinal and altitudinal variations, provided the altitude range is selected to be within the less disturbed height regimes of the solar wind ionosphere interaction. The short—term variability ascribed to solar EUV interaction may also be modeled, although this will require a careful sorting out of both ion and neutral processes. On the other hand, very short—term fluctuations (day to day), such as noted within nightside variability are not practically amenable to modelling. REFERENCES
1.
H. A. Taylor, Jr., H. C. Brinton, T. C. G. Wagner, B. H. Blackwell, and C. R. Cordier, Bennett ion mass spectrometers on the pioneer venus bus and orbiter, IEEE Transactions on Geoscience and Remote Sensing, GE—l8, 45, 1980 a.
2.
H. A. Taylor, Jr., H. C. Brinton, S. J. Bauer, R. E. Hartle, P. A. Cloutier, R. E. Daniell and T. A. Donahue, Ionosphere of venus: first observations of day—night variations of the ion composition, Science, 205, 96, 1979 a.
3.
H. A. Taylor, Jr., H. C. Brinton, S. J. Bauer, R. E. Hartle, P. A. Cloutier, and R. E. Daniell, Global observations of the composition and dynamics of the ionosphere of venus: implications for the solar wind interaction, J. Geophys. Res., 1980 b (in press).
4.
S. J. Bauer, T. M. Donahue, R. E. Hartle, and H. A. Taylor, Jr., Venus ionosphere photochemical and thermal diffusion control of ion composition, Science, 205, 109, 1979.
.
Ionospheric Composition of Venus
51
5.
A. F. Nagy, T. E. Cravens, S. Smith, H. A. Taylor, Jr. and H. C. Brinton, Model calculations of the dayside ionosphere of venus: ionic composition, J. Geophys. Res., 1980 (in press).
6.
H. A. Taylor, Jr., H. C. Brinton, S. J. Bauer, R. E. Hartle, P. A. Cloutier, F. C. Michel, R. E. Daniell, Jr., T. N. Donahue and R. C. Naehl, Ionosphere of venus: first observations of the effects of dynamics on the dayside ion composition, Science, 203, 755, 1979 b.
7.
H. A. Taylor, Jr., R. E. Daniell, R. E. Hartle, H. C. Brinton, S. J. Bauer and F. L. Scarf, Dynamic variations observed in the thermal and superthermal ion distributions in the dayside ionosphere of venus, Advances In Space Exploration, 1980 c (in press).
8.
R. E. Hartle, H. A. Taylor, Jr., S. J. Bauer, L. H. Brace, C. T. Russell and R. E. Daniell, Dynamical response of the dayside ionosphere of venus to the solar wind, J. Ceophys. Res., 1980 (in press).
9.
S. J. Bauer and R. E. Hartle, Venus ionosphere: an interpretation Mariner—lO observations, Geophys. Res. Lett., 1, 7, 1974.
of
10.
W. C~Knudsen, K. Spenner, K. L. Miller and V. Novak, Transport of ionospheric 0 ions across the venus terminator and implications, J. Geophys. Res., 1980 (in press).
11.
H. C. Brinton, H. A. Taylor, Jr., H. B. Niemann and H. C. Mayr, time hydrogen bulge, Bull. Am. Astr. Soc., 11, 538, 1979.
12.
H. B. Niemann, W. T. Kasprzak, A. E. Hedin, D. M. Hunten and N. W. Spencer, Mass spectrometric measurements of the neutral gas composition of the thermosphere and exosphere of venus, J. Geophys. Res., 1980 (in press).
13.
H. C. Mayr, I. Harris, H. B. Niemann, H. C. Brinton, N. W. Spencer, H. A. Taylor, Jr., R. E. Hartle, W. R. Hoegy and B. M. Hunten, Dynamic properties of the thermosphere inferred from pioneer venus mass spectrometer measurements, J. Geophys. Res., 85, 1980 (in press).
14.
S. Kumar, D. M. Hunten and A. L. Broadfoot, Non—thermal hydrogen in the venus exosphere: the ionospheric source and the hydrogen budget, Planet Space Sci., 26, 1063, 1978.
15.
S. Kumar, D. N. Hunten and H. A. Taylor, Jr., H abundance in the upper atmosphere of venus, Advances In Space Explora~ion, 1980 (in press).
16.
G. M. Keating, Short term cyclic variations and diurnal variation of the venus thermosphere and exosphere, J. Geophys. Res., 1980 (in press).
17.
H. A. Taylor, Jr., Observed solar geomagnetic control of the ionosphere: implications for reference ionospheres, Space Research XII, Akademie—Verlag, Berlin 72, 125, 1972.
18.
H. A+ Taylor, Jr., H. G. Mayr, S. L. Hsieh, and C. R. Cordier, The signature of H in the equatorial anomaly: an empirical model, Revs. Geophys. Space Phys., 16, 276, 1978.
