The Fe/H ratios in the peaks of high speed streams (HSS) during the decline of Solar Cycle 20 and the following minimum (October 1972-December 1976) were analyzed. The response of the 50-200 keV ion channel of the APL/JHU energetic particle experiment (EPE) on IMP-7 and 8 to solar wind iron ions at high solar wind speeds and Fe measurements were compared with solar wind H and He parameters from the Los Alamos National Laboratory (LANL) instruments on the same spacecraft. In general, the Fe distribution parameters (bulk velocity, flow direction, temperature) are found to be similar to the LANL He parameters. Although the average Fe/H ratio in many steady HSS peaks agrees within observational uncertainties with the nominal coronal ratio of 4.7 x 10(-5), abundance variations of a factor of up to 6 are obtained across a given coronal-hole associated HSS. Over the period 1973-1976, a steady decrease in the average quiet-time Fe/H ratio by a factor of about 4 is measured on both IMP-7 and 8

Bame, S. J.

Mitchell, D. G.

Roelof, E. C.

English

NASA Technical Reports Server

SPATIALVARIATIONOF IRONABUNDANCEIN THE
HIGHSPEEDSOLARWIND,1972 - 1976
D. G. Mitchell I, E. C. Roelof I and S. J. Bame2
IApplied Physics Laboratory
Johns Hopkins University
Laurel, Maryland 20707
2Los Alamos National Laboratory
Los Alamos, NewMexico 87545
ABSTRACT
Wehave analyzed the Fe/H ratios in the peaks of high speed streams (HSS)
during the decline of Solar Cycle 20 and the following minimum(October 1972-
December1976). Weutilized the response of the 50-200 keY ion channel of the
APL/JHUenergetic particle experiment (EPE) on IMP-7 and 8 to solar wind iron
ions at high solar wind speeds (V > 600 km sec-l), and comparedour Fe meas-
urements with solar wind H and He_parameters from the Los Alamos National
Laboratory (LANL) instruments on the same spacecraft In general, the Fe
distribution parameters (bulk velocity, flow direction, temperature) are found
to be similar to the LANLHe parameters. Although the average Fe/H ratio in
many steady HSS peaks agrees within observational uncertainties with the
nominal coronal ratio of 4.7 x 10-5 , abundancevariations of a factor of up to
6 are obtained across a given coronal-hole associated HSS. Over the period
1973-1976, a steady decrease in the average quiet-time Fe/H ratio by a factor
of about 4 is measured on both IMP-7 and 8. A more detailed discussion of
these findings, as well as Fe/H variations correlated with solar flare
activity, can "Defound in Mitchell et al. (i983).
INTRODUCTION
Contemporary plasma detectors (electrostatic analyzers and mass
spectrometers) are limited in measuring iron ions at high solar wind speeds (>
600 km sec -I) with their attendant high kinetic temperatures. An alternate,
though unexpected, technique has been shown in Mitchell and Roelof (1980) and
Mitchell et al. (1981), hereafter referred to as Papers 1 and 2, to be capable
of high time resolution measurements of solar wind iron under precisely these
conditions. The technique uses the 50 - 200 keV ion channel of the Energetic
Particle Experiments (EPE), D. J. Williams, Principal Investigator, on the IMP
7/8 spacecraft. Another measurement of iron in high speed solar wind, includ-
ing charge state determination, was reported by Ipavich et al. (1983) who also
employed a solid state detection system. In addition to being able to measure
iron at high bulk velocities, the uniqueness of the observations presented
here lies in their continuity over time scales of hours, days, or years so
that we can investigate variations in abundance and other distribution func-
tion parameters in individual streams, and even the evolution of streams
themselves over the decline of the last solar cycle.
605
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-- 1 O0 I
500 550 600 650 700 750 800
IMP 7 helium velocity (km s-1)
85O
FIGURE 1 (a) Scatter plot of the logarithm of the EPE L1 detector iron
response (5.5 minute averages) normalized by the LANL hydrogen
flux, versus the LANL hydrogen velocity for all V H > 500 km s-_
during 1973. (b) Scatter plot of the logarithm of the EPE L1
detector iron response normalized by the LANL helium flux, versus
the LANL hellum velocity. Note the reduced scatter above
VHe ~ 660 km s-I.
606
TECHNIQUE
As described in Papers 1 and 2, the iron is seen primarily in Sectors 9
and I0, the sunward-looking sectors of the 16 sectors in the ecliptic plane.
In Paper 2, we describe in detail a method whereby for certain events we can
derive both the iron bulk flow velocity (VFe) and its thermal velocity (VF^),
assuming a convected Maxwellian thermal iron distribution. Using the mo_el
from Paper 2 for the instrument response based upon detector calibrations and
geometry, we can use the measured VFe and VFe to obtain the iron flux, JFe =
