Correlation of plasma velocity with indices of solar and terrestrial activity - cosmic radiatio

Neugebauer, M.

Rao, U. R.

Snyder, C. W.

English

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THE SOLAR WIND VELOCITY AND ITS CORRELATION WITH GEOMAGNETIC, SOLAR
AND COSMIC RAY ACTIVITY
by
Conway W. Snyder and M. Neugebauer
Jet Propulsion Laboratory, California
and
U. R. Rao *
Graduate Research Center of the Southwest
GPO PRICE $
CSFTI PRICE(S) $
Hard copy (HC) _
PRESENTED BY: U. R. RAO
Microfiche (MF)_
(1653 July 65
1. INTRODUCTION
r-~
In this report we discuss the correlation of plasma velocity measured
by Mariner II with various indices of solar and terrestrial activity during more
than 4.5 solar rotations covering the period 28 August through 31 December 1962
(day 240 to 365). The daily mean and the six-hourly mean plasma bulk velocities
have been derived from approximately 40,000 spectra received from' the plasma
l
probe which consisted of a single electrostatic spectrometer pointing within 0.1
degrees of the center of the sun.
2. EXPERIMENTAL RESULTS
Figures la and Ib show the plot of the daily mean plasma velocity, the
planetary indices Kp and the sunspot and C. R, activity. \The plasma velocity
does not show any correlation either with the cosmic ray diurnal amplitude and
time of maximum or with the overall solar activity as measured by the sunspot
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* On leave from Physical Research Laboratory, Ahmedabad, India
https://ntrs.nasa.gov/search.jsp?R=19650021123 2020-03-17T01:51:05+00:00Z
Rao - Page 2
number and 10.7 cm flux. However, a remarkable correlation exists between daily
Z.Kp and daily mean plasma velocity with no time lag, except after day 352 when
both the "expected" and the observed time lags were approximately one day. Every
major peak and trough in plasma velocity (indicated in Figure by vertical lines)
was associated with a corresponding peak or trough in fKp , the correlation
coefficient between the two for the entire period being 0.73 - .04 .
Figure 2 shows the six-hourly mean plasma velocity and K_ for three
selected periods. Even when the plasma velocity was continuously high for a
few days, Kp did not become small. The correlation between six-hourly mean
plasma velocity and Kp was 0.65 - 0.04 . Preliminary analysis shows no corre-
lation between Kp and dv/dt, the flux or the kinetic pressure. Our results
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£ 4OQ
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919 320 321 322 323 524 329 32S 327
TIME IN DAYS
DAILY MEAN PLASMA VELOCITY (KM/SEC)
conclusively prove that 1C is a measure of the plasma velocity and not of the
time rate of change of plasma velocity.
The relationship between K_ and the daily mean plasma velocity is illus-
trated in Figure 3. The best fit for the data is given by the straight line
V (km/sec) = (8.44 ± 0.74) £1^ + (330 t 17)
V = 330 km/sec, corresponding to £K^ = 0 represents the minimum plasma
velocity which can excite disturbances in the geomagnetic field. Applying this
relation we find that the mean plasma velocity during 1957 was 510 km/sec. Com-
paring the extrapolated velocity of 475 km/sec with that measured by Explorer X
just at the boundary of the magnetosphere on 26 March 1961, we conclude that the
velocity gets attenuated by a factor of 1.6 across the shock front ahead o,f the
cavity.
Each of the peaks shown in Figure 1 exhibits a very strong 27-day
. . . * Rao - Page 3
recurrence tendency (see Figure 4). The peaks are divided into five groups,
A to E, where each member of a group
represents an encounter with the same
long-lived plasma stream. Only two
of these were associated with Sc
storms, the rest being associated with
M-region storms. Particularly inter-
esting are A series peaks which were
associated with a region of the photo-
sphere which, time after time con -
tained from one to three calcium
plages. Beginning in rotation number
DAYS OF SOLAR ROTATION
7 390 36O
TIME IN DAYS FROM UAN 0
1766 with McMath plage number 6504, this active region was still visible in
rotation 1777. The complete solar and terrestrial relationships of these peaks
are summarized in Table I.
3. CONCLUSIONS
1. There was always a measurably large plasma flow from the directionof
the sun. Plasma velocity varied from day to day.
2. Plasma velocity is not correlated with the cosmic-ray diurnal amplitude
or time of maximum or with the overall solar activity.
3. Plasma velocity is very strongly correlated with 1L. The relationship
between Kp and velocity can be represented by the equation
V (km/sec) = 8.44^ Kp + 330
where V and 2_Kp are both daily values. Using this empirical relation, we
find that (a) the daily mean plasma velocity averaged over a year varies from
«-v 510 km/sec to /v330 km/sec from sunspot maximum to sunspot minimum, and (b)
the plasma velocity measured near the earth by Explorer X was a factor of^ 1.6
lower than the velocity of the interplanetary plasma on the day of observation.
4. The plasma velocity showed a very strong 27-day recurrence tendency
and a close association with M-region storms, which indicates that M-regions
»
are emitters of high velocity plasma. '*
A full account of these results will appear in the December issue of J. G. R.
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The solar wind velocity and its correlation with geomagnetic, solar and cosmic ray activity

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