Venus night-
02731177/81/03010037$05.OO/O
TEMPORAL AND SPATIAL VARIATIONS OBSERVED IN THE IONOSPHERIC COMPOSITION OF VENUS: IMPLICATION FOR EMPIRICAL MODELLING H. A. Taylor Jr., S. J. Bauer, R. E. Daniell, H. C. Brinton, H. G. Mayr and R. E. Hartle NASA/Goddard Space Flight Center, Greenbelt, Maryland, USA
ABSTRACT In Situ measurements of the thermal ion composition of the ionosphere of Venus have been obtained for a period of two Venus years from the Bennett rf ion mass Spectrometer on the Pioneer Venus Orbiter. Ion measurements within an altitude interval of 160 to 300 kilometers, corresponding to an overall latitude interval of about _40 to 34°N, are assembled from the interval December 1978 to March 1980. This time interval corresponds to two revolutions of Venus about the Sun, designated as two “diurnal cycles”. The distributions of several ion species in this data base have been sorted to identify temporal and spatial variations, and to determine the feasibility of an analytical representation of the experimental re; suits. Th~first results from the sorting of se~era~ promine~tions including 0 02 , and H and several minor ions including CO 2 , C , and H2 reveal significant diurnal variations, with superimposed modulation associated with solar activity and solar wind variations. The diurnal variation consists of strong day to night contrast in the ion concentrations, with differences of one to two orde~s o~magn$— tude,~dependingupon ion mass and altitude. The concentrations of 02 , 0 , CO2 and C peak throughout the dayside decreasing sharply at the terminators to night— side levels, lower by one to two order~of mag~ituderelative to the dayside. The diurnal variations of the light ions H and H7 peak during the night, exhibiting asymmetric nightside bulges favoring the pre—aawn sector, near 0400 solar hour angle. Superimposed upon the diurnal distributions are modulation signatures which correlate well with modulation in the F10 ~ index, indicating a strong influence of solar variability on the ion production ah~distribution. The influence of solar wind perturbations upon the ion distributions are also indicated, by a significant increase in the scatter of the observations with increasing altitude as higher altitudes, approaching 300 kilometers, are sampled. Together, these temporal and spatial variations make the task of modelling the ionosphere of Venus both very interesting and challenging. INTRODUCTION The first in situ exploration of the composition of the ionosphere of Venus began on December 5, 1978, with the insertion of the Pioneer Venus Orbiter (PVO) into orbit about the planet. Since encounter, the PVO has orbited the planet at a rate approximately equal to one earth day (Period 24.03 hours). The Bennett rf ion mass spectrometer (OINS) instrument on the PVO continues to provide measurements of the thermal ion composition on a daily basis. The purpose of this paper
37
38
1-LA. Taylor et al.
is to describe some of the first results of an examination of the global characteristics of a selected portion of the ion composition and its variation with respect to temporal and spatial parameters. The OIMS provides information on the bowshock, the ionosheath, the ionopause, and the detailed composition of the main body of the ionosphere. The instrument measures directly the composition and concentration of thermal ions, and in addition contributes data on superthermal plasma and ion drifts. Details of the instrument operation are given elsewhere (Taylor, et al., 1980 a [1]). Earlier papers have provided results on the overall characteristics and aeronomic implications of the ion composition, including day to night variability (Taylor, et al, 1979 a [2]; Taylor, et al., 1980 b [3]; Bauer, et al., 1979 [4]; Nagy, et al., 1980 [5]), and evidence and implications of strong interaction between the solar wind and the ion composition (Taylor, et al, 1979 b [6]; 1980 b [3]; 1980 c [7]; Hartle, et al., 1980 [8]). In general, these results have shown the Venus ionosphere to be both noticeably repeatable (e.g., at lower altitudes on the dayside), and highly responsive to variations in the solar wind (e.g. at higher altitudes, and throughout the nightside). Owing to this variability, in order to identify underlying aeronomic characteristics of the ion composition and its structure, it is necessary to examine a large body of data, and to sort out the observations as a function of temporal and spatial parameters. In this paper, we describe early results obtained from the sorting process, which will subsequently be used to provide inputs required for determining the feasibility of representing the composition of the Venus ionosphere by an empirical model. EMPIRICAL DATA BASE The data which comprise the empirical model data base were accumulated in the period December 14, 1978 to March 8, 1980 during the first two revolutions of Venus about the sun. The sidereal period for one revolution of the planet about the sun is about 224 earth days, during which, due to its retrograde rotation, a fixed point on Venus rotates relative to the sun approximately twice. In the present work planetary variations (e.g., longitude effects) are not considered, and a single Venus revolution about the sun is designated as the “diurnal” cycle. In terms of time at the earth, the first and second diurnal cycles at Venus correspond to the periods December 5, 1978 to July 26, 1979 and July 27, 1979 to March 8, 1980. Of the 448 orbits which occurred during the two diurnal cycles, data from a total of 310 orbits were inserted into the data base for the empirical model. The remaining orbits were omitted for a variety of reasons, including gaps in coverage, incorrect instrument mode, and inappropriate spacecraft data format for tie OINS. During the second diurnal cycle, a significant gap in the data coverage in the dusk region of the planet (19—22 hrs SHA) results from a period of superior conjunction when spacecraft operations and data recovery were precluded for an interval of 37 days (orbits). In the same cycle, a smaller gap (15.30 — 16.30 SHA) results from incorrect instrument mode configuration. The PVO orbit is nearly polar, with an inclination at insertion of 105°relative to the equator. Periapsis was initially located at a latitude of about 17°N,with the in~oundleg approaching the planet across the north polar region and with the outbound leg crossing the equator and exiting from the planet across the south polar region. Periapsis altitude drifts upward, and is periodically lowered by on— board propulsion so that the height of periapsis averages to about 160 km. Because of excursions in periapsis height, the latitude of selected orbit heights on the Inbound and outbound passes varies by a few degrees, as will be discussed in the results. In selecting the data for the model, the majority of all available orbits were included, even though on limited occasions data Is obtained for either only the inbound or outbound passes. This Imbalance, however, applies to only about 10% of the orbits within the total, so that no significant bias is introduced.