NFe VFe. This work will concern itself primarily with the quantity JFe/J_ =
NFe VFe/NH V H _ NFe/NH, the iron abundance relative to hydrogen in the solar
wlnd. We sNall use the notation Fe/H for this abundance ratio through the
remainder of this paper.
It has been shown (Ogilvie, 1980; Bochsler and Geiss, 1982) that minor
ions up to oxygen, and perhaps iron, behave similarly to each other (same bulk
velocity and thermal velocity) but dissimilarly to hydrogen (different,
usually higher bulk velocity) in the non-collisional solar wind. In Figure
la, we plot 5.5 minute averages of the sum of the count-rates in Sectors 9 and
I0 on IMP-7 normalized by hydrogen flux, versus the hydrogen bulk velocity
from the Los Alamos National Laboratory (LANL) solar wind detector on IMP-7.
In Figure Ib we plot the sum of Sectors 9 and I0 normalized by the LANL helium
flux, versus the LANL helium velocity. It can readily be seen that VHe orders
the data much better than VH, in agreement with VHe = VFe @ VH. TKroughout
the remainder of this paper, we assume VFe = VHe and use VHe to remove the
detector efficiency velocity dependence.
IRON ABUNDANCE AS A FUNCTION OF HELIOGRAPHIC SOURCE LONGITUDE
Since we are interested in the coronal source regions and their influence
on Fe/H, we wish to examine the variation of Fe/H as a function of the
heliographic location of its source in the corona. To this end we shall map
the tlme-ordered data into bin-averaged 2° wide bins in heliographic source
longitude, using the constant radial velocity approximation from the corona to
1AU.
In Figure 2, we show two consecutive recurrences of a particularly broad
stream, with the first occurrence in light lines (VH) and dots (VHe), the
second in heavy lines and dots. The second panel displays Fe/H normalized by
the coronal value of 4.7 x 10-5 of Withbroe (1971), which is also nearly the
same as the solar wind value of 5.3 x 10-5 obtained (at lower solar wind
velocity and kinetic temperature than those at which we are working) by Bame
et al. (1979) for interstream solar wind. The absolute normalization of Fe/H
is not well determined; flight spare detectors were used in the detector cali-
brations, and though there is good qualitative agreement between IMP-7, IMP-8,
and the spare detectors, when simultaneously sampling data from a particular
stream IMP-7 and 8 disagree with one another by a factor of ~ 3 in absolute
response. On the basis of that consistent pattern, we have removed that
factor of 3 difference in this paper, multiplying all IMP-7 data by 3 before
plotting.
Although the stream's leading edge has moved westward approximately 40 °
on the second occurrence, the average value of Fe/H has not changed signi-
ficantly, and, in fact, can be considered a stable signature of this source
region. This stream is unusual in its breadth and the good coverage by IMP-7
607
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225 270 315
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FIGURE 2 IMP-7 LANL solar wind parameters and the EPE iron abundance for a
coronal hole-assoclated high velocity stream, days 50 - 62, 1973,
(light lines and dots), plus its recurrence on the following
rotation (heavy lines and dots) plotted as a function of hello-
graphic source longitude, assuming constant radlal flow from the
corona to 1 AU. The top panel shows VH (solid llne) and VHe
(dots). Second panel ls the logarithm of the measured iron
abundance (Fe/H) normalized by a coronal value of 4.7 x 10-5 •
608
on consecutive rotations, but the repeatability of the Fe/H profile is a
common feature of other stable corotating streams associated with coronal
holes.
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FIGURE 3 Average of velocities and Fe/H over 5 solar rotations in 1974,
plotted versus source longitude.
LONG TERM TRENDS
Exploiting the repeatability of the corotatlng stream associated Fe/H
ratios, in Figure 3 we average five consecutive solar rotations of data in
mid-1974 to obtain a more continuous, representative longitude record of Fe/H
than a single rotation with its attendant data gaps (due to the magnetospheric
passes of the spacecraft) can provide. The stream on the right is fairly
constant in Fe/H at a level lower by a factor of ~ 2 than the stream shown in
Figure 2. The stream from 0 ° to 170 ° has a longitude-dependent Fe/H ratio,
decreasing from a value a factor of ~ 2 above the stream centered at 260 ° to a
value perhaps a factor of 3 below that value, for a total change of a factor
609
of ~ 6 from one edge of the stream to the other. The stream between 0= and
170° is associated with the equatorward extension of the south polar coronal
hole. We suggest here that the topology of the coronal source region may be
related to the systematic decrease in Fe/H from west to east in this stream,
in which the south-easterly orientation of the eastern boundary of the coronal
