Interplanetary scintillation observations from 1971-1975 show that the average solar wind speed increases away from the solar equator, with a mean gradient of 2.1 km/s per degree. These results are compared with spacecraft observations over the + or - 7 deg attainable in the ecliptic and with those deduced from comet tails. The role of temporal variations, especially those caused by latitude dependent solar wind streams, is emphasized, and this points to the need for extensive ecliptic and ground-based observations during an out-of-the-ecliptic spacecraft mission

Coles, W. A.

Rickett, B. J.

English

NASA Technical Reports Server

IPS OBSERVATIONS OF THE SOLAR WIND SPEED OUT 
OF THE ECLIPTIC 
by 
W. A. Coles 
and 
6. J. Rickett  
APPLIED PHYSICS & INFORMATION SCIENCE DEPARTMENT 
UNIVERSITY OF CALIFORNIA, SAN DIEGO 
LA JOLLA, CALIFORNIA 92093 
[ Inv i ted  paper presented a t  the NASAIESRO Symposium "The Sun and Interplanetary 
Medium i n  Three Dimensions" G.S.F.C., Hay 19753. 
September, 1975 
https://ntrs.nasa.gov/search.jsp?R=19760017037 2020-03-22T15:45:11+00:00Z
Abstract  
In te rp lane ta ry  s c i n t i l l a t i o n  (IPS) observations from 1971-1975 show 
t h a t  the average so la r  wind speed increases away from the s o l a r  equator, 
w i t h  a mean gradient  o f  2.1 km/s per degree. These r e s u l t s  are compared 
w i t h  spacecraft  observations over the i 7O a t t a i nab le  i n  the e c l i p t i c  and 
w i t h  those deduced from cornet t a i l s .  The r o l e  o f  temporal va r ia t ions ,  
especial  l y  those caused by l a t i t u d e  dependent so la r  wind streams, i s  
emptlasized. This po in t s  t o  the need f o r  extensive e c l i p t i c  a ~ d  ground- 
based observations dur ing an out-of- the-ecl  i p t i c  spacecraft mission. 
I n t r oduc t i on  
The s o l a r  wind bo th  i n  and o u t  o f  the e c l i p t i c  can be s tud ied  from 
the  ea r t h  by the  method o f  I n t e rp l ane ta r y  S c i n t i l l a t i o n s  (IPS). The 
method was pioneered by Hewish and h i s  colleagues, who deduced the  s o l a r  
wind speed from m u l t i p l e  s t a t i o n  observations. I n  1966 Dennison and 
Hewish (1967) found an increased speed out  o f  the  e c l i p t i c ,  wh i l e  i n  1967 
Hewish and Symonds (1969) found no such increase. The i r  mu1 t i p l e  s t a t i o n  
observat ions then ceased. I n  t h i s  paper we r e p o r t  s o l a r  wind speeds 
deduced from 74 MHz IPS observations made a t  UC San i l iego from May 1971 
through t o  A p r i l  1975. The observing system was described by Armstrong 
and Coles (1972) and by Coles e t  a1 (1974). Resul ts from 1972 were repor ted 
by Coles and Maagoe (1972). We w i l l  a1 so mention b r i e f l y  the  relevance 
o f  I P S  observat ions simultaneous w i t h  an out-of- the-ecl  i p t i c  spaceprobe 
mission. 
IPS Method 
The s c i n t i l l a t i o n  s igna l  i s  the  sum of waves scat tered along the 1 i n e  
o f  s i g h t  from a g iven rad io  source. Most o f  the sca t t e r i ng  occurs where 
the  l i n e  o f  s i gh t  i s  c l oses t  t o  the sun, because o f  the steep decrease w i t 5  
s o l a r  distance i n  the  s t reng th  o f  the e l ec t r on  dens i ty  c i i c ros t ruc tu re  which 
causes IPS.  For a sphe r i ca l l y  symmetric s o l a r  wind a weight ing f l ~ n r t i o n  can 
be def ined and the I P S  "mid-point" speed (Coles and Maagoe 1972) can be 
shown t o  be a s p a t i a l  average of the  so la r  wind speed centered on the p o i n t  
o f  c loses t  approach. However, i n  the presence o f  so l a r  wind streanis spher ica l  
symmetry does no t  apply. 
The l f f e c t  o f  the  spa t i a l  averase through such streams ;;as been invesbigated 
by conipar-ing the I P S  observat ion w i t h  those expected by 1:iapping p o i n t  observa- 
t i ons  made or, the  IMP-7 spacecraf t  out  alot ig the l i n e  of s i g h t  i n  question. 
Harmon (1975) and Coles e t  a l .  (1975) demonstrated a close agreement 
between I P S  "mid-point" speed and the IMP-7 data mapped t o  the po in t  where 
the l i n e  o f  s igh t  i s  c losest  t o  the sun, More de ta i led  comparisons are i n  
progress, i nves t i  gat ing the precise form o f  the spat ia l  weighting caused 
by streams. However, f o r  the present purposes the compari son demonstrates 
tha t  each IPS observation i s  representative o f  the so la r  wind speed a t  the 
po in t  o f  c losest approach, 
Results 