Ionospheric Composition of Venus
39
Th~ion~entered ~n the data base are J4+, H
2+, He+, O~, C+, N+, 0+, 0l8+~CO+(N2+) NO , 0~ , and CO2 , including the prominent and secondary species measured throughout the ionosphere. Measurements at the mass positions 17, 24, 40, and 56 AMU are not included in the sorted results, on the basis of either infrequency of observations or current lack of evidence sufficient to firmly establish the presence of the particular i~n. ~n t~epresent study, the so~ting~rocedure+is applied only to the major ions 0 , 02 , H , and the minor ions, C , CO9 , and H9 as a preliminary test of the sorting and analysis procedure. Subsequent work wili extend to the other ions, including the lowest concentration constitutents, for which sorting parameters may require further adjustment and fine tuning. To construct the data base, inbound (15° to 34°N)and outbound (—3°S to 15°N) observations were assembled within the altitude range of periapsis to 300 km. These data were in turn sorted according to latitude, altitude, and diurnal cycle, as a function of solar hour angle (SHA), using intervals for these parameters found to be most appropriate from examination of preliminary sorting results. OBSERVATIONS FROM SORTING DATA BASE Diurnal Variations. In general, the results of the measurements of diurnal variations of the ion composition reflect a prominent, stable distribution of ions throughout the dayside relative to a reduced, highly irregular nightside distribution, consistent with earlier observations based upon more limited data (Taylor, et al., 1980 b [3]). The contrast in day to night variability+is exemplified in the distribution of the prominent upper ionosp~ereconstitutent 0 , shown in Figure 1. In this figure, the diurnal variation of 0 observed at 220 km (+ 2 km) is constructed from observations from both inbound and outbound passes, irrespective of latitude, and the results from both diurnal revolutions are combined. ALT: 220km(±2km)
PV
OIMS
REV: Iond2
LAT:
—
21—30°N l~— 9°N
10
I::! ~
~
—
7
~
= ~ ;i: 10 Z
7~4~7~ —
~Io
3.~’
7*
~‘
~
~r
7
,~ ‘7
(j
7
—;7
~ lot 100
—
I
12
jig
24
SOLAR4HOUR ANGLE Fig. 1 The diurnal variation of 0 observed for the period of two planetary revolutions of Venus about the Sun, occurring between December 1978 and March 1980. Data from latitude intervals of 21° to 30°Nand 10° to 9°N are combined over the altitude interval 220 km + 2 Ion. Relative lack of observations in dusk—midnight sector results from data gap during superior conjunction.
40
1-l.A. Taylor et
al.
N~ar the dawn and dusk terminators, the day to night decrease in concentrati~nof 0 is s~en to be approximately an order of magnitude, 4dropping3from about 10 ions/cm during the day to an upper limit of about 10 ions/cm during the night. Through the night, the degree of variability in concentration is intense, exhibiting a range of two orders of magnitude, w~ile the dayside variation is only about a factor of two or three. In the case of 0 , the altitude of 220 kin (-I- 2 kin) is selected for best data coverage, and least evidence of scatter; a similar tuning of the sorting altitude range is applied to each ion distribution which fo~lows. 4he diurnal variations in the concentrations of two other dominant ions÷ 0 , and H are given in Figures 2 and 3, respectively. The distri~utionof302 a~200 km pe~ks on the dayside at a concentration of about 5 x 10 ions/cm . In the case of 02 , the diurnal variation exhibited at 200 km (±2 kin) is of the order of 100:1, again with considerably more variablity exhibited during night relative to day.
ALT: LAT: P.V.
OIMS
REV: I and2
—
200km(±2km) 20°— 30°N 1°— 9°N
10
~
10
~ThT~
0
~
12 SOLAR HCUP ANGLE
iS
24
Fig. 2 The diurnal variation of 02+ for the same conditions as Figure 1, except altitude window is 200 km + 2 kin, and latitude intervals are 20° to 3O°Nand 1°to 9°N. The light ion H+, sorted at 2~Okin (±2 km) has a diurnal distribution essentially anticorrelated with that of 0 , with strong concentrations during the night relative to the day. The extreme in the diurnal variation of a factor of approximately 100 to 1 occurs between the pre—dawn region and the n~onquadrant and reflects a pre—da~nbulge ~n the nightside distribution, wher~H reaches a maximum of about 5 x 10 ions/cm The ratio of the dawn to dusk H concentration observed at 220 km is approximately 5:1 in favor of the pre—dawn region, although the average density level in both of the nightside regions is difficult to characterize due to the considerable variability encountered ther~. As in the case of each of the other ions examined, the variability in the H concentration is typically greater during the night, than the day.
