Diurnal variations of atomic hydrogen in thermosphere derived from Explorer 32 data and correlated with solar cycle, solar rotation, and earth rotatio

Brinton, H. C.

Mayr, H. G.

English

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TEMPORAL VARIATIONS OF
THERMOSPHERIC HYDROGEN DERIVED
FROM IN SITU MEASUREMENTS
HENRY C. BRINTON
HANS G. MAYR
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PREPRINT
TBIPOPAL VARIATIONS OF THE-PMOSPH RIC HYDROGEN
DERIVED FRUM IN SITU MEASURETENTS
by
Henry C. Brinton and Hans G. Mayr
Laboratory for Planetary Atmospheres
May 1971
GODDARD SPACE FLIGHT CENTER
Greenbelt, Maryland
r
TEMPORAL VARIATIONS OF THERMOSPf ERIC HYDROGEN
DERIVED FROM IN SITU MEASUREMENTS
BY
Henry C. Brinton and Hans G. Mayr
Laboratory for Planetary Atmospheres
The atomic hydrogen content of the earth's thermosphere has been
investigated by a number of experimental techniques with only moderate
success. Direct mass spectrometer measurements have proved difficult
to interpret [Reber et al., 1970; Harpold and Horowitz, 19711. All
airglow techniques suffer to various degrees from the fact that they
measure integral effects; interpretation of the observations in terms
of hydrogen densities requires prior knowledge of the hydrogen global
distribution. In particular, Lyman-a is not a sensitive indicator
of thermospheric hydrogen concentration because of the optical thick-
ness of the exosphere and the consequent importance of high-altitude
scattering [Meier, 1970]. Ground-based observations of Balmer-a	 f
emission provide a better measurement of thermospheric hydrogen vari-
ations [Tinsley, 1970], but determination of absolute hydrogen	 t
concentration is difficult due to uncertainties in the intensity of
the solar Lyman-s line and in the integrated hydrogen content along
the line of observation.
This paper presents early results of a correlative study of
Explorer 32 data obtained in the period June 1966 - January 1967,
which has provided the first in situ determination of the temporal
1
iP
I
variations of thermospheric hydrogen. The measurements were made in
the altitude range 280-450 San, within a limited range of latitude
(23° to 47°) and longitude (-65° to -124°). The hydrogen concentra-
tion has been derived from the chemical equilibrium relationship
n (H) = 9 n(H+	 n(0)	 (1)
[Hanson and Ortenburger, 1961], using n(H +) measurements by the
Explorer 32 Bennett RF ion mass ,pectrometer (described in Brinton
et al., 1969). Simultaneous measurements of n  by two cylindrical
Langmuir probes on the same satellite (L. H. Brace, experimenter)
provided n(O+ ) values (in the altitude range of interest n  = n(O+)
to a close approximation). The values of n(0) were obtained from
the model atmosphere of Jacchia [1964], computed for the solar and
geophysical conditions existing at the time of each n(H +) and n  mea-
surement. The validity of the model for this time period, altitude
range, and geographic location has been affirmed by Explorer 32
density gage measurements, as reported by Newton [1970]. (A more
recent model [Jacchia, 1970] includes n(0) values which are at most
10a higher than those of the 1964 model.)
The upper panel of Figure 1 shows the n(H) values derived from
data obtained under quiet magnetic conditions (a p ; 10, six hours
prior to the measurement). To simplify subsequent analysis the
hydrogen concentrations have been normalized to an altitude of 350
1Jn, assuming a scale height of 600 km which corresponds to a temp-
erature of 900 0 , the average of the day and night values for this
period. (The exospheric temperature at the time of each measure-
F
i
2
ment has been calculated using the model equations of Jacchia [1964].)
i	
During the eight-month period of measurement the satellite orbit
phased through two diurnal cycles while maintaining almost constant
altitude at a given geographic location, a characteristic of the
Explorer 32 orbit which makes it ideal for study of tle diurnal vari-
ation in n(II). The progression of local time is indicated by the scale
at the top of Figure 1; note that local time decreases from left to right.
The daily and S4-day mean values of the 10.7 can solar flux for
the period of the n(H) observations are shown in the lower panel of
Figure 1. The overall decrease in n(H) during the period of observa-
tion correlates with the steady increase in solar activity; super-
imposed on this long-term behavior is a less pronounced periodic
variation of n(H) arising from the 27-day solar activity variation
associated with solar rotation.
A curve fitting technique has been used to resolve the complex
variation of n(H) shown in Figure 1 into a number of density compo-