This e f f e c t i v e  observing po in t  changes i n  so la r  l a t i t ude ,  longitude 
and rad ia l  distance as the sun rotates and the ear th  o r b i t s  the sun. Thus 
for  radiosources not i n  the e c l i p t i c ,  about two months o f  high so la r  l a t i t u d e  
data can be obtained each year. The geometry, however, i s  such tha t  the 
high l a t i t udes  occur together w i t h  small so la r  distances. The distance 
dependence can be separated out by s t d y i n g  the four  ec l  i p t i c  radio-sources, 
f o r  which the l a t i t u d e  remains w i t h i n  10' o f  the equator; Figure 1 shows 
the solar  wind speeds averaged i n t o  i n te rva l s  of 0.1 AU i n  rad ia l  distance 
during 1971-1975. We conclude tha t  there i s  no s i g n i f i c a n t  va r i a t i on  o f  
average solar wind speed w i th  rad ia l  distance between 0.4 AU. and 1.1 AU. 
Figure 2 shows a s im i l a r  p l o t  f o r  a l l  sources versus l a t i t . . .  and, because 
there i s  no rad ia l  distance dependence, i t  can be in terpreted as showing 
the solar wind speed as a funct ion o f  l a t i r zde .  The v e r t i c a l  bars are 
2 2 standard deviations i n  the average solar  wind speed over l a t i t u d e  i n t e r -  
vals ind icated by the hor izonta l  bars. The r.m.5. var ia t ion  i n  a s ing le  
speed observation i s  remarkably constant a t  about 120 km/s, showing no 
s ign i f i can t  change w i th  la t i tude.  Tke ve r t i ca l  e r ro r  bars are la rger  a t  
high l a t i t udes  because there are fewer data points a t  h igh la t i tudes .  ( I n  a 
3 / \ 9 
typ ica l  year, 300 observations from 0-1o0Ii decreasing t o  25 observations 
from 5 0 ~ - 7 0 ~ ~ \ .  The major conclusion i s  tha t  there i s  a systematic increase 
o f  solar wind seed w i th  la t i tudes both nor th  and south o f  the solar equator. 
This i s  evident i n  each year from 1971 through 1975 as wel l  as i n  the grand 
average o f  a l l  data. The average gradient i s  close t o  2.1 km/s per degree 
of la t i tude.  However, the curves are not qu i t e  syranetrical but centered near 
l o O ~ ,  g iv ing an apparently steeper gradient i n  the south than i n  the north. 
This asymnetry i s  only marginally s ign i f i can t  and we are s t i l l  checking f o r  
second order systematic er rors  which could cause this.  
Discussion 
--
Our observed la t i tude  gradient must be compared w i th  other data. As 
a1 ready mentioned, the Cambridge IPS  observations detected a 1 a t i  tude gradient 
i n  1966 but not i n  1967. Whereas t h i s  could conceivably be influenced by 
solar cycle effects, i t  i s  more l i k e l y  tha t  the small number o f  observations 
a t  high la t i tudes i s  responsible. Their measurements included only about 
30 days each year which corresponded t o  la t i tudes above 20'; the long term 
average behavior could we1 1 be masked by the day-to-day and mon th-to-mon t h  
v a r i a b i l i t y  found i n  the solar wind speed. 
Spacecraft observations have been analyzed t o  look for ef fects due t o  
the f 7' la t i tude  range avai lable i n  the e c l i p t i c  (e.g. Hundhausen e t  al. 1971 ). 
Smith and Rhodes (1974) and Rhodes and Smith (1975) deduced large apparent 
gradients (10-15 km/s per degree) by comparing solar wind speeds observed 
near Earth (Explorer 33,34,35) and a t  Mariner 5. They analyzed data from 
nearly s i x  solar ro ta t ions over l a t i t ude  differences from 0' t o  6'. We 
suggest that  a possible explanation o f  t h e i r  large gradient over a few degrees 
o f  la t i tude  comes from solar wind streams. From our I P S  data i t  i s  c lear tha t  
so lar  wind streams often e x i s t  f o r  several so lar  ro ta t ions w i th  steep la t i tude  
gradients near the equator (see f o r  example the wide southerly stream i n  
Figure 13 of Coles e t  al. 1974). It i s  l i k e l y  t h a t  such features co~ l t r i bu te  
strongly t o  a s i x  month average. Figure 2 shows t ha t  i n  1972 the 
apparent gradient between f 5' was + 4 km/s per degree, while i n  1973 the 
apparent gradient reversed t o  - 3 km/s per degree. The inf luence o f  
steep l a t i t ude  gradients from spec i f i c  recurrent streams was probably the 
cause f o r  such large values. 
More d i f f i c u l t  t o  reconci le are the resu l t s  from comet t a i l  observa- 
tions. Brandt e t  al.  (1975) have analyzed 678 comet observations spread 
over 75 years and conclude tha t  the l a t i t ude  gradient i s  - 0.9 + 0.7 km/s 