Ionospheric
PV 1
OIMS
Composition of Venus
REV: Iand2
41
ALT:
220km(~2km)
—9
N
-~
H
-
Ii
io~_J’
1~I~~~%,
~
~
~510
I
o~~1
____
____
10
I
0
6
12
18
24
SOLAR HOUR ANGLE
Fig. 3 The diurnal variation of H+, for the same conditions as Figure 1. As a test of the resolution capabili~y of th~sorting procedure for examining minor ion species, the measurements of CO2 and H2 are sorted, and the resulting diurnal variations are illustrated in Figures 4 and 5, respectively. In the cage of 112+, sorted at 220 km (±2 Ion) the dawnside bulge in the distribution of H is also exhibited in the distribution of this trace ion, as shown in Figure 4. Allowing for the large variability superimposed on+the distribution, a ratio of at least lO:~is ascribed to the concentra~ionof H2 , between dawn and dusk. The peak in 112 concentration of about 30/cm occurs near a solar hou~ angle of 04 hours in approximately the same positio~as the pre—dawn peak in the H distribution. Near noon, the concentration gf 112 frequently drops below the instrument sensitivity limit of about 1 ion/cm The diurnal variation of the minor ion CO9+, shown in Figure 5, is relatively stable,3exhibiting a rather uniform daysi8e distribution averaging about 5 x 10 ions/cm , and dropping by two orders of magnitude or more at the terminators. For this ion, which has a relatively low scale height, the altitude range of 180 (±2 kin) is used to obtain a reasonably smooth distribution. The dayside CO2 distribution exhibits evidence of some modulation, which is also evident to a varying degree in the other ions. This modulation is discussed in a later section. Variations Associated with Latitude and Altitude. In order to identify one example of the degree of ion concentration+variations associated with latitude, we compare the combined diurnal profiles of 0 at a fixed altitude of 220 km (+ 2 kin), for the two separate ~atitude bands traversed on inbound and outbound passes. In Figure 6, profiles of 0 are separated into inbound (latitude 21° to 30°N)and outbound (latitude = 1° — 9°N) groups to illustrate latitude related effects. Within each latitude band, small latitude variations from orbit to orbit (of at most 3 — 4°) are not yet considered in the sorting.
42
H.A. Taylor et a7~.
ALr: P.V~ CIMS __________
=
I8Okm(t2km)
~
REV: I and2 __________
19°— 30°N 0°— 9°N
____
+
CO2
clot
41
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z o
to
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z
2 ~
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-
~
F
F
F
~
F
FF~W
FF
~ F~ F~F
r
100
ThT~m~rnfl~ 6 12 -
18
SOLAR HOUR ANGLE
24
Fig. 4 The diurnal variation of CO + for the same conditions as Figure 1, except altitude window is ~8O kin + 2 Ion, and latitude intervals are 19° to 30°Nand 0°to 9°N.
P.V
OIMS
REV: I and2
ALT:
220km(±2km)
LAT:
21°— 1°—30°N 9°N
—
+
-H °~ U N
Z 2 ~j10 ~
a
:0 (1)
-~
2
~
a
z I-.
—~
°~ F---
--ia~ -
10
~
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I~
-~
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2~r~2~
a
~
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(2
al
2 2
2
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—r
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Z
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~
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~
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aj
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~
~\~2
-
2 2,1 22 2 2
2 2 22è2~
22 2
Ii~
~°
~2
2 °~-~i~mff~iflThT 18
24
SOLAR HOUR ANGLE
Fig. 5
The diurnal variation
of 112+ for the same conditions
as Figure 1.
Ionospheric Composition of Venus
43
Considering first the expected consequence of the variation in solar zenith angle (SZA) as a function of latitude, we note that across the dayside, where the rela— + tively stable ion distributions permit comparison, the average concentrations of 0 in the 21°— 30° latitude interval are estimated to be lower by about 10 — 20% relative to the corresponding ion concentrations at 4 — 9°latitude. This relationship is associated with a difference of about 20° SZA near the sub—solar point, between these latitude regions.
P.V. OIMS
ALT: 220 kmfi2km) LAT AS SNOWN
REV: I and2 0
0
21 —30 LAT
~ INBOUND
~ ~
‘~‘
(0~ ~
,~,
~
o~
,
~
lot -
(.1) -
z 0
10
~
to:
OUTBOUND
i°—90 LAT
~ ~
ID’
100
-
I
I
I
I
I
l~2 I
I
I
1
118
I
I 24
SOLAR HOUR ANGLE
Fig. 6 A comparison of the diurnal variation in 0+ observed at the separate latitude intervals: 21° to 30°Nand 1°to 9°N. The altitude range is 220 km + 2 km. The diurnal period corresponds to the first Venus—Sun revolution, in the interval December 1978 to July 1979. The solar zenith angle at noon is about 27° for the upper distribution and about 6°for the lower distribution. A second aspect of latitude related variations is the possibility of spatial and temporal effects in the coupling of the solar wind with the ionosphere. Further inspection of Figure 6 reveals that, aside from the foregoing SZA effect, the structure of the mid and low latitude 0 profiles is generally quite similar, indicating that in this sample, the degree of interaction between the solar wind and the ionosphere is relatively similar in the two latitude regions. It may also be
44
H.A. Taylor et al.
assumed from the observed similarities that there are no pronounced planetary latitudinal effects, such as might result from anomalies in the distribution of an intrinsic magnetic field, were such to exist. Although not illustrated, the distributions of some of the other ions also indicate a lack of pronounced latitudinal variations. However, more sorting and analysis of the data will be required to support this initial observation. Although sorting and analysis of the data base is not complete for all ions, a feature typical of all species examined is the significant increase in variability in concentration encountered with increasing height. The height dependence÷of the variability is illustrated in Figure 7, where the diurnal distribution of 0 is illustrated for the height ranges 220, 260 and 298 kilometers (±2 km). As the altitude of sampling is increased, the variability increases dramatically across the dayside until at the 300 km level the dayside variability becomes equivalent to that observed at night. Similar trends are observed for other ions not shown. ALT: AS
P.V.
