Solar irradiance in the extreme ultraviolet flux (EUV) has been monitored for 11 years by the Pioneer Venus Orbiter (PVO). Since the experiment moves around the Sun with the orbital rate of Venus rather than that of Earth, the measurement gives us a second viewing location from which to begin unravelling which irradiance variations are intrinsic to the Sun, and which are merely rotational modulations whose periods depend on the motion of the observer. Researchers confirm an earlier detection, made with only 8.6 years of data, that the EUV irradiance is modulated by rotation rates of two families of global oscillation modes. One family is assumed to be r-modes occupying the convective envelope and sharing its rotation, while the other family (g-modes) lies in the radiative interior which as a slower rotation. Measured power in r-modes of low angular harmonic number indicates that the Sun's envelope rotated about 0.7 percent faster near the last solar maximum (1979 thru 1982) than it did during the next rise to maximum (1986 to 1989). No change was seen in the g-mode family of lines, as would be expected from the much greater rotational inertia of the radiative interior

Hoegy, Walter R.

Wolff, Charles L.

English

NASA Technical Reports Server

SOLAR IRRADIANCE OBSERVED FROM PVO 
AND INFERRED SOLAR ROTATION 
Charles L. Wolff and Walter R. Hoegy 
NASA Goddard Space Flight Center 
Greenbelt,MD 20771, U.S.A. 
ABSTRACT 
Solar irradiance in the EUV has been monitored for 11 years by the Pioneer Venus Orbiter 
(PVO). Since the experiment moves around the Sun with the orbital rate of Venus rather 
than that of Earth, the measurement gives us a second viewing location from which to 
begin unravelling which irradiance variations are intrinsic to the Sun, and which are 
merely rotational modulations whose periods depend on the motion of the observer. 
We confirm an earlier detection, made with only 8.6 years of data, that EUV irradiance is 
modulated by rotation rates of two families of global oscillation modes. One family is 
assumed to be r-modes occupying the convective envelope and sharing its rotation, while 
the other family (g-modes) lies in the radiative interior which has a slower rotation. 
Measured power in r-modes of low angular harmonic number (I 5 7) indicates that the 
Sun's envelope rotated about 0.7% faster near the last solar maximum (1979 thru 1982) than 
it did during the next rise to maximum (1986 thru 1989). No change was seen in the g-mode 
family of lines, as  would be expected from the much greater rotational inertia of the 
radiative interior. 
THE EUV DATA 
The Langmuir probe on the Pioneer Venus Orbiter (PVO) measures a photoemission 
current which is a proxy for the solar extreme ultraviolet flux (EUV) when PVO is outside 
the ionosphere of Venus (see Brace et al., 1988). The measured flux covers the wavelength 
range from about 30 to 130 nm. A measurement of the quantum efficiency of the probe 
surface material, Rhenium, performed a t  the National Institute of Standards and 
Technology, showed that the contributions to the measured photoelectron current are: 
56% (from H Lya, 121.6 nm), 28% from the continuum between 30 and 110 nm, 
4% (from He I1 a t  30.4 nm), 2% (He I, 58.4 nm), 2% ( 0  V, 62.9 nm), 
4% (C 111, 97.7 nm), 2% (H Lb, 102.6 nm), and 2% ( 0  VI, 103.2 nm). 
For details of the instrument see Krehbiel et al. (1980). Nearly 11 years of daily values of 
the flux were available, from day 343 of 1978 to day 328 of 1989, except for times of superior 
conjunction when data transmission from Venus was not possible. Brace et al. (1988) 
have given translations of the Langmuir probe current (Ipe, unit amps) into EUV flux 
in photon cm-2 s-' and also into the often used 10.7 cm flux value. They also discussed the 
utility of the Venus-based solar measurements for studies of how solar variability affects 
the ionosphere, atmosphere, and magnetosphere of Venus. A shorter segment of the EUV 
https://ntrs.nasa.gov/search.jsp?R=19910003152 2020-03-24T06:52:40+00:00Z
