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Neutral winds and ion drifts in the polar ionosphere caused by convection electric fields—2
Neutral winds and ion drifts in the polar ionosphere caused by convection electric fields-2 HIROSHI MAEDA Geophysical Institute, Kyoto University, Kyoto 606, Japan (Receioed 5 January 1977; in revised form 16 February 1977) Abstrati-The same equations of motion for ions and neutrals as those in Paper 1 are solved numerically using realistic models of the polar ionosphere in a disturbed state. It is found for the F-region that the ion drifts have a simple E X B drift pattern without depending upon the distrubution of ion density, and that the neutral winds have magnitudes somewhat smaller than the drifts and directions shifted clockwise by about 30” from the drifts. For the E-region, the ion drifts have smaller magnitudes than and similar patterns to those in the F-region, with directions shifted clockwise by about 60”, whereas the neutral winds have a complicated pattern such that the winds in the aurora1 zone are towards the equator around noon and midnight and towards the pole around dawn and dusk, with a reverse tendency in the polar cap. From a comparison with observation, it may be concluded that the ion drifts are in general agreement with the observed drifts, but the neutral winds (of electric-held origin alone) can not account for the observed winds. This means that the observed winds would be much influenced by winds of thermal origin, especially in and near the aurora1 zone during the night. 1.
~ODU~ON
In Paper 1 (MAEDA, 1976) we calculated the distribution of ion drifts and neutral winds in the polar ionosphere in a quiet state caused by convection electric fields, and the following main results were obtained: (1) In the F-region the drifts have a simple E x J3 pattern, whereas the winds have magnitudes smaller than and directions shifted from the drifts. (2) The relation between the wind and drift in the E-region is strongly affected by the distribution of ion density, suggesting different relations for different stages of disturbance. (3) The winds of electric-field origin are of the same order of magnitude as those of thermal origin, so that both of these should be taken into consideration for the comparison with observation. Quite recently, high-resolution auroral-zone wind measurements were made by incoherent scatter radar at Chatanika, Alaska (BREKKE et al., 1973, 1974; RINO er al., 1976) and very interesting results have been obtained. RINK et al. (1976) observed detailed height-dependent structures of the E-region winds and showed that the daytime winds had a general pattern of the global pressuredriven winds below 110 km but strongly influenced by ion drag of electric-field origin above 110 km, whereas the night-time winds seemed to be much influenced by auroral-zone heating and also by ion drag at higher altitudes. COMFORT et al. (1976) reanalyzed the time-dependent and heightaveraged E-region winds derived from Chatanika I
by the method developed by BREKKE et al. (1973) and compared with theoretical calculations for the same conditions. They concluded that ion drag alone could not account for the observations and that the accelerations due to pressure gradients required to resolve the discrepancy between calculated and observed winds were of the same order of magnitude as those due to ion drag of electric-field origin. This paper deals with a disturbed state in which the enhancement of ion density in the aurora1 zone is taken into account, and two models for each of the convection electric field and the aurora1 zone are employed. The calculated results are presented in some detail and discussed in comparison with observation. data
2. MODELS
(a) Uectric field It has been shown (e.g. I-IEPPNER, 1972) that the dawn-to-dusk electric field observed from a sequence of OGO-6 orbits over the northern hemisphere is symmetric or asymmetric with respect to the geomagnetic pole, perhaps in association with the inte~lanetary magnetic field. Therefore, we employ two profiles (symmetric and asymmetric) of the electric fields as is shown in Fig. 1. Further, for the asymmetric fields, two cases are considered, i.e. the electric field in the dawn side is more (Case 1) or less (Case 2) enhanced than that in the dusk side.
849
HIROW (SYMMETRIC)
DUSK
(b) Ion density
DAWN I
....... 80’
I 70’
I 900
I 8Q
--30
1 70’
(ASYMMETRIC)
DUSK
Although considerable efforts have been made to model the polar ionosphere, we do not yet have any detailed information about the threedimensional profile of the ion-density distribution all over the polar regions, especially in disturbed states. However, we have some information about the enhancement of ion density in the aurora1 E-region from radio and rocket observations, so that we assume here a profile for the F-region and two profiles for the E-region as shown in Fig. 2(a, b). The figures show the latitudinal distribution of the ion density at noon (1200 h) and midnight (0000 h). The distributions at other local times have been interporated from those at 1200 h and 0000 h by the help of theoretical consideration.
POTENTIAL I 80’
80’
DAWN
I 60_
-FIELD
(c) AuroraE zone
.““...’ POTENTIAL
w 60’
MAEDA
I
I
I
I
1
70’
80’
90’
80’
70”
60’
LATITUDE
Fig. 1. Symmetric (top) and asymmetric (bottom) dawnto-dusk electric fields (full line) and their potentials (dotted line), as inferred from OGO-6 observations (after HEPPNER, 1972).