OIMS
REV: I
SHOWN
LAT: -~s—o N
106
~ o~
298km (±2km)
I 3 10
~
~ ~
4
~
-~ ~
‘ 7
‘
-I
‘
102J
a
~,
~
I
v’:’
‘
~,‘
-
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i~
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~
~
‘j’~ a
10
4
1 ,
F~
10~ ~
8
iø
Z 0
~
~
-
I
~:
‘
220km ~(±2km)
106
10
-~j
~
-1 r
,
~,,i,,
l~ ~
~
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~ ~0
777
7 ,
, ‘~ ‘,
-
2~4
-
260km ~,(±2km)
~TTI
‘
I
I
ITrFi
24
SOLAR F-FOUR ANGLE
Fig. 7 An indication of the increase in variability in 0+ concentration on the dayside, as a function of increasing altitude, for the levels 220, 260 and 298 km (±2 kin). Observations are from the —3° to 9°Nlatitude interval of first diurnal revolution (December 1978 to July 1979).
Ionospheric
Composition
45
of Venus
Temporal Variations. In comparing data from the first and second diurnal cycles, temporal variations are observed which significantly modify at least the dayside portions of the distr$buti~nsof+certai~ions, as shown in Figures 8 and 9. The dayside profiles of 0 , 02 , CO 2 and C shown in Figure 8 exhibit a distinct modulation with relative maxima near 7, 10, and 13 hours SHA. This modulation appears to be a continuation of a perturbation sequence which first became noticeable near noon in the first diurnal cycle, shown in Figure 9. This modulation, which features concentration differences as large as a factor of 2 — 3, is evident in the distributions in each of the latitude intervals sampled, although the amplitude of the modulation appears to vary as a function of both mass and latitude. Although
P.V. OIMS
REV: 2
-~
220km (±2km)
P
I’ —7°N
0
10~ I0~
~
+
-
200krn(±2km)
a
10
3
F
b~,1~f
I
~÷
~.
~f’~ ~22Okm’±2km) ~I°—7°N 1
5
o2
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!b0
180km (±2km)a ~
~
~
~ ~
~to2
II—-
IO7 I
-.-~‘.
•L
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~
.. S..-
-too 200
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M
SOLAR HOUR ANGLE
Fig.~8 ~odu4~tion of ~he dayside portions of the diurnal profiles of 0 , 0~ , C , and CO 9 for the second diurnal revolution, corresponding to July 1979 to March 1980. Vertical lines indicate relative peaks and troughs in concentrations+and ~ssocia~ed variations in the solar F10 ~ index. Observations ~f 0 , 02 and C are from the latitude interval 10°to 7°N. The CO2 results are from 14° to 22°N. Altitude levels vary according to mass, as noted. The data is plotted beginning with orbit 235 (July 29, 1979) at 1700 SHA, with orbit number and date increasing with SHA to orbit 440 (February 17, 1980), at 1500 SHA. The data gap between about 1500 SHA and 1700 SHA results from incorrect instrument configuration; the gap between about 1900 SHA and 2300 SHA results from superior conjunction. The F10 7 data taken at earth has been transformed to the location of Veniis, as described in text. The F107 magnitudes shown are those observed at earth.
46
H.A. Taylor et al.
the phasing of planetary longitude relative to solar hour angle has not been taken into account, it seems clear from the period of the observed modulation in the ion concentrations that the source of the modulation is a temporal and not a planetary inherent effect. Although the modulation observed in the ion distributions during both the first and second diurnal revolutions is complicated by short term orbit—to—orbit variations and small latitude variations resulting from orbit periapsis movement, it is possible to estimate the period of the modulation across the dayside. In Figures 8 and 9, the indicated approximate positions for successive peaks and troughs in the concentrations shows that the source responsible for the variation is varying with a period of about 27—28 earth days, approximately the rotation period of the Sun. In order to relate the observed diurnal modulation to possible solar effects, we have also included a plot of the F 10 ~ solar radiation parameter in Figures 8 and 9. To compare the F107 data (takenat earth) with the OIMS data (taken at Venus)
P.V. OIMS
REV: I
~~~~2Okrn(±2km)
,0
a1O
T
+
~
ID~
200km (±2km)
02
3°—9°N
P
220 km (±2km) 10~
o2
CO2
(±2km) =
~
‘~,I5°—23°N
F107 •
III
-
I~ ~
I~Il
2 0~
~,o2 300
2Nz ~
T
~
. ~.
•
...
200
~ ~
111112111111181111
-
IO22__~_
100
M
24
SOLAR HOUR ANGLE
Fig. 9
Modulation of diurnal variations of 0~, 02+~C+ and CO
2~, for the first diurnal revolution, corresponding to December 1978 to July 1979. The data is plotted beginning with orbit 10 (December 15, 1978) at 1700 SHA, and orbit number and date increase with SHA to orbit 234 (July 27, 1979) completing the cycle at 1700 SHA. Since the F10 ~ fluxes on the first and last orbits of the cycle are different, there is a discontinuity~in the ~lO ~ at 1700 SHA, which is also possibly indicated in the 0 and CO2 • prcfiles.
