Three small antennas will be added to the OVRO interferometer to form a five-element solar-dedicated array. This would provide up to 7 or 10 baselines (compared to the present 1 or 3). This would be sufficient to apply microwave diagnostics to most active region and burst sources. By using frequency-synthesis it would also provide an imaging capability comparable to that of an approximately 100 baseline interferometer. Expansion of the array is discussed

Gary, D. E.

Hurford, Gordon J.

English

NASA Technical Reports Server

N89-15862 i 
PLANNED IMPROVEMENTS 
TO TEE 
OWENS VALLEY FREQUENCY-AGILE INTERE'EROMETeR 
FOR MAX'91 
G. J. Hurford, D. E. Gary 
( C a1 t e ch ) 
SCIENTIFIC MOTIVATION FOR MICROWAVE SPECTROSCOPY ................................................ 
* MEASUREMENT OF CORONAL MAGNETIC FIELDS 
Gyroresonance opacity creates s p e c t r a l  f ea tu re s  t h a t  
can be used t o  accurately measure t h e  s t rength  of 
magnetic fields at t h e  base of t h e  corona. (Figure 1) 
This provides t h e  basis for a practical coronal 
magnetograph. 
* DIAGNOSTICS OF SOLAR EmRGETIC ELECTRONS 
Theoretical models of brightness  temperature spec t r a  show 
t h a t  microwaves can cleanly d is t inguish  between thermal 
and nonthermrl gyrotsynchrotron emission on the  basis of 
shape of t h e  spectra. The peak br ightness  tsmperature 
and peak frequency then def ine t h e  plasma and p a r t i c l e  
parameters. (Figure 2) 
98 
https://ntrs.nasa.gov/search.jsp?R=19890006491 2020-03-20T03:39:00+00:00Z
WHY HASN'T THIS BEEN DONE BEFORE ?? ................................... 
The diagnos t ic  content of microwaves is i n  terms of t h e i r  
b r ightness  temperature spec t ra  (surface b r igh tness ) .  
To measure br ightness  temperature a t  any frequency, it i s  
necessary t o  s p a t i a l l y  resolve t h e  source. 
The physics of gyroresonance emission and t h e  character of 
b u r s t  spec t r a  imply t h a t  s p e c t r a l  reso lu t ion  of about 10% 
is required over severa l  octaves of frequency. 
THERE IS NO EXISTING FACILITY THAT COMBINES BOTH ADEQUATE 
SPECTRAL COVERAGE AND HIGH SPATIAL RESOLUTION IMAGING. 
(The VLA does not have t h e  s p e c t r a l  coverage; t h e  present  
Owens Valley interferomster  does not have enough base l ines  
t o  image complex sources . )  
WHAT'S TXE PLAN ?? ------------------ 
To add 3 small antennas to the 0-0 interferometer  t o  form 
a 5-element SOLAR-DEDICATED array.  
This would provide up t o  7 or 10 baselines (compared t o  
t h e  present  1 or 3). This would be s u f f i c i e n t  t o  apply 
microwave diagnostic8 to mo8t a c t i v e  region and b u r s t  sources. 
By using frequenCy-8ynthOsis it would also provide an imaging 
capab i l i t y  comparable t o  t h a t  of a -100 base l ine  interferometer  
for morphological ob8ervation8. 
(See Figures. ) 
99 
WHAT'S INVOLVED I N  EXPANDING THE ARRAY ?? ......................................... 
* The p r e s e n t  system i s  based on a pair  of 27-meter p a r a b o l i c  
antennas,  equipped wi th  f requency-agi le  r e c e i v e r s .  
By r a p i d l y  c y c l i n g  i n  frequency t h e  system f u n c t i o n s  
e f f e c t i v e l y  as a microwave spec t rometer  over  t h e  range  
1 t o  18 GBz. Base l ines  up t o  1 .25  km are a v a i l a b l e  so 
t h a t  b u r s t  and a c t i v e  r eg ion  sources  can be r e s o l v e d  a t  
a l l  f r equenc ie s .  
* To expand t h e  a r r a y ,  w e  w i l l  add 3 small (2 m diameter) 
an tennas ,  equipped w i t h  f requency-agi le  receivers of 
e s s e n t i a l l y  t h e  c u r r e n t  des ign .  
* Large an tennas  are n o t  necessary  because by doing i n t e r f e r o m e t r y  
between 2 meter and 27 m e t e r  diameter an tennas ,  t h e  
effective s e n s i t i v i t y  i s  t h e  same as t h a t  w i t h  a pa i r  
of 7 m e t e r  an tennas  (geometr ic  mean of 2 and 2 7 ) .  This 
i s  quite s a t i s f a c t o r y .  