per degree nor th  o r  south; tha t  i s  no t  s i g n i f i c a n t l y  d i f f e r e n t  from zero. 
Their observations are concentrated i n  the range 0' t o  50~15 (as are the 
IPS observations) and are scattered f a i r l y  we1 1 through the phases o f  the 
so lar  cycle. It would be o f  i n t e res t  t o  see i f  the high l a t i t ude  data 
were uniformly d is t r ibu ted  over the phases of the so la r  cycle. Except for  
so lar  cycle e f fec ts  we cannot suggest any simple explanation f o r  the 
discrepancy; though long term systematic o r  random temporal var iat ions could 
be responsible. 
Concl usi  ons 
We have presented strong evidence that  during 1971 t l~rough 1975 the 
average solar wind speed increased out o f  the equatorial plane g iv ing an 
average gradient o f  2.1 km/s per degree o f  l a t i t ude  e i the r  north o r  south. 
Our observations show tha t  stream and also slower var ia t ions can obscure the 
average l a t i t ude  behaviour i n  the so lar  wind (as they do also f o r  averagc 
propert ies i n  the e c l i p t i c ) .  I n  planning out-of- the-ecl ipt ic spacecraft 
missions such changes must be expected. Jup i ter  "swing-by" c.iissions would 
give 12-18 ~,lonths a t  nore than 30' from the equator. During t h i s  t i ne  i t  w i l l  
be irnportant t o  maintain regular observations i n  the e c l i p t i c  i n  order t o  
disentangle temporal and la t i tude  effects. A continued program o f  IPS  
observations throughout the period would cover a range o f  longitudes and 
lat i tudes and further help bu i l d  a picture o f  the spat ia l  and temporal 
structure i n  the solar wind. I n  addit ion t o  the average l a t i t ude  behaviour 
such j o i n t  observations would allow the la t i tude  structure o f  individual 
streams t o  be explored. 
Ac know1 edgemen t s 
We are very grateful t o  a1 1 the people who have helped make the I P S  
observations a t  UCSD since 1971. I n  par t icu lar  we thank John Armstrong, 
John Harmon, Jay Kaufman, Steffen Maagoe, Steve Scott and David Sime. This 
research i s  supported by the Atmospheric Sciences Division o f  the W.S.F. 
under grant DES 75-13451. 
References 
Armstrong, J. W. and W. A. Coles, Analysis of Three-Station 
Interplanetary Scintillation, J. Geophys. Res., 77, 4602, 1972. 
Brandt, J. C., R. S. Harrington, and R. G. Rosen, Interplanetary 
Gas. XX. Does the Radial Solar Wind Speed Increase with Latitude?, 
Astrophys. J., 196, 877, 1975. 
Coles, W. A., B. J. Rickett, and V. H. Rumsey, Interplanetary 
Scintillations, Solar Wind Three (Edited by C. T.  uss sell), 
published by IGPP, UCLA, 1974. 
Coles, W. A. and S. Maagoe, Solar Wind Velocity from IPS Observations, 
J. Geophys. Res., 77, 5622, 1972. 
Coles, W. A., J. K. Harmon, A. J .  Lazarus, and J. D. Sullivan, 
Comparison of 74 MHz IPS and IMP 7 Observations of the Solar 
Wind during 1973, Trans. A.G.U., EOS, - 56, 440, 1975. 
Dennison, P. A. and A. Hewish, The Solar Wind Outside the Plane of 
the Ecliptic, Nature, - 213, 343, 1967. 
Harmon, J. K., Scintillation studies of density microstructure in 
the solar wind plasma, Ph.D. Thesis, UC San Diego, 1975. 
Hewish, A. and M. D. Symonds, Radio Investigation of the Solar 
Plasma, Planet. Space Sci., l7, 313, 1969. 
Hundhausen, A. J., S. J. Bame, and M. D. Montgomery, Variations 
of Solar Wind Properties: Vela Observations of a possible 
heliographic latitude dependence, J. Geophys. Res., - 76, 5145, 1971 
Rhodes, E. J., Jr., and E. J. Smith, Multispacecraft study of the 
solar wind velocity at Interplanetary Sector boundaries, J. 
Geophys. Res., 80, 917, 1975. 
Smith, E. J. and E. J .  Rtlodes, Jr., kridence of a Velocity Gradient 
in the Solar Wind, Solar Wind Three (Edited by C. T. Russell), 
published by IGPP, UCLA, 1974. 
Figure Captions 
Figure 1. Solar wind speeds from 1971-1975 measured within 10' 
of the  s o l a r  equator, averaged i n t o  0.1 AU i n t e r v a l s  
of r a d i a l  distance. The v e r t i c a l  e r r o r  bars  a r e  2 
twice the  standard deviat ion i n  the mean. 
Figure 2. Solar wind speeds from 1971 through 1975 averaged 
in to  l a t i t u d e  in t e rva l s  shown by the  horizontal  bars.  
Ver t ica l  bars  a r e  + twice the standard deviat ion 
i n  the  mean. The lower r i gh t  graph is the o v e r a l l  
average from the o ther  f i ve  graphs. 
Figure 1. 
Figure 2. 


IPS observations of the solar wind speed out of the ecliptic

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