high heliographic latitude observations of electron heat flux
TRANSCRIPT
[16.15] 194th Meeting of the AAS
High Heliographic Latitude Observations ofElectron Heat Flux
J.E. Littleton, A.E. Badeau, Jr., J.A. Milsom, and E.E. ScimeWest Virginia Univ. Dept. of Physics
High Heliographic Latitude Observations of Electron Heat Flux*J.E. Littleton, A.E. Badeau, Jr., J.A. Milsom, and E.E. Scime, West Virginia Univ. Dept. of Physics
ABSTRACT
At high heliographic latitudes, sunlight contaminates the SWOOPS electron detectors on the Ulysses
spacecraft by generating large numbers of photoelectrons inside the instrument as the detector
aperture sweeps past the direction to the Sun. We have developed an algorithm that removes this
contamination. The electron distribution function is corrected by interpolating across the
contaminated azimuthal elements using data obtained at adjacent spin angles after taking account of
the spacecraft potential. The electron heat flux, the third moment of the distribution function, is of
particular interest because observations at low heliographic latitudes suggest a correlation between
the electron heat flux and plasma instabilities.
We report on our application of this algorithm to data from the high latitude passage of the Sun by
Ulysses in 1994-95 to determine if there appears to be any latitude dependence of the electron heat
flux. This research was supported by NASA.
*Based, in part, on the B.S. Honors Senior Thesis of AEB.
The Ulysses spacecraft's Solar Wind Observations Over the Poles of the Sun (SWOOPS)
experiment measures the three-dimensional electron distribution function (Bame et al. 1992, A&AS,
92, 237). The heat flux, in the frame moving with the electrons, is calculated from the distribution
function as
vvvvvvq������� 3
e2e
e )())((2
dfm
������� � .
The electron heat flux is carried mainly by the solar wind's halo electrons (McComas et al. 1992,
Geophys. Res. Lett., 19, 1291). That this flux is nonzero implies an asymmetric electron velocity
distribution.
Observations of the electron heat flux near the Sun's equatorial plane, made during the outbound trip
to Jupiter in 1990-91, showed that the heat flux varied approximately as R-3, with R the distance
from the Sun (Scime et al. 1994, J. Geophys. Res., 99, 23401). For expansion along the magnetic
field with no loss, the exponent of R should be �-2. The observed variation indicates that solar wind
electrons are losing heat flux as they expand into interplanetary space. A suggestion by Gary et al.
(1994, J. Geophys. Res., 99, 23391) to explain this loss is that plasma waves (whistlers) generated in
the expanding solar wind extract energy from the electrons.
Evidence for the production of these low frequency plasma waves is found in correlations between
heat flux variations and plasma wave power measured by the Unified RAdio and Plasma (URAP)
wave experiment on Ulysses. The figures show the waves generated by shocks in the solar wind
during a 30-day period as Ulysses moved from heliographic latitude -11� to +11�.
The sudden increases in electron density in the first figure indicate the passage of shock waves. The
shock passages are correlated with increases in plasma wave activity. In the middle figure we plot
wave activity at 1.77 Hz as an example. Increases in plasma wave activity at the same time as the
shocks are obvious at this frequency and are seen at other frequencies as well. Also correlated with
the shock passages and the plasma wave activity are the increases in electron heat flux shown in the
bottom figure. These correlations support the hypothesis that plasma waves affect the electron heat
flux in the solar wind.
0
5
10
15
20
50 55 60 65 70 75 80
Electron Num ber Dens ity versus T im eN
e (
cm-3
)
Day of 1995
0
3
6
9
12
15
50 55 60 65 70 75 80
qe versus T im e
qe (�
W/m
2)
Day of 1995
0
2 104
4 104
6 104
8 104
50 55 60 65 70 75 80
W ave Activity at 1.77 Hz versus T im e
Ma
gn
etic
Fie
ld S
tre
ng
th (
10-1
5 T
esl
a)
Day of1995
The most significant feature of the Ulysses mission, however, is its observations out of the ecliptic
plane. These observations can help us determine if the same phenomena occur at higher solar
latitudes.
The SWOOPS detectors are mounted on the side of the spacecraft whose rotation axis always points
toward Earth, as shown in the figure. At high heliographic latitudes the detector apertures are
directly illuminated by sunlight that results in photoelectrons being ejected in the detectors as they
sweep past the Sun.
�.
SWOOPS
electron access�
165 eV
-50
0
50
100
150
200
60 120 180 240 300 360 420 480
�����
ccou
nts ����
����
��� ��
17 eV
0
200
400
600
800
1000
60 120 180 240 300 360 420 480
�����
ccou
nts
����
����
��� ��
These figures show the contamination of the
electron count for three energy values and
several polar angles � on 1994 October 29 when
Ulysses was 1.97 AU from the Sun and at a
latitude of -72.5�.
This contamination of the electron data must be
eliminated before useful heat fluxes can be
calculated for high latitudes.
