A large number of solar maps were obtained with the use of Itapetinga 45 ft antenna at 22 GHz and 44 GHz. A statistical study of these maps, reduced using original techniques, permitted the establishment of the solar radius with great accuracy at the two frequencies. It is found that 22 GHz and 44 BHz radiation originates at 16,00 km and 12,500 km above the photosphere, respectively. Excess emission due to active regions was clearly identified at lower solar latitudes above and below the equator, extending up to 26,000 km and 16,500 km above the photosphere, at 22 GHs and 44 GHz, respectively

Costa, J. E. R.

Homor, J. L.

Kaufmann, P.

English

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SINGLE-DISH HIGH SENSITIVITY DETERMINATION OF SOLAR LIMB
EMISSION AT 22 AND 44 GHz
J. E. R. Costa, J. L. Homor, and P. Kaufmann
INPE: Instituto de Pesquisas Espaciais, CNPq, C.P. 515, 12.200 S_o Jos_
dos Campos, SP, Brazil
ABSTRACT
A large number of solar maps were obtained with the use of the Itapetinga 14-m (45-ft)
antenna at 22 GHz and 44 GHz in the 1978-1982 period. A statistical study of these maps,
reduced using original techniques, permitted the establishment of the solar radius with great
accuracy (<1.5 arc s r.m.s.) at the two frequencies. It has been found that 22 GHz and 44 GHz
radiation originates at 16 000 km and 12 500 km, respectively, above the photosphere. Excess
emission due to active regions was clearly identified at lower solar latitudes above and below the
equator, extending up to 26000 km and 16 500 km above the photosphere at 22 GHz and
44 GHz, respectively.
DISCUSSION
During 1978-1982 the Itapetinga 14-m (45-ft) radome-enclosed antenna was intensively
used in a large number of solar runs in connection with SMY and real-time operations with SMM
satellite measurements and by other observatories. Each observing session (a few days up to two
months) was carried on at 22 GHz and 44 GHz linear polarization, simultaneously, or at 22 GHz,
right- and left-handed polarization. Only one run was carried on at 22 GHz, two orthogonal
linear polarizations (May 1980). Details on the experimental setup are given elsewhere(Kaufmann
et al., 1982). During each run, at least one full-Sun map was obtained per day. Every full-Sun
map was obtained in a timescale relatively short (6 min) compared to timescales required by other
large antennas (1 to 2 hr). The sensitivity attained was very high: 10 to 20 K in antenna tem-
perature. The antenna half-power beamwidth was 4 arc min at 22 GHz and 2 arc rain at 44 GHz,
but the spatial definition of "hot spots" of more than 15 percent above the quiet Sun was better
than 15 arc s.
The present study was based on 353 full-Sun maps obtained at 22 GHz, and 79 maps
obtained at 44 GHz. Each map was obtained using the drift-scan technique, with scans in right-
ascension separated by 2 arc min in declination. For the study of the solar limb, we determined
one isotherm contour corresponding to the antenna temperature that was one-half of the value
of the most probable antenna temperature found in the solar disc (assumed as representative of
the quiet Sun). An arbitrary circle was adjusted by least-squares fit to the points that generated
that contour, and the center of the radio-Sun was determined. With this technique we eliminate
uncertainties due to any systematic pointing error. The radius was then determined as the mean
distance from the center to the coordinates defining the isotherm contour previously established.
This method was found to be highly accurate. In terms of solar photospheric radius Ro,
we obtained
- Solar radius at 22 GHz: (1.02305 + 0.00082) Ro
- Solar radius at 44 GHz: (1.01922 + 0.00135) Ro
201
https://ntrs.nasa.gov/search.jsp?R=19860015160 2020-03-20T15:25:56+00:00Z
or, correspondingly, the height above the photosphere of the 22 GHz emission was of 16 000 +
600 kin, and at 44 GHz it was of 12 500 + 1000 km, determined from 353 maps and 79 maps,
respectively. The corresponding accuracy was of less than 1.5 arc s r.m.s. These results have
greater accuracy and are consistent with measurements obtained by various authors at frequencies
above 10 GHz (Figure 1). It was possible to determine, for example, the apparent variation of
the solar radio-radius with the variation of the distance of the Earth to the Sun (i.e., the varia-
tion of the apparent size of the optical solar disc), as shown in Figure 2.
The solar limb features were analyzed in terms of departures from the mean solar radio-
radius, as it was described before. In Figure 3 we show the mean departures for 353 maps at
22 GHz, in solar heliocentric polar coordinates. It consists of one point every 0.5 deg (720 points
over 360 deg). The principal findings were the following ones:
- The contribution from active regions clearly concentrate around 18 deg North and