flux data was used in two earlier studies of solar periodicities: Hoegy and Wolff (1989), 
Wolff and Hoegy (1989, herein: "Paper I"). 
The upper panel of Figure 1 shows the 11 year record of solar EUV flux. To de-emphasize 
the larger fluctuations a t  solar maximum, we divided the data by i ts  400 day running 
mean to produce the more uniform sequence plotted below it. The lower panel i s  the data set 
that  will be Fourier-analyzed. 
First 4 Yrs. 
I I I I I I I I I 
79 80 81 82 83 84 85 86 87 88 89 
Year - 1900 
Figure 1--The upper panel shows the probe current which is proportional to solar EUV flux. It declines by 
about 35% after the maximum of the 11 year solar cycle. Dividing this data by its 400-day running mean 
produces the data set in the lower panel. 
FOURIER SPECTRUM 
Figure 2 shows a Fourier spectrum of the 11 year EUV data set. We plot amplitude (the 
square root of the power). The upper scales emphasize that a peak on this graph 
representing a solar rotation period will be seen a t  one period in the Venus experiment and 
a t  a shorter period if viewed from Earth, due to the orbital motions. However a peak caused 
by an intrinsic solar fluctuation happening a t  all longitudes, will be seen a t  the same 
period and frequency from any planet. 
Four features stand out in the observed spectrum. 
1. The broad cluster of peaks between 350 and 460 nHz is caused, in part, by solar surface 
rotation rates. The two tallest peaks may be due to prominent, long-lasting features 
fixed to the solar surface. If so, they would be seen from Earth as 27.0 and 28.0 day periods. 
2. Amplitudes near those two tall peaks appear to have a weak first harmonic; see cluster 
between 780 and 830 nHz. 
3. There is a steady rise in amplitude as frequencies decrease below 130 nHz. This is caused 
by the dense collection of g-mode beat frequencies identified in the sunspot record by Wolff 
(1983). The measured amplitudes would increase all the way to the origin except that our 
analysis suppressed the lowest frequencies when dividing by the 400-day running mean. 
4. As in Paper I, we attribute the very strong peaks near 105 nHz and 10512 to the lowest 
harmonic r-mode (I = 1). This determines the mean sidereal rotation rate of that mode, 
Vrl = -1.0 + 1.5 nHz, (1) 
and thus updates the values, 106 nHz and -1.5 + 1.5 nHz , found in Paper I using a shorter 
time interval. A characteristic of the observed spectrum is the aliasing of many lines by 
f105 nHz due to the nonlinear interaction between this r-mode and other oscillation modes. 
Finally, lines in the spectrum tend to be split by a distracting fine structure; the average 
spacing between maximums is only 4.1 nHz. A considerably larger average spacing 
(=I0 nHz) was found in an earlier analysis of most of this data (Paper I). The fine 
structure is caused by aliasing between strong signals in the two solar maximum periods 
contained in the present data set, whereas the older analysis terminated in 1987 and 
contained only one solar maximum. The insert on Figure 2 shows how the fine structure 
distorts the signature of the I= 1 r-mode. Without the ripples, the cluster of power between 
95 and 115 nHz might be a fairly smooth peak with a maximum a t  2F. The measured 
spectrum near 2F is consistent with an r-mode having two active longitudes per rotation 
(symmetry, S = 2) and having a larger amplitude near the 11 year solar maximums. We 
adopt this interpretation. The width of the cluster implies that the amplitudes are strong for 
roughly three years a t  a time and significantly weaker during the intervening solar 
minimum. The presence of power a t  F too shows that the S=l  component is not a t  all 
negligible for this mode. 
- Period a t  Earth,  if due  to  ro ta t ion  
100 50 30 25 20 15 
I I I 
200 400 600 800 
Frequency (nHz) Observed at Venus 
Figure 2 -- Fourier spectrum of the 11 years of normalized EUV flux shown on bottom panel of Figure 1. Below 130 nHz, the 