For the E-region, we take two profiles of the aurora1 zone; one is a zone (Fig. 3a) concentric with the geographic latitude, and. the other is a zone (Fig. 3b) inferred from aurora1 observations (e.g. FELUSTEIN, 1963). It is supposed that the enhancement of ion density as shown in Fig. 2(a, b) occurs only along the aurora1 zone as shown in Fig. 3(a, b), respectively. The distribution of the electric fields for these two auroral-zone models is calculated from the dawn-to-dusk distribution of electric potentials as shown in Fig. 1 (see Paper 1 for details), and the results are shown in Fig. 4(a, b) for the two models. It is also assumed that the geomagnetic axis coincides with the rotational axis of the Earth.
F-LAYER
E-LAYER T E lb2
r-1
‘\
90'
80’ LATITUDE
70’
60’
I 600
I
(bl
700
LATITUDE
Fig. 2. Latitudinal distributions of ion density at ncxm (1200 h) and midnight (0000 h) in the F- and E-layers for two auroral-zone models (a) and (b) as is shown in Fig. 3.
851
Neutral winds and ion drifts in the polar ionosphere
Fig. 3. Two auroral-zone models (a) and (b). The ion density in shaded aurora1 zones is enhanced as is shown in Fig. 2.
6”
6”
(a) Fig. 4. Distribution
of convection electric fields for the auroral-zone models (a) and (b), calculated from the potentials as is shown in Fig. 1.
where
3. METHOD OF CALCULATION
We have used the following equations zontal motion for neutral and ion gases:
of hori-
Neutral gas; au at=2nvcose+3u-v)
av
(la)
~=-2nUcose+3o-v) Ion gas;
at4 e
,,=~(E,+uB,)+vi(u-u)=o au e ,,=--$Ey-uB,)+y(V-u)=O
(lb)
U, V =Southward and eastward components of the wind velocity, 4 o = southward and eastward components of the drift velocity, IV, ni = neutral and ion number densities, = collision frequency of an ion with neutrals, = angular frequency of Earth’s rotation, ;;: e, m, =charge and mass of an ion, E,,E,=southward and eastward components of the electric field, = upward component of the geomagnetic BZ field. The pressure and viscous terms for the neutral gas are neglected, and the inertial, pressure and Coriolis terms for the ion gas are also neglected.
MAFiDA HIROSHI
852
The equations (la) and (lb) are solved numerically by a method of successive approximation as was described in Paper 1, and we have a final solution for about 20 iterations. 4. RESULTS
Figure 5 shows the distribution of ion drifts (left) and neutral winds (right) at 300 km (top) and 120 km (bottom) calculated for the symmetric electric fields and the auroral-zone mode1 (a). It is found for the F-region that ion drifts have a simple E x I3 drift pattern very similar to those in Paper 1 (Fig. 7), and therefore the difference in the distribution of ion density does not have any significant effect on the ion drift. However, the neutral winds are much more influenced by the distribution of ion density in such a manner that the velocity becomes larger (about three times) than that in Paper 1, and the direction shifts clockwise by
about 30” from that of drift. For the E-region, the drift and wind velocities are larger (about three times) than those in Paper 1 (Fig. 9). The distribution of drifts has a similar pattern to that of winds in the F-region, but the directions of the former are shifted some 30” in a clockwise direction relative to the latter. It is also to be noted that the distribution of winds in the E-region has a similar complicated pattern to that in Paper 1 (Fig. ll), but the directions are much more shifted relative to each other, especially in the aurora1 zone. Figure 6 shows the distribution of ion drifts (left) and neutral winds (right) at 120 km for the asymmetric electric field, where Case 1 and Case 2 correspond to the enhancement of the electric field in the dawn and dusk sides, respectively. It is seen that the effect of asymmetry in the electric field is more clear in the ion drift than in the neutral wind, though the wind velocity becomes larger due to this effect, especially in the polar cap.
Fig. 5. Distribution of ion drifts (left) and neutral winds (right) at 300 km (top) and 120 km (bottom) in the polar ionosphere, calculated for the symmetric electric field.
Neutral winds and ion drifts in the polar ionosphere 12”
853
12”
Fig. 6. Distribution of ion drifts (left) and neutral winds (right) at 120 km in the polar ionosphere, calculated for the asymmetric electric fields such that the electric field in the dawn side is more (Case 1) or less (Case 2) enhanced than that in the dusk side.