South, at both East and West limbs. A clear minimum occurs at the solar equator.
- The active regions' emissions at 22 GHz extend up to 26 000 km above the photo-
sphere (or up to 10 000 km above the radio-radius at 22 GHz).
- A "darkening" was noticed to be more pronounced at the south solar pole. No
physical interpretation has been attempted yet. It might be meaningful to point out that for the
353 maps used in this study, the southern pole was towards the Earth for most of them, with a
mean inclination of-1.2 deg for all the collection.
In Figure 4 similar data are shown for 44 GHz, for the solar limb departures from the
mean radio-radius. The plot is considerably noisier than the 22-GHz plot due to the smaller
number of maps used in the statistics (79). The 44-GHz emission due to active regions are suggested
at the East and West limbs, extending up to about 16 500 km above the photosphere (or up
to 4000 km above the radio-radius at 44 GHz). A depression or "darkening" is indicated near
the NE limb, for which no explanation has been found. The south pole limb seems to be slightly
"darker" than the north pole limb (the mean inclination was +2.0 deg for the 79 maps).
Finally, from the data shown in Figures 3 and 4, we confirm that the best estimate of the
solar radius can be obtained at limb coordinates corresponding to 45 deg West and East of the
poles, when the reference solar radio-radius is not established in advance (Furst, Hirth, and
Lantos, 1979).
Acknowledgments. q"nis research was partially supported by the Brazilian research agencies
FINEP and FAPESP. One of the authors (PK) is a Guest Investigator on SMM.
202
REFERENCES
Coates, R. J., 1958, Proc. 1RE, 46, 122.
Furst, E., Hirth, W., and Lantos, P., 1979, Solar Phys., 63, 257.
Home, K., Hurford, G. J., Zirin, H., and de Graaw, Th., 1981, Astrophys. J., 244, 340.
Kaufmann, P., Strauss, F. M., Schaal, R. E., and Laporte, C., 1982, Solar Phys., 78, 389.
Kislyakov, A. G., Kuznetsov, I. V., Kuznetsova, N. A., Chernyshev, V. I., and Serov, N. V., 1975,
Soy. Astron., 18, 612.
Labrun, N. R., Archer, J. W., and Smith, C. J., 1978, Solar Phys., 59, 331.
Pelyushenko, S. A. and Chernyshev, V. I., 1983, Soy. Astron., 27, 340.
Swanson, P. N., 1973, Solar Phys., 32, 77.
Wrixon, G. T., 1970, Nature, 227, 1231.
203
30000 I t I t I I I ! I ! I I I | I ! I
a HORNEET AL.(1981)
x LABRUN-ET-_. (1978) - ECLIPSE
I o SWANSON-TTg"73) ECLIPSE
KISLYAKOVET AL. (1975)
25000 I • COATES(195-BT--
PELYUSHENKOANDCHERNYSHEV(1983)
I + WRIXON (1970) ECLIPSE
? FDRSTET AL. (1979)
-I- • PRESEN'F-IIESIJLTS
1,,, 20000L_"-t-
O
°"3--t_.
,,, 15000
"-r
I--
"'
C)
° Tc:_ 10000
LO
"-r-
5000 _ _
0 ! _ I I I I I I t t I I ! | | I I -
10 1O0 1000
FREQUENCY(GHz)
Figure 1. Some determinations of the height above the photosphere of radio emission at frequencies above 10 GHz, reported by
various authors, indicated at the top-right comer, using different techniques. The present results are considerably
improved in accuracy, and fit well to the general trend in the range 10 to 70 GHz.
22 GHz
17.5 T 17.5 - 44 GHz
I 353 DATA POINTS 79 DATA POINTS
17.0--- __ 17.0
," ,.__
,_.__ _.,.._r.: ._ _._
1" o'_ % °t "l °%'_ • °
.. _ • . :,
•/,,,,,,._. t :.:_;. ..
lO.O _ 16.0 -
15.5 I I I 1 _5.5 I I I I
15.50 15.75 16.00 10.25 16.50 15.50 15.75 16.0D 16.25 16.50
PHOTOSPHERICRADIUS ( ARC14IN) PHOTOSPHERICRADIUS ( ARCHIN )
(A) (B)
Figure 2. Plots of the measured solar radio-radius (ordinates) against the apparent optical disc radius
t_ (abcissae), for data obtained at 22 GHz (A) at 44 GHz (B).
ON
MEAN SOLAR LIT':BAT 22 GHz (353 MAPS)
ACTIVE REGIONS _ E _ W
\)
::
, 10000 KM
," 16.4 . __ 1 MEANRADIUS (22 GHz)
16000 KM16.2
< !
16.D [ ] 111 [ __ OPTICAL RADIUS
N E S W N
POSITIONANGLE(a)
Figure 3. Mean departures from the mean solar radius at 22 GHz as a function of position angle referred to the optical disc
set at 16 arc min. Active regions' contributions clearly concentrate at about 18 deg North and South of the equator,
at both East and West limbs, extending up to 26 000 km above the photosphere. A "darkening" was evidenced
on the southern polar limb, which might be attributed to a net negative inclination of the Sun for the 353 maps.
NMEANSOLAR LIMB AT 44 GHz (79 MAPS) /_
17.D /CTIVE REGION_ E S_ W
_: 16.B
f,_..)
v 16.6-
t/')
= _
J IUS (44 GHz.O
_ 16.2
_ 0
POSITION ANGLE (a)
Figure 4. Mean departures, from the mean solar radius at 22 GHz, similarly to Figure 3. Data are "noisier" due to the smaller
number of maps (79). Active regions are suggested to be concentrated at lower solar latitudes and extending up to
o_a about 16500 km above the photosphere. A depression near the NE limb is of unknown origin.


Single-dish high sensitivity determination of solar limb emission at 22 and 44 GHz

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