[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