* Although an tennas  of t h i s  diameter are r e a d i l y  available, 
comnerc ia l ly  available antenna mounts do  n o t  combine 
adequate sky coverage w i t h  reasonable  cost. Therefore 
w e  w i l l  b u i l d  t h e  antenna mount in-house,  u s ing  a simpler 
des ign  well-matched t o  our  requirements .  
(See 1 / 4  scale model.) 
* To avoid  t h e  expense of new cab l ing ,  t h e  small an tennas  w i l l  be 
l o c a t e d  a t  e x i s t i n g  antenna s t a t i o n s .  (See below.) 
QD 
0 
N 
I' 
Interferometer configuration, showing !he possible locations of the 27 m antennas ( X ) relative to 
(lie 40 m telescope (T) and the control room (C). 
* Normally a s i g n i f i c a n t  expense i n  N-antenna a r r a y 8  
is t h e  need f o r  N x N c o r r e l a t o r s  and a s s o c i a t e d  systems.  
* This expense w i l l  be avoided by c o n s t r u c t i n g  a 5-to-3 l i n e  
m u l t i p l e x e r  and t h e n  t ime-mult iplexing 
l i n e s ,  runplifiers, correlators and data system. 
e x i s t i n g  de lay  
* The r e s u l t i n g  system w i l l  be capable  of f u l l y  sampling 5 
antennas  whi le  changing f r equenc ie s  up t o  1 0  times per 
second. 
100 
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k x  
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0 
o x  
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Figure 1. The upper panel shows model line-of-sight spectrum calculated using a 
three-dimensional potential field model of sunspot magnetic fields in a constant 
conductive flux modelled atmosphere. The discontinuities are a result of the interplay 
of gyroresonance opacity (at the 2nd and 3rd harmonics) with the sharp temperature 
gradient at the transition zone. x and o represent emission in the extraordinary and 
ordinary modes respectively. The dashed line shows the effect of increasing the model 
magnetic field by 10%. 
The lower panel shows a typical observed line-of-sight spectrum near an isolated 
sunspot. As suggested by the model, the frequencies at which the spectral breaks occur 
(particularly in the extraordinary mode) can be readily interpreted in terms of the 
magnetic field where the line of sight intersects the base of the corona. For this line of 
sight (4000 km displaced from the center of the sunspot), at 300.000 K the 3rd harmonic 
of gyrofrequency corresponds to 7.6 GHz, itnplying a field of 900 gauss (ref Hurford 
and GUY, NASA-CP-2442, ~317) .  
101 
Universal Spectra for Homogeneous Sources 
Frequency [CHZ) 
Figure 2. Universal curves, from theory, for enlissions by various mechanisms from a 
homogeneous source. The arrows indicate the magnitude and direction in which the 
spectrum would shift for a factor of 2 increase of the indicated parameter. The top 
panels show brightness temperature spectra (requiring measurements with spatial 
resolution) and the bottom panels show the corresponding flux density spectra for a 
source of fixed size. The arrow labelled B in the thermal gyroresonance spectrum 
actually represents the quantity, B (nL)"" sin(q)0.66. Theta, the angle between the field 
line and line of sight, can be independently determined from circular polarization. 
102 
TP spectra near peak 
I I I I I I I I I  
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X 
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18: 4:lO 
X 
I 
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Frequency (GHz) 
Figure 3. The microwave spectrum in right- and left-hand circular polarization for a 
burst with demonstrably simple spatial structure and interferometrically determined size 
of 8 arcseconds. The overlaid curves assume a homogeneous source with a thermal 
gyrosynchrotron spectrum. Thus the only degrees of freedom for the fits are the peak 
flux and frequency. Corresponding x-mode (solid) and o-mode (dashed) fits are shown. 
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Planned improvements to the Owens Valley frequency-agile interferometer

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