Figure 7 shows the distribution of ion drifts (left) and neutral winds (right) at 120 km for the symmetric electric field and the two auroral-zone models (a) and (b). It is seen that the distributions of drift and wind in and near the amoral zone are much influenced by the auroral-zone models used. Generally speaking, this effect is more conspicuous in the wind than in the drift. Because the model (b) is more realistic than the model (a), it should be noted that the enhancement of ion density along the aurora1 zone has an important effect on the distribution of winds, not only inside, but also near the outside of the zone. 5. COMF’ARISIONWITH OBSERVATION AND DISCUSSION
As no detailed observations of drift and wind have recently been made for the polar F-region, we compare the present results with the observed val-
ues cited in Paper 1. It is generally
found that the agreement between calculated and observed results is better for ion drift than for neutral wind. Furthermore, it should be noted that the drift velocities have the same magnitude and pattern as those in Paper 1 in spite of a different profile of ion density, whereas the wind velocities have a different magnitude and pattern from those in Paper 1. This means that the neutral wind is more influenced by the distribution of ion density than the ion drift in such a manner that as the ion density increases, the neutral wind speed increases and becomes closer to the ion drift velocity. For the E-region, RINO et al. (1976) have recently published interesting results of highresolution auroral-zone drift and wind measurements by the incoherent scatter radar at Chatanika, Alaska (L = 5.6, A = 65”). Some of their results are reproduced in Figs. 8-10.
HIROSHI MAEDA
854
6”
I
Fig. 7. Distribution of ion drifts (left) and neutral winds (right) at 120 km in the polar ionosphere, calculated for the symmetric electric field and for the auroral-zone models (a) and (b).
5,
C
120Km teal.)
H
200
e _ T g
-2oo-
-
o-‘-..
119Km lobs.1 _YzE,
20
22
00
02
Local time (hr)
04
06 20
22
Local
00
02
04
06
time (hr)
Fig. 8. The eastward (a) and northward (b) components of the drift velocity at 125 km (top) and 119 km (bottom) observed at Chatanika, Alaska, during the night of 6 November 1974 (after RINO et al., 1976). The calculated results at 120 km in the aurora1 zone (full line) and polar cap (dotted line) are also shown for comparison.
Neutral winds and ion drifts in the polar ionosphere
Figure 8 shows the eastward (a) and northward (b) components of the drift velocity observed at 125 km (top) and 119 km (bottom) during the night of 6 November 1974, where the calculated results at 120 km in the aurora1 zone (full line) and polar cap (dotted line) are also shown for comparison. It is seen that the observed drifts at 125 km are in general agreement with the calculated ones at 120 km in the aurora1 zone, except for some phase differences (about 3 hr). Figure 9 shows the neutral-wind clock-dial plots observed during the night of 6 November 1974. It is seen that the wind velocities increase with increasing altitudes (from about 70 m/see at 102 km to about 300 mlsec at 125 km), and the wind directions are generally equatorward from 2000 to 0100 AST (Alaska standard time) and poleward from 0100 to 0600 AST in which a negative bay of about 500 gammas has been developed. Figure 10 shows the neutral-wind clock-dial plots observed during the day of 25-26 July 1976. It is seen that the wind velocities slightly increase with increasing altitudes (from about 130 m/set at 98 km to about 250 mlsec at 120 km), and the wind directions are generally poleward with some fluctuations at higher altitudes. In this period small magnetic disturbances (less than 200 gammas) have been developing. In order to compare these observations, we have calculated the drift and wind distributions at every 10 km in the E-region. Figure 11 shows the distribution of ion density at noon (1200 h) and midnight (0000 h) from 100 km (a) to 150 km (f). In NEUTRAL
VELOCITY
6 NOVEMBER
3
ALTITUDE = 120 km i-
/
lml
855
I- 0600
i
.i !
\
‘A__/,’ hllDNlGHT
*
LOCAL TIME
1250
scale
Fig. 10. Same as those in Fig. 9 for 25-26 July 1975 (after RINO ef al., 1976).
1974
0600 AST
1800 AST
iTUDE
= 102 km
TITIJDE
= 119 km
_
ITUDE
._-.L__
I-
I
!-
___-_I/..-
= 125 km
Fig. 9. Neutral-wind clock-dial plots for 6 November 1974, observed at Chatanika by the incoherent scatter radar (after RINO et al., 1976).
Fig. Il. Latitudinal distribution of ion density at noon (1200 h and midnight (0000 h) at every 10 kms in the polar E-region.
I
h
Fig. 12. Distribution of neutral winds at every 10 km in the polar E region calculated for the symmetric efecttic field and the aurora1 zone model fb). this calculation we have used the symmetric electric model (b). The calculated wind distributions are shown in Fig. 12. It is generally found that the wind velocities are very small at 100 km and increase with increasing altitudes (from about 50 mlsec at 110 km to about field and the auroral-zone
150 mfsec at 150 km), and that the wind patterns in and near the aurora1 zone do not show any remarkable change with altitude. In the aurora1 zone, the winds are toward the equator around noon and midnight and toward the pole around dawn and dusk, but this tendency is reversed in the polar cap.
Neutral winds and ion drifts in the polar ionosphere
Phase
(degree)
Fig. 13. Height-variations of the amplitude (full line) and phase (dotted line) of the neutral wind and ion drift at 0000 h, 0600 h, 1200 h and 1800 h in the polar cap (6 = 1O*)and aurorat zone (0 = 20“).