Ionospheric Composition of Venus
47
the relative positions of the earth and Venus must be taken into account. This was accomplished by assuming that the solar activity was localized in solar longtitude so that a particular active region seen at earth on a given day would have been seen a number of days earlier (or later) at Venus. Thus, to determine the flux corresponding to an OIMS observation at a particular hour angle, the angular separation of Venus and earth at the time of the 01145 observation is determined. This angle, combined with the relative angular velocities of the earth, Venus, and solar surface, determines how much earlier (or later) the same face of the sun would have been seen at earth. (No attempt has been made to account for the temporal variation of solar activity between its observation at earth and its impact on the Venus atmosphere.) In this manner, we have been able to plot the daily earth based F 10 ~ flux data at 15 minute SHA intervals on Figures 9 and 10, revealing the excellent correlation between the 27 day modulations in solar activity and the ion concentrations. ALT 2ookmI~2km)
P.V. OIMS
REV: 2
LAT4—26N
in F
H I
io~ 2 2
~ I~7~’ ~
~ ‘, A~ II,
— —
+
j1,I1
II
~till
‘I
t~
I
,
Io
~
_~IoI
100
...,
-
300 ‘10.7
0
I
I
1
1
I
.../• ......... 11 Il........... 12 1 II Ill SOLAR HOUR ANGLE
•~—.I
18 I
•...— III
....
w
I~.24200 100
Fig. 10 The diurnal variation of H+ for the second Venus/Sun revolution, with the corresponding variation of F 10 ~ index. Plot constructed as in Figure 8.
Relative to the dayside, it is more difficult to identify possible modulation ef— fect~ in the variable nightside ion distributions. As an example, the distribution of H for the second diurnal revolution is plotted with the corresponding F10 7 distribution in Figure ~O. It may be seen that there is a subtle hint of an anti— correlation between n(H ) and F10 7 at several intervals across the dayside and a possible indication of a positiveS correlation near 0600 SHA where the abrupt upsurge occurrs in F10 ~• These observations are obviou~1yprecarious, however, in view of the consideragle variability involved in the H distribution.
48
H.A. Taylor et al. DISCUSSION AND INTERPRETATION
The global features of the variation of the ion composition of the Venus ionosphere are characterized in two general categories: spatial (latitudinal, altitudinal) and temporal (Diurnal, short term) variations. In the case of the spatial variations, we note that neither the latitudinal nor the altitudinal variations in the ion distributions sampled appear to deviate significantly from general theoretical predictions made prior to the in situ exploration of the ionosphere. With respect to altitude effects, the increased variability of 0+ as a function of altitude on the dayside illustrates that as one moves upward above the region dominated by chemical equilibrium, the upper ionosphere becomes increasingly subject to interaction with the solar wind. As a consequence, the compressive and expansive forces generated by the solar wind interaction can result in significant scale height variations (Hartle, et al., 1980 [8]) which in turn produce an increasing scatter in the distributions of a given ion within a fixed altitude window. With decreasing altitude, the ion distributions should increasingly reflect the influence of the ion—neutral chemistry and variations in the neutral gas density. Although we have not yet performed a complete analysis of the height variations for each mass, it is likely that the present results include height variations associated with changing conditions in both solar radiation and solar wind parameters, as discussed later. Regarding latitude variations, again typified by the 0+ distributions (Figure 6) the results obtained from the first two diurnal revolutions are also not surprising in view of current theoretical concepts of ~he formation and maintenance of the dayside ionosphere. The concentration of 0 at 220 km, i.e., near its “peak”, which was identified earlier as an “F 2 ledge” (Bauer and Hartle, 1974 [9]), depends on both the photochemical production rate (q) and plasma transport processes. To a first order, the average daytime concentration near this peak should be related to cos ~ where X is the solar zenith angle (SZA) si~ce the photoionization process has this SZA dependence, w~ile the concentration n(O ) enters linearly in the balance equation, q = div [n(O )], where v is the plasma transport velocity. 0 (NoteThus, that for average the a photochemical level of equi1i~rium the 0 concentration layer, E or at F~iigher , N latitudes ~ i.e.,(21°cos — 30°) which reflects a 26° SZA should be correspondingly (o~10%) lower than that at 1° — 9°N with a 5° SZA, as found in the observations (Figure 6). Aside from the SZA effect, the general lack of any pronounced latitudinal variability in the ion profiles is attributed both to the relatively small latitude interval encompassed between the inbound and outbound measurements as well as the lack of important latitudinal variations in any intrinsic planetary magnetic field, if such exists.
x).