If
nents, each of which is associated with a primary component of the
exospheric temperature. The expressions for the temperature compo-
nents are those of Jacchia [1964], and include contributions associ-
ated with 11-year solar cycle, 27-day solar rotation, diurnal varia-
tion, geomagnetic activity, and semi-annual variation.
The results of this analysis for the n(H) components associated
with solar activity are shown in the left-hand panel of Figure 2. The
light solid line represents the n(H) component due to 11-year solar
cycle; its behavior indicates that during the eight months of mea-
3
surement the hydrogen content of the thermosphere decreased by a
factor of approximately 2.5 due to the long-term increase in thermo-
spheric temperature associated with increasing solar activity.
Superimposed on the solar cycle component is the n(II) contribution
associated with the 27-day variation of solar activity due to solar
c	
rotation. This periodic component accounts for n(II) variations of
as much as a factor of 1.3, with hydrogen density again varying
inversely with the associated temperature component.
The c.iurnal component of the n(H) variation is shown, superimposed
on the solar cycle contribution, in the right-hand panel of Figure 2.
The diurnal effect is observed as approximately a factor of two
increase in n(H) between day and night. Further, the error in our
curve-fit analysis is minimized when a phase lag of approximately
three hours between the diurnal variation of model exospheric temperature
and hydrogen density is introduced. There also are indications that
the magnitude of the phase lag is different at night than during day,
as has been suggested by Patterson [1966]; this question is currently
under investigation.
Table 1 compares the n(H) diurnal variation determined from Explorer
32 with several models currently in use and with recently reported
airglow results.	 (The models chosen are for F 10.7 = 100.) The
quantities compared are the night-to-day thermospheric hydrogen den-
sity ratio and the diurnal density-temperature phase shift. The
Explorer 32 in situ observations are in considerable disagreement
with the COSPAR International Reference Atmosphere 1965 (CIRA, 1965)
4
and the Jacchia [1964] model. However, our results are within the
limit of four on the diurnal density ratio deduced from Lyman-a mea-
surements by Meier and Mange [1970] and are in excellent agreement
with the Balmer-a observations of Tinsley [1970] and with the theo-
retical predictions of Patterson [1966] and McAfee [1967] whose models
c	 include the effect of lateral hydrogen flow.
Our thermospheric hydrogen densities are approximately a factor
of three higher than those given by the Kockarts and Nicolet [1962,
1963] theoretical model. Prior [1971] has derived two high altitude
n(II) values from satellite drag data obtained in the period March-May
1967; his densities, derived for altitudes above 2300 km and for local
times near 0600 hours, also exceed the Kockarts and Nicolet values by
about a factor of three.
ACCURACY OF RESULTS
Determination of the relative variation of n(H) by the method
employed in this study is extremely accurate, since systematic errors
associated with instrument calibration and performance affect only
the derivation of absolute hydrogen density. The primary sources of
inaccuracy in the relative variations of n(II) are random factors such
%
as data reading error. The amplitude of this "noise" on the n(H)
variation, based on estimated uncertainties in n(H + ) and n(0+ ) and
substantiated by the curve-fit analysis, is +150. This random error
has been filtered out of the n(H) components plotted in Figure 2.
The possible inaccuracy of the absolute hydrogen density is
primarily due to an uncertainty of +25% in the experimental verifi-
r
5
cation of the model n(0) values 'Newton, personal conmiunication].
Inflight calibration of the ion spectrometer for fi + using ne data11
from the companion cylindrical Langmuir probe [Rrinton et al., 19691
eliminates possible errors due to mass discrimination in the spec-
trarieter and to aerodynamic effects in the satellite sheath. Further,
any systematic error in the probe measurement of n  is canceled out,
since it is the ratio n(li+)/n(0+) which enters in the derivation of
n(II). A small systematic error, of magnitude 0 to +100, arises £rori
the use of equation 1 when T i ¢ T  [Banks, 1967]. In summary, the
absolute uncertainty of our n(H) values is (+35/-25)0.
SUMMARY
The concentration of atomic hydrogen at 3S0 km altitude has been
derived from in situ measurements of ion composition and model n(0)
values; the selected measurements were made within a limited geo-