The height variations of the amplitude and phase of drift and wind are shown in Fig. 13, where the phase is measured counter-clockwise from southward. It is seen that the amplitudes of drift and wind rapidty decrease below 120 km, and that the phase differences between the drift and wind are larger at higher altitudes than at lower altitudes with some exceptions (at 0000 h in the aurora1 zone, and 1200 h in the polar cap). If we compare Fig. 12 and 13 with Fig. 9 and 10, it is found that the observed wind velocities are about two times larger than the calculated ones, and that the observed wind directions do not agree with the calculated ones in the aurora1 zone but agree with those in the polar cap. These results may suggest that the observed winds are equally affected by both electric fields and pressure gradients as mentioned in Paper 1 and by COMFORT et al. (1976). In fact, the neutral winds of thermal origin in the polar E-region have been estimated to have magnitudes of about 100 mlsec and directions from noon to midnight (see e.g. TARPLEY, 1970) on quiet days. Although the wind system on disturbed days is not yet clear, it seems that the enhancement in magnitudes (a few times) and the change in directions (from equatorward to poleward in the nighttime aurora1 zone) of the winds may be expected in association with aurora1 zone heating. HEAB and MEGILL (1975) and STRAUS and SCHULZ (1976) have recently estimated circulations in the polar thermosphere due to electric fields and Joule heating, and showed the importance of these
857
two effects in addition to the effect of EUV heating during the day. The main purpose of this series of papers is to make clear the contribution of convection electric fields to the neutral wind in the polar ionosphere. For detailed comparison with observation we must take into consideration all possibte effects that drive the neutral air into motion, and also the difference between the magnetic and rotational axes of the Earth. Furthermore, it should be noted that in the present analysis the effect of electronion collisions has been neglected. This is due to the reason that the coefficient of momentum transfer through collisions between ions and electrons 6,( = n,m,v-,), in the E-region isvery small as compared with that between ions and neutrals, @,( = niqvi,), and the drift velocities of ions and electrons are almost the same in the F-region. However, in regions between E and F the effect of 0, may not be negligible. 6. CONCLUSIONS From the results obtained in this paper, following conclusions may be drawn.
the
For the F-region (1) The ion drifts have simple EX B drift pattern, even if the distribution of ion density is disturbed. (2) The velocity of the wind is generally smaller than and is displaced from that of the drift, with a tendency that the larger the ion density, the smaller the difference between them. (3) The agreement between calculated and observed results is better for ion drift than for neutral wind (see Paper l), suggesting an important effect due to winds of thermal origin. For the E-region (1) The effect of asymmetric electric fields is more clear in the drift than in the wind, even though the wind field is generally enhanced by this effect. (2) The effect of different models of the aurora1 zone is more remarkable in the wind than in the drift. (3) The drifts have a similar pattern to those in the F-region, but shift clockwise by about 60”. (4) The winds have again a complicated pattern similar to those in Paper 1, such that the winds in the aurora1 zone are toward the equator around noon and midnight and toward the pole around dawn and dusk, with a reverse tendency in the polar cap.
858
Hmosru
(5) The drift and wind velocities generally increase with increasing altitudes, and the phase differences between those are also increased with increasing altitudes. (6) The observed drifts at Chatanika are in general agreement with the calculated ones in the aurora1 zone, except for some phase differences. (7) The observed winds are generally larger than the calculated winds, and the wind directions do not agree with the calculated ones, again suggesting an important contribution of thermal-origin winds. Finally, we would like to emphasize the need for further development and continuation of highresolution wind measurements by incoherent scatter radar and other techniques, because of their important role in understanding detailed dynamical
USEDA
structures of the ionosphere and thermosphere, especially in the aurora1 zone and polar cap. Note added In proof-The ratio of the term of ion-electron collision to that of ion-neutral collision may roughly be estimated as shown in the following table, and so it seems that the effect of ion-efectron colbsion can be neglected: Altitude (km) 100 120 140 170
%&I 4.87E-6 6.64B-5 2.18E-4 6.72E-4
Ui-Ve/Vi
-V”
@*(Vi
9.37E-t.l 9.25E-1 7.10E-2 l.OOE-2
1973 1974 1976
.l. geophys. Res. 78, 8235. .I. geophys. Res. 79, 2448. Planet. Space Sci. 24, 541.
1963 1975 1972 1976 1976 1970
Geomag. Aeronomy 3, 183. J. geophys. Res. 80, 1829. J. geophys. Res. 77, 4877. J. atmos. rerr. Phys. 38, 197. J. geophys. Res. 81, 5822. Plant. Space Sci. ia, 1091.
1976
Preprint for J. geophys. Res.