Considering the long term temporal variations, the diurnal variation exhibits important observational results relative to theoretical predictions. As indicated in earlier results from the OINS (Taylor, et al, 1979 a [2]; Taylor et al., 1980 b [3]) and as underscored by these more complete model results, the very significant concentration of ionization often observed at night is surprising in view of the long Venus night and absence of photoionization across the nightside. The variability of the nightside ionization is analogous to that observed in the auroral
regions of earth and as such, is supportive of arguments for both dynamic transport of dayside ionization as well as particle ionization as processes contributing to the maintenance of the nightside ionosphere. Owing to the highly structured ion profiles generally encountered at night, it is difficult to make definitive tests of competing transport and impact ionization hypotheses. Nevertheless, it has been shown (Taylor, et al, 1980 a [1]; Knudsen, et al, 1980 [10]) that ion flow velocities observed near the dusk terminator are of sufficient magnitude to provide the transport of ionization from the dayside necessary to maintain the observed nightside ionosphere. Such being the case, the variability in the solar
Ionospheric Composition of Venus
49
wind which drives the day—to—night transport is consistent with the high degree of variability observed in the nightside ionization. Such variability does not, how-ever, rule out the possible effects of impact ionization, and studies of these competing processes for maintaining the nightside ionosphere will continue to be pursued. Other significant ~eatures+of the diurnal variation include nightside bulges in th~ distri~utionsof H +and H The observed predawn density maxima in the species H and H 2 and also He (alt~oughnot shown) are in striking contrast to the re~ative— ly more symmetrical diurnal variations of the heavier ions. Assuming that H is in chemical e~ui1ibr4uinnear periapsis, Brinton et al., 1979 [11] have used measurements of H and 0 , combined with measurements from the PVO neutral mass spectrometer (ONMS) to determine the distribution of atomic hydrogen, which also peaks during early morning hours (4.00 LT). Moreover, the diurnal variation in neutral He (Niemann, et al., 1980 [12]) shows a similar behavior which is significantly different in character from the more symmetrical daytime maxima in CO2 and 0 around noon. On the basis of these results, it appears that at lower altitudes, the gross diurnal variations in the ionospheric composition reflect the behavior of the neutral atmosphere. This in turn,7can2be un~erstood if one postulates large eddy diffusion coefficients (K 3 x 10 cm sec ) and thermospheric superrotation rates wits periods between 5 and 10 days (Mayr, et al., 1980 [13]). The presence of the H2 bulge is also consistent with theoretical predictions for the distribution of neutral H2 on the nightside (Kumar et al., 1978 [14]) and is the subject of a companion paper in these proceedings (Kumar et al., 1980 [15]). Superimposed on the diurnal variation, the short—term 27 day modulation is interpreted as the result of variable heating and ionization of the neutral atmosphere, coupled perhaps with a variable interaction between the solar wind and the upper ionosphere. Evidence for the importance of the solar radiation as the perturbation source is provided by the fact that the phasing of the modulation is consistent with fluctuations of similar time scales in the F10 ~ flux. The observed correlation between F1 and the ion concentration is again a direct consequence of the important varia~iiityof the neutral (ionizable) constituents with changes in solar EUV flux. First, the ETJV radiation is ~ primary souce of ionization. For chemical equilibrium, the ion concentration N (X ) F~0~ .exp (K/F10 7)~ leading to a more 1ieionizing radiation and the concentration of neutral constituents involved complex dependence on ratio the solar flux due to tfie~actthat tl in production and loss processes vary with solar activity. The solar radiation is also a source of energy for the neutral atmosphere, where the temperature, T , measured by the ONMS, shows a 27 day periodicity in phase with F 10 7 [Niemann, pr~vatecommunication]. Any such change in T affects the neutrat ~as composition, which is closely tied to the photochemistry o~the ion composition. Owing to the complexity of these coupled effects, we have not yet made a quantative analysis of the response of the ion concentrations to the solar F10 7 variations. We note also that planetary wave motions have been identified in the neutral den— city measurements (Niemann, et al., 1980 [12]; Keating, et al., 1979 [16]), although the period reported for these planetary waves is about 5 — 6 days, which is well separated from the 27day periodicity observed in the modulation of the ion distributions. Although this secondary source of short term perturbation may in some instances couple with the solar radiation effects to perhaps modify the patterns in the ion and neutral distributions, a more refined analysis of the ion data and careful correlation with corresponding neutral data will be required to identify such presumably subtle effects. As indicated earlier, the nighteide ion variability makes identification of pos— ible modulation effects of the 27 day solar variation difficult. While there are hints of relationships between the solar variability and fluctuations in the
50
H.A. Taylor at al.
nightside ion distributions, such as might be suggested from the H+ distribution in Figure 10, we have as yet no firm evidence. Identification of such relationships would tend to support the case for ion transport as the principal mechanism for maintaining the nightside ionosphere. With this objective, we will refine the sorting and analysis of the data base in an attempt to confirm any such relationship in the nightside data. Venus—Earth Relationships and Empirical Modelling. The foregoing results reveal important differences in the ionospheres of Venus and Earth, which have significant implications for empirical modelling. First, the fact that the effect of the solar variation is rather easily identified in the ion distributions apparently relates in part to the relative uniformity of the lower ionosphere. In the apparent absence of an intrinsic magnetic field at Venus, the ion distributions are not regulated on a large scale, in contrast to the Earth ionosphere, where the strong dipole magnetic field significantly influences the ion distributions. At Earth, magnetic effects are sufficiently pronounced to produce distortions of seasonal variations in the ion distributions, leading to “solar—geomagnetic season” effects (Taylor, 1972 [17]), and to distortions of altitude and latitude variations, e.g., in the equatorial anomaly (Taylor, et al., 1978 [18]). Such distortions are evidently not present at Venus, and the lower ionosphere apparently reflects the solar variations in a more straightforward way than at earth. A second implication of the Venus—Earth relationships is that the upper portion of the dayside ionosphere and the overall nightside ionosphere of Venus are relatively more closely linked to the solar wind than in the case of Earth. As a consequence, these regions exhibit a degree of variability which is a particular challange for both data sorting and modelling techniques. In summary, the presence of both long and short—term variability observed in the ion distributions is a factor influencing the objective of and approach toward developing an -empirical model of the Venus ionosphere. The repeatable day—to—night gradients in the ion distributions appear to be adaptable to parametric modelling. This also applies apparently to the observed dayside latitudinal and altitudinal variations, provided the altitude range is selected to be within the less disturbed height regimes of the solar wind ionosphere interaction. The short—term variability ascribed to solar EUV interaction may also be modeled, although this will require a careful sorting out of both ion and neutral processes. On the other hand, very short—term fluctuations (day to day), such as noted within nightside variability are not practically amenable to modelling. REFERENCES
1.