spectrum is influenced by g-mode beat frequencies and by the r-mode for f = 1 which causes very strong lines at F and 2F (see insert). 
THE INTERIOR MODES @-MODES) 
Beat frequencies from the rotation of g-modes in the Sun's radiative interior seem to have 
influenced the sunspot record of the last two centuries. Wolff (1983) observed the 
frequencies and determined the sidereal rotation rate of each mode, multiplied by a 
symmetry factor S whose value was either 1 or 2. Since then, the symmetry S = 2 has fit 
observations best and has been adopted. For I > 1, Table 1 lists the sidereal rotations found 
in 1983 for all nonlinear g-modes which our EUV data is able to resolve. The value for I = 1 
is less certain and was suggested in Paper I based on most of the EUV data. 
TABLl3 1 
Standing Solar g-modes 
Sidereal Rotation Main EUV Fluctuation Ex~ected 
I Frequency Period Frequency Period 
As pointed out in Paper I, we do not expect to see g-mode rotation rates easily a t  the surface 
because their amplitude declines exponentially in the convective envelope. Rather, we 
expect to see the results of eruptions stimulated when their active longitudes near the base of 
the convection zone rotate past those of prominent global modes in the envelope. Since the 
d= 1 mode in the envelope should be the most important for our experiment (which integrates 
over the full solar disk), the most detectible frequencies in the rotational range should be, 
vb = 2(vg - vrl) , (2) 
where Vg is the g-mode rotation rate (second column of Table I) ,  and the factor 2 is the 
symmetry. Imposing equation (I), fixes the frequencies Vb. The right side of Table 1 lists 
them and corresponding periods, vb-'. The seven frequencies plus their aliases a t  f105 
nHz make a set of 21 theoretical lines. We would expect to detect them all if there were not 
confusion from many other aliases and another family of modes in part of the same range 
(r-modes, below). Even so, only 5 frequencies (marked X) are not detected in the EUV 
spectrum, magnified on Figure 3. The detection criterion is that a theoretical line be closer 
to an obsereved maximum than to a minimum. The binomial expression gives probability 
of accidental a g r e e c e n t  as c 1% which confirms the detection of much higher statistical 
significance (Wolff, 1983) based on beat frequencies rather than rotation rates. 
The three theoretical lines with frequencies <500 nHz are due to the g-mode for I= 1. They 
cause strong observed lines which seem to be broader and, therefore, more conspicuously 
subdivided by the fine structure. Perhaps, their actual effect would be modelled better by 
smooth peaks, about 10 nHz wide, fitted to the three clusters of observed power in the vicinity 
of each. Independent confirmation is needed that these are indeed due to the I= 1 g-mode. 
The periods in the right column of Table 1 should appear with identical values whether 
detected from Venus or Earth since they are intrinsic periods of the Sun. 
Figure 3--The degree of correspon- 20 
dence of g-mode frequencies (Table 1) 
with magnified portions of the E W  o, 
spectrum (Figure 2). Of the 21 lines, a 5 
only 5 (marked X) are not detected. c, .4 +
The g-mode spectrum appears 3 times 
due to aliasing at f 105 nHz caused by 
interaction with the lowest harmonic 
r-mode. 10 
0 
300 400 
Frequency (nHz) Observed at Venus 
600 700 800 
Frequency (nHz) Observed a t  Venus 
THE ENVELOPE MODES (r-MODES) 
Many observers have reported changes in the rate of solar surface rotation a t  a given 
latitude. To see if the entire convective envelope might vary its mean rotation with time, 
we analyzed data from three, partly overlapping four year periods. Figure 4 shows the 
Fourier spectrum of each. The centroid of the high amplitude cluster near 400 nHz seems to 
occur a t  lower frequencies during the last four years (bottom panel). This region is typical 
of solar surface rotation since a rotation period seen from Venus as (400 n ~ z ) - l  would be 