Reference is also made to the following unpublished material: RINO C. L., BIUXKE A. and BARON M. J.
v~)/ei,(Vi
-un)
4.56E-4 6.14E-5 1.55E-5 6.74E-6
Acknowledgements-The author would like to thank A. and C. L. RINO for helpful comments on the original version of this paper. BREKKE
REFERENCES BREKKEA., DOUPNIKJ. R. and BANKS P. M. BREKKE A., DOUPNIKJ. R. and BANKSP. M. COMFORTR. H., Wu S. T. and SWENSONG. R. &LDSrErN Y. I. HEAPS M. G. and MEGILL L. R. HEPPNERJ. P. MAEDA H. STRAUSJ. M. and SCHULZM. TARPLEYJ. D.
-
~ODU~ON
In Paper 1 (MAEDA, 1976) we calculated the distribution of ion drifts and neutral winds in the polar ionosphere in a quiet state caused by convection electric fields, and the following main results were obtained: (1) In the F-region the drifts have a simple E x J3 pattern, whereas the winds have magnitudes smaller than and directions shifted from the drifts. (2) The relation between the wind and drift in the E-region is strongly affected by the distribution of ion density, suggesting different relations for different stages of disturbance. (3) The winds of electric-field origin are of the same order of magnitude as those of thermal origin, so that both of these should be taken into consideration for the comparison with observation. Quite recently, high-resolution auroral-zone wind measurements were made by incoherent scatter radar at Chatanika, Alaska (BREKKE et al., 1973, 1974; RINO er al., 1976) and very interesting results have been obtained. RINK et al. (1976) observed detailed height-dependent structures of the E-region winds and showed that the daytime winds had a general pattern of the global pressuredriven winds below 110 km but strongly influenced by ion drag of electric-field origin above 110 km, whereas the night-time winds seemed to be much influenced by auroral-zone heating and also by ion drag at higher altitudes. COMFORT et al. (1976) reanalyzed the time-dependent and heightaveraged E-region winds derived from Chatanika I
by the method developed by BREKKE et al. (1973) and compared with theoretical calculations for the same conditions. They concluded that ion drag alone could not account for the observations and that the accelerations due to pressure gradients required to resolve the discrepancy between calculated and observed winds were of the same order of magnitude as those due to ion drag of electric-field origin. This paper deals with a disturbed state in which the enhancement of ion density in the aurora1 zone is taken into account, and two models for each of the convection electric field and the aurora1 zone are employed. The calculated results are presented in some detail and discussed in comparison with observation. data
2. MODELS
(a) Uectric field It has been shown (e.g. I-IEPPNER, 1972) that the dawn-to-dusk electric field observed from a sequence of OGO-6 orbits over the northern hemisphere is symmetric or asymmetric with respect to the geomagnetic pole, perhaps in association with the inte~lanetary magnetic field. Therefore, we employ two profiles (symmetric and asymmetric) of the electric fields as is shown in Fig. 1. Further, for the asymmetric fields, two cases are considered, i.e. the electric field in the dawn side is more (Case 1) or less (Case 2) enhanced than that in the dusk side.
849
HIROW (SYMMETRIC)
DUSK
(b) Ion density
DAWN I
....... 80’
I 70’
I 900
I 8Q
--30
1 70’
(ASYMMETRIC)
DUSK
Although considerable efforts have been made to model the polar ionosphere, we do not yet have any detailed information about the threedimensional profile of the ion-density distribution all over the polar regions, especially in disturbed states. However, we have some information about the enhancement of ion density in the aurora1 E-region from radio and rocket observations, so that we assume here a profile for the F-region and two profiles for the E-region as shown in Fig. 2(a, b). The figures show the latitudinal distribution of the ion density at noon (1200 h) and midnight (0000 h). The distributions at other local times have been interporated from those at 1200 h and 0000 h by the help of theoretical consideration.
POTENTIAL I 80’
80’
DAWN
I 60_
-FIELD
(c) AuroraE zone
.““...’ POTENTIAL
w 60’
MAEDA
I
I
I
I
1
70’
80’
90’
80’
70”
60’
LATITUDE
Fig. 1. Symmetric (top) and asymmetric (bottom) dawnto-dusk electric fields (full line) and their potentials (dotted line), as inferred from OGO-6 observations (after HEPPNER, 1972).
For the E-region, we take two profiles of the aurora1 zone; one is a zone (Fig. 3a) concentric with the geographic latitude, and. the other is a zone (Fig. 3b) inferred from aurora1 observations (e.g. FELUSTEIN, 1963). It is supposed that the enhancement of ion density as shown in Fig. 2(a, b) occurs only along the aurora1 zone as shown in Fig. 3(a, b), respectively. The distribution of the electric fields for these two auroral-zone models is calculated from the dawn-to-dusk distribution of electric potentials as shown in Fig. 1 (see Paper 1 for details), and the results are shown in Fig. 4(a, b) for the two models. It is also assumed that the geomagnetic axis coincides with the rotational axis of the Earth.