61 eV
0
50
100
150
200
250
60 120 180 240 300 360 420 480
�����
ccou
nts ����
����
��� ��
We have developed an algorithm that automatically detects the contamination by sunlight. The
contaminated data are then smoothed for all of the energy divisions, �E, and for all of the colatitude
divisions, ��, by averaging the values of the electron density over the two azimuthal angle elements,
��, on each side of the contaminated element. The most significant contamination occurs at high
energy as seen in the preceding figures. Since the high energy electrons make the greatest
contribution to the electron heat flux, the algorithm corrects the data by the largest relative amounts
at the highest energies. The effect of this smoothing on the electron distribution function can be seen
in the following two figures.
These figures are contour plots of the distribution function for all the solar wind electrons before and
after applying the correction described above. The coordinates for the plots are the components of
the electron velocities parallel, v||, and perpendicular, v�, to the interplanetary magnetic field after
subtracting the bulk velocity of the electrons.
|)(|v;||)(v |||| vvvBBvv���
��
��
�����������
The magnetic field is the average field during the two-minute interval that elapses as the electron
distribution is being measured by SWOOPS.
The contamination by photoelectrons has been removed in the corrected distribution. Its overall
shape is unchanged and results in a more accurate value for the heat flux.
-20000 -10000 0 10000 20000
20000
10000
0
-10000
-20000
Parallel Velocity (km s-1)
Per
pend
icu
lar
Vel
ocity
(km
s-1)
-72.5 -70.0 -67.5 -65.0 -62.5 -60.0
ln[fe(v) (sec 3 cm -6)]
Uncorrected Electron Distribution Function
-20000 -10000 0 10000 20000
20000
10000
0
-10000
-20000
Parallel Velocity (km s-1)
Per
pend
icu
lar
Ve
loci
ty (
km s-1)
-72.5 -70.0 -67.5 -65.0 -62.5 -60.0
ln [fe(v) (sec 3 cm -6)]
Corrected Electron Distribution Function
The electron heat flux is computed for high latitudes after removal of the contamination by sunlight
to determine if there is any latitude dependence of the flux. We consider the flux to be acceptable if
the deviation of the magnetic field from the average is low during the acquisition of the distribution
function and if the resultant heat flux is approximately parallel or antiparallel to the magnetic field
(Scime et al. 1994, J. Geophys. Res., 99, 23401). This figure shows the magnitude of the heat flux
plotted as a function of heliographic latitude �. The heat flux seems to depend on latitude.
1.00
10.00
-80 -40 0 40 80
Heat F lux versus Latitude
qe
(�W
m-2
)
Heliographic Latitude (degrees)
When Ulysses is at the highest latitudes, however, it is also farthest from the Sun. Account of this
can be taken by scaling the flux by some power of its distance from the Sun. Since the analysis of
Scime et al. found an approximate R-3 decrease, we scale the flux by the factor R3. We have also
prepared a plot of flux scaled by the factor R2 to see if it provides a better fit since heat flux
expanding radially with no divergence would vary as R-2, the steepest variation with no losses. The
first of the following two plots is the R3 scaling and the second is the R2 scaling. The latter seems to
provide a better fit for the regions with |�| > 25�. There appears to be little dependence on the
latitude for these regions.
1.00
10.00
-80 -40 0 40 80
Heat F lux N orm alized by R3 versus Latitude
qeR
3 (�
Wm
-2A
U3)
Heliographic Latitude (degrees)
1.00
10.00
-80 -40 0 40 80
Heat Flux N orm alized by R2 versus Latitude
qeR
2 (�
Wm
-2A
U2)
Heliographic Latitude (degrees)
A decrease as R-2 in the electron heat flux for the high latitude zones implies little loss in electron
heat flux. The next figure shows plasma wave activity in the 1.44 Hz band for high heliographic
latitude. During the interval shown in this figure, Ulysses's heliographic latitude changed from
-72.8� to -70.3�. The wave activity in this plot reveals none of the jumps in plasma wave power
prominent in the figure for low heliographic latitudes. Electron heat flux is also plotted in the next
figure and shows no correlation with variations in the plasma wave power. Plasma wave activity in
this figure is at the level seen during the time intervals when there are no shocks in the earlier plot.
The same holds true for the electron heat flux.
Shock waves are not observed in the high latitude solar wind. It is reasonable to assume that the
generation of shock waves enhances electron heat flux losses by stimulating plasma instabilities near
the solar equatorial plane. At high heliographic latitudes, where the solar wind is steadier, the lack
of shocks inhibits the generation of plasma instabilities and consequently there is little unexplained
loss of electron heat flux. Our on-going analyses of the data for greater heliocentric distances and
higher latitudes should clarify the dependence of the heat flux on R. Additional observations by
Ulysses during its next solar passage at solar maximum are expected to increase our understanding
of energy generation and transport by the solar wind.
0
4000
8000
1.2 104
1.6 104
0.5
1
1.5
2
2.5
3
301 302 303 304 305 306 307 308
W ave Activity at 1 .77 Hz and qe versus T im e
Magnetic Field Strength at 1.77 Hz Heat Flux
Ma
gn
etic
Fie
ld S
tre
ng
th (
10
-15 T
esl
a)
qe (
�W
/m2)
Day of 1994