graphic region between June 1966 and January 1967. The variation of
n(H) between approximately 4 x 10 S and 8 x 104 cm -3 during this
period has been resolved into densit y contributions associated with
the primary exospheric temperature components. The results for the
three most significant components are summarized beluiv:
(1) Solar Cycle: The increase in average solar activity
(F 10 7) from 100 to 140 and resultant atmospheric temperature increase
caused a factor of 2.5 decrease in n(H).
(2) Solar Rotation: The 27-day variation in solar activity and
the corresponding temperature camponent were correlated with an
inverse n(1f) variation of as much as a factor of 1.3.
r
6
(3) Diurnal: 'Me cor.centration of hydrogen increased by
approximately a factor of two between day and night, with a phase
delay of about three hours bctlaeen model exospheric temperature and n(11).
Variations in n(H) associated with magnetic storms and *L^
atmospheric semi-annual variation are currently tinder investigation.
ACKNOI'IEDCMENTS
This work is the result of an extensive correlative stud y of
Explorer 32 data; the contributions to this effort made by Larry H.
Brace and George P. Ne%rttm are gratefully acknowledged.
L
7
f
REFFRINCES
Banks, P. Ni., The temperature coupling of ions in the ionosphere,
Planet. Space Sci., 1S, 77, 1967.
Brinton, H. C., R. A. Pickett, and H. A. Taylor, Jr., Diurnal and
seasonal variation of atmospheric ion composition; correlation
with solar zenith angle, J. Geophys. Res., 74, 4064, 1969.
COSPAR Working Group IV, COSPAR Into rnationa? Reference Atmosphere,
1965 (CIRA, 1965), North Holland Pub. Co. (Amsterdam), 1965.
Hanson, W. B., and I. B. Ortenburger, The coupling between the proton-
osphere and the normal F-region, J. Geophys. Res., 66, 1425, 1961.
Harpold, D. N., and R. Horowitz, Minor gases measured with the OGO-6
neutral mass spectrometer, paper presented at AGU Spring Meeting,
Washington, D. C., 1971, Trans. AGU, Sr, 294, 1971.
.Jacchia, L. G., New static models of the thermosphere and exosphere
with empirical temperature profiles, Special Report 313, Smithsonian
Institution Astrophysical Observatory, Cambridge, Mass., May 1970.
Jacchia, L. G., Static diffusion models of the upper atmosphere with 	 ^
empirical. temperature profiles, Special Report 170, Smithsonian
Institution Astrophysical Observatory, Cambridge, Mass„ December
1964.
Kockarts, G., and M. Nicolet, Le probli'me aeronomique de 1'helium et
de 1'hydrogene neutres, Ann. de Geophys., 18, 269, 1962.
Kockarts, G., and M. Nicolet, L'helium et l'hydrogene atomique au
tours d'un minimum d'activite solairr, Ann. de Geophys., 19, 370,
1963.
8
McAfee, J. R., Lateral flow in the exosphere, Planet. Space Sci., 15,
599, 1967.
Meier, R. R., Observations of Lyman -n and the atomic hydrogen distri-
button in the thermosphere and exosphere, Space Res., 10, 572,
1970.
Meier, R. R., and P. Mange, Geocoronal hydrogen: an analysis of the
Lyman-alpha airglow ohserved from OCO-4, Planet. Space Sci., 18,
803, 1970.
Newton, G. P., Latitudinal dependence of the diurnal density
variation, J. Geophys. Res., 7S, 5510, 1970.
Patterso:l, T. N. L., The diurnal variation of the atomic hydrogen
concentration at the base of the exosphere, Planet. Space Sci., 14,
425, 1966.
Prior, E. J., Hydrogen drag deduced from simmiltaneous measurements
near 2300 ]an and 2800 kin, paper presented at AGU Spring ?Meeting,
Washington, D. C., 1971.
Reber, C. A., D. N. Harpold, A. F. Hedin, N. W. Spencer, G. R. Carignan, 	 11
and D. R. Taeusch, Neutral atmospheric composition data from the
quadrupole mass spectrometer, paper presented at AGU Spring Meeting,
Washington, D. C., 1970, Trans. AM, 51, 586, 1970.
Tinsley, B. A., Variations of Ba Lner -a emission and related hydrogen 	 #
distributions, Space Res., 10, 582, 1970.
9
•.
LOCAL TIME (HOURS)
6	 24	 18	 12	 6	 24	 18	 12	 6
6.0
5.0
4.0
	 ,0 • •	 ^
3.0 •	 ^. 1	 .	 ..'	 1	 •
Z	 2.0	 '	 •	 •''	 •	 " • ' i
n(H)	 ..	 ' •	 '
x10 5 cm- 3 	 4ft	 •	 ^• .	 •
1.0
	 ._ . •
0.8
	 •
0.6
0.5
JUNE	 JULY	 AUG.	 SEPT. i OCT.	 NOV.	 DEC.	 JAN.
1966
	 1967
,on
i
160
SOLAR 140
Fio.7	 120
100
80
JUNE I JULY I AUG. I SEPT. I OCT. I NOV
1966
DEC. I	 JAN.
1967
Figure 1. Concentration of atomic hydrogen at 3S0 tan altitude,
derived from in situ measurements of ion composition
and model n(0) values, for period June 1966-January
1967. Variations of daily and 54-day mean values of
F10 . 7 solar flux during measurement period are also
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Temporal variations of thermospheric hydrogen derived from in situ measurements

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