F-LAYER
E-LAYER T E lb2
r-1
‘\
90'
80’ LATITUDE
70’
60’
I 600
I
(bl
700
LATITUDE
Fig. 2. Latitudinal distributions of ion density at ncxm (1200 h) and midnight (0000 h) in the F- and E-layers for two auroral-zone models (a) and (b) as is shown in Fig. 3.
851
Neutral winds and ion drifts in the polar ionosphere
Fig. 3. Two auroral-zone models (a) and (b). The ion density in shaded aurora1 zones is enhanced as is shown in Fig. 2.
6”
6”
(a) Fig. 4. Distribution
of convection electric fields for the auroral-zone models (a) and (b), calculated from the potentials as is shown in Fig. 1.
where
3. METHOD OF CALCULATION
We have used the following equations zontal motion for neutral and ion gases:
of hori-
Neutral gas; au at=2nvcose+3u-v)
av
(la)
~=-2nUcose+3o-v) Ion gas;
at4 e
,,=~(E,+uB,)+vi(u-u)=o au e ,,=--$Ey-uB,)+y(V-u)=O
(lb)
U, V =Southward and eastward components of the wind velocity, 4 o = southward and eastward components of the drift velocity, IV, ni = neutral and ion number densities, = collision frequency of an ion with neutrals, = angular frequency of Earth’s rotation, ;;: e, m, =charge and mass of an ion, E,,E,=southward and eastward components of the electric field, = upward component of the geomagnetic BZ field. The pressure and viscous terms for the neutral gas are neglected, and the inertial, pressure and Coriolis terms for the ion gas are also neglected.
MAFiDA HIROSHI
852
The equations (la) and (lb) are solved numerically by a method of successive approximation as was described in Paper 1, and we have a final solution for about 20 iterations. 4. RESULTS
Figure 5 shows the distribution of ion drifts (left) and neutral winds (right) at 300 km (top) and 120 km (bottom) calculated for the symmetric electric fields and the auroral-zone mode1 (a). It is found for the F-region that ion drifts have a simple E x I3 drift pattern very similar to those in Paper 1 (Fig. 7), and therefore the difference in the distribution of ion density does not have any significant effect on the ion drift. However, the neutral winds are much more influenced by the distribution of ion density in such a manner that the velocity becomes larger (about three times) than that in Paper 1, and the direction shifts clockwise by
about 30” from that of drift. For the E-region, the drift and wind velocities are larger (about three times) than those in Paper 1 (Fig. 9). The distribution of drifts has a similar pattern to that of winds in the F-region, but the directions of the former are shifted some 30” in a clockwise direction relative to the latter. It is also to be noted that the distribution of winds in the E-region has a similar complicated pattern to that in Paper 1 (Fig. ll), but the directions are much more shifted relative to each other, especially in the aurora1 zone. Figure 6 shows the distribution of ion drifts (left) and neutral winds (right) at 120 km for the asymmetric electric field, where Case 1 and Case 2 correspond to the enhancement of the electric field in the dawn and dusk sides, respectively. It is seen that the effect of asymmetry in the electric field is more clear in the ion drift than in the neutral wind, though the wind velocity becomes larger due to this effect, especially in the polar cap.
Fig. 5. Distribution of ion drifts (left) and neutral winds (right) at 300 km (top) and 120 km (bottom) in the polar ionosphere, calculated for the symmetric electric field.
Neutral winds and ion drifts in the polar ionosphere 12”
853
12”
Fig. 6. Distribution of ion drifts (left) and neutral winds (right) at 120 km in the polar ionosphere, calculated for the asymmetric electric fields such that the electric field in the dawn side is more (Case 1) or less (Case 2) enhanced than that in the dusk side.
Figure 7 shows the distribution of ion drifts (left) and neutral winds (right) at 120 km for the symmetric electric field and the two auroral-zone models (a) and (b). It is seen that the distributions of drift and wind in and near the amoral zone are much influenced by the auroral-zone models used. Generally speaking, this effect is more conspicuous in the wind than in the drift. Because the model (b) is more realistic than the model (a), it should be noted that the enhancement of ion density along the aurora1 zone has an important effect on the distribution of winds, not only inside, but also near the outside of the zone. 5. COMF’ARISIONWITH OBSERVATION AND DISCUSSION
As no detailed observations of drift and wind have recently been made for the polar F-region, we compare the present results with the observed val-
ues cited in Paper 1. It is generally
found that the agreement between calculated and observed results is better for ion drift than for neutral wind. Furthermore, it should be noted that the drift velocities have the same magnitude and pattern as those in Paper 1 in spite of a different profile of ion density, whereas the wind velocities have a different magnitude and pattern from those in Paper 1. This means that the neutral wind is more influenced by the distribution of ion density than the ion drift in such a manner that as the ion density increases, the neutral wind speed increases and becomes closer to the ion drift velocity. For the E-region, RINO et al. (1976) have recently published interesting results of highresolution auroral-zone drift and wind measurements by the incoherent scatter radar at Chatanika, Alaska (L = 5.6, A = 65”). Some of their results are reproduced in Figs. 8-10.