seen from Earth as 27.6 days. If some of this shift in the centroid is due to a slowing of the 
mean sidereal rotation, V, , of the convective envelope, then the sequence of r-mode 
rotation rates, 
is reduced proportionally. The frequencies expected a t  Venus are twice the synodic rates, 
Figure 4--Fourier spectrums of the first, 
middle, and last 4 years of EUV data. They 
are quite different because 4 years is not 
nearly long enough to capture the full 
complexity of solar behavior. Typical 
solar surface rotation rates fall within the 
broad cluster of peaks near 400 nHz. The 
cluster is shifted to lower frequencies in the 
last four years, suggesting a slower average 
rotation during that interval. 
20 4 Years starting 1978.04 
4 16 
12 
4 
n 
" 
0 200 400 600 800 
Frequency (nHz) 
0 
0 200 400 600 800 
Frequency (nHz) 
4 Years starting 1982.41 
I I I I 
24 - I I I I - 
- - 
4 Yeara starting 1986.88 - 
f 18 
t: 
4 
0 
0 200 400 600 800 
Frequency (nHz) 
and also the aliases, Vobs f 105 nHz. The sequence 2 5 15 7 gives a group of 18 lines to test 
whether Vm has changed. (Frequencies for I > 7 are too close to be resolved by this four year 
data set and the I = 1 mode is independent of a change in va, to first order.) Of the 18 lines, 
we detected respectively, 13  or 14 using the values of va, shown in Table 2. They are listed 
as sidereal rates and then converted to synodic values. 
Table 2 
Mean Rotation of Convective Envelope 
Observed Interval 
First 4 Years, 1978.9 - 1982.9 
Last 4 Years, 1985.9 - 1989.9 
Sidereal Rate 
(nHz) 
460.0 f 2. 
456.7 + 2. 
Apparent from Earth 
Rate Period 
(nHz) (davs) 
428.3 27.02 
425.0 27.23 
For an observer on Earth, the table shows that the mean rotation period of the convective 
envelope (as averaged by the r-modes) was 27.02 days during the first four years and 27.23 
in the last four years. For the last four years, the deviation of each observed line from a 
predicted line is shown on Figure 5; vertical separations are for clarity. The four lines 
that failed our detection criterion are the two most extremely left or right. The very high 
degree of clustering argues for the reality of the fit since randomly chosen frequencies 
would appear with equal probability anywhere along the horizontal range of 11 nHz which 
is the mean separation of maximums on Figure 4. 
These fits are the best we found and the only ones close to 27 days. But, if i t  is conceivable 
that the Sun's convective envelope could change its speed by 4% or more during the 11 year 
interval, then several other solutions are possible. They are of lesser quality and more 
likely to be statistical accidents but we can't rule them out from the EUV data alone. 
Range for random numbers ,-+ 
Frequency Deviation (nHz) 
Figure 5--Deviation of peaks in the observed spectrum from the 18 frequencies caused by the rotation 
of solar r-modes. Concentration toward the central half of the plot confirms the r-mode interpretation 
since an irrelevant model, or random frequencies, would populate the entire horizontal range. 
REFERENCES 
Brace, L.H., Hoegy, W.R., and Theis, R.F., "Solar EUV Measurements a t  Venus Based 
on Photoelectron Emission from the Pioneer Venus Lanpmuir Probe", 
J. Geophys. Res., vol. 93, no. A7,7282-7296,1988. 
Hoegy, W.R. and Wolff C.L., "A Seven Month Solar Cvcle Observed with the L a n v u i r  
Probe on Pioneer Venus Orbiter", J .  Geophys. Res., vol. 94, no. A7, 8663-8671,1989. 
Krehbiel, J.P., Brace, L.H., Theis, R.F., Cutler, J.R., Pinkus, W.H., Kaplan, R.B., 
"Pioneer Venus Orbiter Electron Tem~erature  Probes", IEEE Trans. Geosci. Remote 
Sens., GE-18,49,1980. 
Wolff, C. L., "The Rotational S~ec t rum of e-modes in the Sun", Astrophys. J., vol. 264, 667- 
676,1983. 
Wolff, C.L. and Hoegy, W.R., "Periodic Solar EUV Flux Monitored near Venus", Solar 
Phys., vol 123, 7-20, 1989, ("Paper I"). 


Solar Irradiance Observed from PVO and Inferred Solar Rotation

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server