H. A. Taylor, Jr., H. C. Brinton, T. C. G. Wagner, B. H. Blackwell, and C. R. Cordier, Bennett ion mass spectrometers on the pioneer venus bus and orbiter, IEEE Transactions on Geoscience and Remote Sensing, GE—l8, 45, 1980 a.
2.
H. A. Taylor, Jr., H. C. Brinton, S. J. Bauer, R. E. Hartle, P. A. Cloutier, R. E. Daniell and T. A. Donahue, Ionosphere of venus: first observations of day—night variations of the ion composition, Science, 205, 96, 1979 a.
3.
H. A. Taylor, Jr., H. C. Brinton, S. J. Bauer, R. E. Hartle, P. A. Cloutier, and R. E. Daniell, Global observations of the composition and dynamics of the ionosphere of venus: implications for the solar wind interaction, J. Geophys. Res., 1980 b (in press).
4.
S. J. Bauer, T. M. Donahue, R. E. Hartle, and H. A. Taylor, Jr., Venus ionosphere photochemical and thermal diffusion control of ion composition, Science, 205, 109, 1979.
.
Ionospheric Composition of Venus
51
5.
A. F. Nagy, T. E. Cravens, S. Smith, H. A. Taylor, Jr. and H. C. Brinton, Model calculations of the dayside ionosphere of venus: ionic composition, J. Geophys. Res., 1980 (in press).
6.
H. A. Taylor, Jr., H. C. Brinton, S. J. Bauer, R. E. Hartle, P. A. Cloutier, F. C. Michel, R. E. Daniell, Jr., T. N. Donahue and R. C. Naehl, Ionosphere of venus: first observations of the effects of dynamics on the dayside ion composition, Science, 203, 755, 1979 b.
7.
H. A. Taylor, Jr., R. E. Daniell, R. E. Hartle, H. C. Brinton, S. J. Bauer and F. L. Scarf, Dynamic variations observed in the thermal and superthermal ion distributions in the dayside ionosphere of venus, Advances In Space Exploration, 1980 c (in press).
8.
R. E. Hartle, H. A. Taylor, Jr., S. J. Bauer, L. H. Brace, C. T. Russell and R. E. Daniell, Dynamical response of the dayside ionosphere of venus to the solar wind, J. Ceophys. Res., 1980 (in press).
9.
S. J. Bauer and R. E. Hartle, Venus ionosphere: an interpretation Mariner—lO observations, Geophys. Res. Lett., 1, 7, 1974.
of
10.
W. C~Knudsen, K. Spenner, K. L. Miller and V. Novak, Transport of ionospheric 0 ions across the venus terminator and implications, J. Geophys. Res., 1980 (in press).
11.
H. C. Brinton, H. A. Taylor, Jr., H. B. Niemann and H. C. Mayr, time hydrogen bulge, Bull. Am. Astr. Soc., 11, 538, 1979.
12.
H. B. Niemann, W. T. Kasprzak, A. E. Hedin, D. M. Hunten and N. W. Spencer, Mass spectrometric measurements of the neutral gas composition of the thermosphere and exosphere of venus, J. Geophys. Res., 1980 (in press).
13.
H. C. Mayr, I. Harris, H. B. Niemann, H. C. Brinton, N. W. Spencer, H. A. Taylor, Jr., R. E. Hartle, W. R. Hoegy and B. M. Hunten, Dynamic properties of the thermosphere inferred from pioneer venus mass spectrometer measurements, J. Geophys. Res., 85, 1980 (in press).
14.
S. Kumar, D. M. Hunten and A. L. Broadfoot, Non—thermal hydrogen in the venus exosphere: the ionospheric source and the hydrogen budget, Planet Space Sci., 26, 1063, 1978.
15.
S. Kumar, D. N. Hunten and H. A. Taylor, Jr., H abundance in the upper atmosphere of venus, Advances In Space Explora~ion, 1980 (in press).
16.
G. M. Keating, Short term cyclic variations and diurnal variation of the venus thermosphere and exosphere, J. Geophys. Res., 1980 (in press).
17.
H. A. Taylor, Jr., Observed solar geomagnetic control of the ionosphere: implications for reference ionospheres, Space Research XII, Akademie—Verlag, Berlin 72, 125, 1972.
18.
H. A+ Taylor, Jr., H. G. Mayr, S. L. Hsieh, and C. R. Cordier, The signature of H in the equatorial anomaly: an empirical model, Revs. Geophys. Space Phys., 16, 276, 1978.
Venus night-