HIROSHI MAEDA
854
6”
I
Fig. 7. Distribution of ion drifts (left) and neutral winds (right) at 120 km in the polar ionosphere, calculated for the symmetric electric field and for the auroral-zone models (a) and (b).
5,
C
120Km teal.)
H
200
e _ T g
-2oo-
-
o-‘-..
119Km lobs.1 _YzE,
20
22
00
02
Local time (hr)
04
06 20
22
Local
00
02
04
06
time (hr)
Fig. 8. The eastward (a) and northward (b) components of the drift velocity at 125 km (top) and 119 km (bottom) observed at Chatanika, Alaska, during the night of 6 November 1974 (after RINO et al., 1976). The calculated results at 120 km in the aurora1 zone (full line) and polar cap (dotted line) are also shown for comparison.
Neutral winds and ion drifts in the polar ionosphere
Figure 8 shows the eastward (a) and northward (b) components of the drift velocity observed at 125 km (top) and 119 km (bottom) during the night of 6 November 1974, where the calculated results at 120 km in the aurora1 zone (full line) and polar cap (dotted line) are also shown for comparison. It is seen that the observed drifts at 125 km are in general agreement with the calculated ones at 120 km in the aurora1 zone, except for some phase differences (about 3 hr). Figure 9 shows the neutral-wind clock-dial plots observed during the night of 6 November 1974. It is seen that the wind velocities increase with increasing altitudes (from about 70 m/see at 102 km to about 300 mlsec at 125 km), and the wind directions are generally equatorward from 2000 to 0100 AST (Alaska standard time) and poleward from 0100 to 0600 AST in which a negative bay of about 500 gammas has been developed. Figure 10 shows the neutral-wind clock-dial plots observed during the day of 25-26 July 1976. It is seen that the wind velocities slightly increase with increasing altitudes (from about 130 m/set at 98 km to about 250 mlsec at 120 km), and the wind directions are generally poleward with some fluctuations at higher altitudes. In this period small magnetic disturbances (less than 200 gammas) have been developing. In order to compare these observations, we have calculated the drift and wind distributions at every 10 km in the E-region. Figure 11 shows the distribution of ion density at noon (1200 h) and midnight (0000 h) from 100 km (a) to 150 km (f). In NEUTRAL
VELOCITY
6 NOVEMBER
3
ALTITUDE = 120 km i-
/
lml
855
I- 0600
i
.i !
\
‘A__/,’ hllDNlGHT
*
LOCAL TIME
1250
scale
Fig. 10. Same as those in Fig. 9 for 25-26 July 1975 (after RINO ef al., 1976).
1974
0600 AST
1800 AST
iTUDE
= 102 km
TITIJDE
= 119 km
_
ITUDE
._-.L__
I-
I
!-
___-_I/..-
= 125 km
Fig. 9. Neutral-wind clock-dial plots for 6 November 1974, observed at Chatanika by the incoherent scatter radar (after RINO et al., 1976).
Fig. Il. Latitudinal distribution of ion density at noon (1200 h and midnight (0000 h) at every 10 kms in the polar E-region.
I
h
Fig. 12. Distribution of neutral winds at every 10 km in the polar E region calculated for the symmetric efecttic field and the aurora1 zone model fb). this calculation we have used the symmetric electric model (b). The calculated wind distributions are shown in Fig. 12. It is generally found that the wind velocities are very small at 100 km and increase with increasing altitudes (from about 50 mlsec at 110 km to about field and the auroral-zone
150 mfsec at 150 km), and that the wind patterns in and near the aurora1 zone do not show any remarkable change with altitude. In the aurora1 zone, the winds are toward the equator around noon and midnight and toward the pole around dawn and dusk, but this tendency is reversed in the polar cap.
Neutral winds and ion drifts in the polar ionosphere
Phase
(degree)
Fig. 13. Height-variations of the amplitude (full line) and phase (dotted line) of the neutral wind and ion drift at 0000 h, 0600 h, 1200 h and 1800 h in the polar cap (6 = 1O*)and aurorat zone (0 = 20“).