hole allows us to sample the composition across the edge (which is not
possible for the more usual merldional orientation). If this is the case, the
implication is that Fe/H is higher for solar wind originating near the inner
edges of the coronal hole boundary region, decreasing as one approaches the
outer edge of the boundary region.
We have taken this process of summingover multiple rotations one step
further. By summingover an entire year, and then taking estimates by eye of
the minimum, maximum,and mean Fe/H values for each year, we can examine
(Figure 4) the yearly trend in Fe/H over the four years 1973-1976 during the
decline of sunspot Cycle 20. It is clear that there is a monotonic decrease
in Fe/H over the period. There is also a decline in He/H over the sameperiod
(upper curve, data from Feldmanet al., 1978), but the iron abundancedecrease
precedes the helium abundance decrease by at least a year. The difference
between the trends in Fe/H and He/H is actually greater than Figure 4
portrays, since the iron data is all from high speed streams while the
downturn in He/H is dominated by the low speed solar wind; in fact, Bameet
al. (1977) reported that He/H at the higher speeds remained remarkably
constant (~ 5%) over this same period in the maxima of high speed streams.
Thus, we find that the iron and helium abundances in high speed streams differ
in their behavior on both intermediate (one stream's duration) and long term
(fraction of a year to fraction of a solar cycle duration) time periods,
though they are sometimes well correlated on a fraction of a day time scale.
SUMMARY AND CONCLUSIONS
Using the 50 - 200 keV ion channel of a solid state detector on the EPE
experiment, we have obtained the only thermal iron measurements at solar wind
speeds over 600 km s-I during the decline of Solar Cycle 20, when the solar
wind structure was dominated by stable corotatlng coronal hole associated high
speed streams. We have found that:
i) The response of the EPE to Fe, when compared to the LANL He
measurements, is consistent with the iron bulk velocity, thermal velocity, and
bulk flow angle being the same (on average) as those for helium.
2) The profile of Fe/H as a function of time or coronal source longitude
is a stable, repeatable feature of a stable corotatlng high speed stream.
3) Fe/H within a corotating stream can vary significantly as a function
of the plasma's coronal source longitude, up to a factor of ~ 6, and we have
identified the edge of a coronal hole with a gradient in Fe/H.
4) A long-term decrease of a factor of ~ 4 is measured in Fe/H from 1973
to 1976 during the decline of solar cycle 20.
610
FIGURE 4
10-
9--
8--
7
6
O
x 5
"l-
T 4
3--
2--
1 --
9
8 -
7
6
_" 5
O
X
T 4
ii
3
2
_ IMP He/H
-e
--Jo
Coronal
7
]
8
7
-- IMP--_
Approximate mean, maximum and
minimum in high speed streams at
all longitudes averaged over a year
Feldman et al, (1978)
• IMP, all SW speeds
O 2 year average
8
7
1 I I I I
1973 1974 1975 1976
Year
Four-year decrease in Fe/H during the decline of Solar Cycle 20.
611
ACKNOWLEDGEMENTS
We wish to thank D. J. Williams for the use of the IMP 7 and 8 EPE data,
and R. D. Zwickl for his assistance with the LANL plasma data. Our effort was
supported in part by NASA and in particular, at JHU/APL through Grant NAGW-
264. The Air Force Geophysics Laboratory also provided partial support via
Task ZFI0 of Contract N00024-83-C-5301 between the U. S. Navy and The Johns
Hopkins University.
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Bame, S. J., J. R. Asbrldge, W. C. Feldman, and J. T. Gosling, Evidence for a
structure-free state at high solar wind speeds, J. Geophys. Res., 82,
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Bame, S. J., J. R. Asbrldge, W. C. Feldman, E. E. Fenlmore, and J. T. Gosling,
Solar wind heavy ions from flare-heated coronal plasma, Solar Phys., 62,
179, 1979.
Bochsler, P. and J. Gelss, Cross- and auto-correlatlons of speeds and kinetic
temperatures of minor ions in the solar wind, EOS, Trans. AGU, 63, 424,
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Feldman, W. C., J. R. Asbrldge, S. J. Bame and J. T. Gosling, Long term varia-
tions of selected solar wind properties: IMP 6, 7 and 8 results, J___t"
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Mitchell, D. G. and E. C. Roelof, Thermal iron ions in high speed solar wind
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Thermal iron ions in high speed solar wind streams, 2. Temperatures and
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variations at speeds > 600 km s-I 1972-1976 J Geophys. Res., submitted
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612


Spatial variation of iron abundance in the high speed solar wind, 1972 to 1976

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