The height variations of the amplitude and phase of drift and wind are shown in Fig. 13, where the phase is measured counter-clockwise from southward. It is seen that the amplitudes of drift and wind rapidty decrease below 120 km, and that the phase differences between the drift and wind are larger at higher altitudes than at lower altitudes with some exceptions (at 0000 h in the aurora1 zone, and 1200 h in the polar cap). If we compare Fig. 12 and 13 with Fig. 9 and 10, it is found that the observed wind velocities are about two times larger than the calculated ones, and that the observed wind directions do not agree with the calculated ones in the aurora1 zone but agree with those in the polar cap. These results may suggest that the observed winds are equally affected by both electric fields and pressure gradients as mentioned in Paper 1 and by COMFORT et al. (1976). In fact, the neutral winds of thermal origin in the polar E-region have been estimated to have magnitudes of about 100 mlsec and directions from noon to midnight (see e.g. TARPLEY, 1970) on quiet days. Although the wind system on disturbed days is not yet clear, it seems that the enhancement in magnitudes (a few times) and the change in directions (from equatorward to poleward in the nighttime aurora1 zone) of the winds may be expected in association with aurora1 zone heating. HEAB and MEGILL (1975) and STRAUS and SCHULZ (1976) have recently estimated circulations in the polar thermosphere due to electric fields and Joule heating, and showed the importance of these
857
two effects in addition to the effect of EUV heating during the day. The main purpose of this series of papers is to make clear the contribution of convection electric fields to the neutral wind in the polar ionosphere. For detailed comparison with observation we must take into consideration all possibte effects that drive the neutral air into motion, and also the difference between the magnetic and rotational axes of the Earth. Furthermore, it should be noted that in the present analysis the effect of electronion collisions has been neglected. This is due to the reason that the coefficient of momentum transfer through collisions between ions and electrons 6,( = n,m,v-,), in the E-region isvery small as compared with that between ions and neutrals, @,( = niqvi,), and the drift velocities of ions and electrons are almost the same in the F-region. However, in regions between E and F the effect of 0, may not be negligible. 6. CONCLUSIONS From the results obtained in this paper, following conclusions may be drawn.
the
For the F-region (1) The ion drifts have simple EX B drift pattern, even if the distribution of ion density is disturbed. (2) The velocity of the wind is generally smaller than and is displaced from that of the drift, with a tendency that the larger the ion density, the smaller the difference between them. (3) The agreement between calculated and observed results is better for ion drift than for neutral wind (see Paper l), suggesting an important effect due to winds of thermal origin. For the E-region (1) The effect of asymmetric electric fields is more clear in the drift than in the wind, even though the wind field is generally enhanced by this effect. (2) The effect of different models of the aurora1 zone is more remarkable in the wind than in the drift. (3) The drifts have a similar pattern to those in the F-region, but shift clockwise by about 60”. (4) The winds have again a complicated pattern similar to those in Paper 1, such that the winds in the aurora1 zone are toward the equator around noon and midnight and toward the pole around dawn and dusk, with a reverse tendency in the polar cap.
858
Hmosru
(5) The drift and wind velocities generally increase with increasing altitudes, and the phase differences between those are also increased with increasing altitudes. (6) The observed drifts at Chatanika are in general agreement with the calculated ones in the aurora1 zone, except for some phase differences. (7) The observed winds are generally larger than the calculated winds, and the wind directions do not agree with the calculated ones, again suggesting an important contribution of thermal-origin winds. Finally, we would like to emphasize the need for further development and continuation of highresolution wind measurements by incoherent scatter radar and other techniques, because of their important role in understanding detailed dynamical
USEDA
structures of the ionosphere and thermosphere, especially in the aurora1 zone and polar cap. Note added In proof-The ratio of the term of ion-electron collision to that of ion-neutral collision may roughly be estimated as shown in the following table, and so it seems that the effect of ion-efectron colbsion can be neglected: Altitude (km) 100 120 140 170
%&I 4.87E-6 6.64B-5 2.18E-4 6.72E-4
Ui-Ve/Vi
-V”
@*(Vi
9.37E-t.l 9.25E-1 7.10E-2 l.OOE-2
1973 1974 1976
.l. geophys. Res. 78, 8235. .I. geophys. Res. 79, 2448. Planet. Space Sci. 24, 541.
1963 1975 1972 1976 1976 1970
Geomag. Aeronomy 3, 183. J. geophys. Res. 80, 1829. J. geophys. Res. 77, 4877. J. atmos. rerr. Phys. 38, 197. J. geophys. Res. 81, 5822. Plant. Space Sci. ia, 1091.
1976
Preprint for J. geophys. Res.
Reference is also made to the following unpublished material: RINO C. L., BIUXKE A. and BARON M. J.
v~)/ei,(Vi
-un)
4.56E-4 6.14E-5 1.55E-5 6.74E-6
Acknowledgements-The author would like to thank A. and C. L. RINO for helpful comments on the original version of this paper. BREKKE
REFERENCES BREKKEA., DOUPNIKJ. R. and BANKS P. M. BREKKE A., DOUPNIKJ. R. and BANKSP. M. COMFORTR. H., Wu S. T. and SWENSONG. R. &LDSrErN Y. I. HEAPS M. G. and MEGILL L. R. HEPPNERJ. P. MAEDA H. STRAUSJ. M. and SCHULZM. TARPLEYJ. D.
-