- Email: [email protected]
About the nature of the Night-time Winter Anomaly effect (NWA) in the F-region of the ionosphere
Planet. Space Sci., Vol. 43, No. 5, pp. 603612,
1995 Elsevier Science Ltd Printed in Great Britain 0032-0633/95 $9.50+0.00
Pergamon
0032-0633(94)00115-4
About the nature abfthe Night-time Winter Anomaly effect (NWA) in the F-region of the ionosphere N. Jakowski’
and M. F6rster’
’ Deutsche Forschungsanstalt fiir Luft- und Raumfahrt e.V., Fernerkundungsstation Neustrelitz, Kalkhorstweg 53, 17235 Neustrelitz, Germany 2 Max-Planck-Institut fur extraterrestrische Physik, AuBenstelle Berlin, Rudower Chaussee 5, 12489 Berlin-Adlershof, Germany Received 15 March 1994 ; revised 11 May 1994 ; accepted 11 May 1994
sector during the low solar activity years 1974-1975. In analogy to the well-known seasonal or winter anomaly effect of the daytime F2-layer the NWA is characterized by a higher night-time F2-layer ionization level during winter months than during summer. Since the daytime winter anomaly effect was not observed during years when NWA occurred and vice versa, both phenomena are decoupled and subjected to different physical mechanisms. In a first attempt to interpret the NWA effect, Jakowski and co-workers (1981, 1983, 1988) assumed that the considerable asymmetry in geomagnetic-geographic relationships at the American longitude sector provide good conditions for an effective interhemispheric plasma transport from the southern (summer) hemisphere to the northern (winter) hemisphere. Due to this asymmetry which is well documented by the displaced geomagnetic and geographic equators, the NWA is not observed at the southern hemisphere winter in June/July. This hypothesis was supported by apparent similarities in the behaviour of the night-time ionization at nearly geomagnetically conjugated regions (Havana, Port Stanley) over 11 months and unusual high slab thickness values at the northern winter night indicating incoming plasmaspheric fluxes. To check the influence of the geomagnetic-geographic relationship, the nocturnal ionospheric behaviour at the Asian sector around 1 = 110”E has been studied by comparing ionosonde data from Irkutsk (L = 2.0) in the northern hemisphere and from Mundaring (L = 1.9) in the southern hemisphere (Jakowski et al., 1990). Although less pronounced than in Havana, the NWA effect was observed in Mundaring too. In analogy to the stations Havana and Port Stanley there were found apparent similarities in the night-time behaviour at the nearly conjugated stations Irkutsk and Mundaring. To study the effect in more detail an adequate physicalnumerical model has been applied to the coupled system ionosphere-plasmasphere-ionosphere along flux tubes (Forster and Jakowski, 1986,1988). Modelling the plasma
N. Jakowski and M. F(irster : NWA in the F-region of the ionosphere
604
along the L = 1.5 flux tube at the American sector there have been found indications that the NWA effect is closely related to interhemispheric coupling processes. The aim of this paper is to provide further evidence that the NWA effect is a regular phenomenon at special geomagnetic-geographic relationships under low solar activity conditions. Numerical modelling is used to get more insight into the physical mechanism of this phenomenon.
2. Observations In order to probe the influence of the geomagnetic-geographic relationship upon the occurrence of the NWA effect, ionospheric data both from the American as well as from the Asian longitude sector are analysed. The stations from which data have been taken are listed in Table 1. With respect to the geomagnetic-geographic relationship the American sector is apparently symmetric to the Asian longitude sector (A = 1lO”E) as it is illustrated in Fig. 1. Thus NWA observations at the American northern hemisphere should correspond with indications of NWA at the southern hemisphere at the Asian longitude sector. Using vertical sounding (@2) and total electron content (&) data the mean ionization level of the night-time ionosphere (03/04 LT) obtained at different stations at both longitude sectors are compared in Fig. 2. The ionospheric electron content NF has been derived from Faraday rotation measurements at linearly polarized VHF signals received in Havana from different geostationary satellites such as ATS-6, SMS-1, and GOES between 1974 and 1983. As Fig. 2 shows, the NWA effect is a persistent feature during years of low solar activity both at the American longitude sector (northern hemisphere) as well as at the Asian longitude sector (southern hemisphere). To study the local time dependence of the NWA effect at the Mundaring ionosonde station, the diurnal behaviour of NmF2 is plotted for January and June 1975 in Fig. 3a. Whereas at the Havana station (L = 1.5) the NWA effect starts already at pre-midnight and ends at sunrise (Fig. 3b), the NWA effect observed in Mundaring (L = 1.9) which is somewhat less pronounced, starts after midnight
Table 1. Table of geographic and geomagnetic coordinates
and corresponding L-values of stations from which data have been used in this paper
Station Havana
Geographic Lat. Long. 23.1
277.5
Geomagnetic Lat. Long. 34.2
L-value
346.0
1.5
9.5
1.6
WAV)
Port Stanley (PST) Irkutsk (IRK) Brisbane (BBI) Mundaring (MUN)
-51.7 52.5
302.2
-40.4
104.0
41.1
174.8
2.0
-27.5
152.9
- 35.7
227.4
1.5
- 32.0
116.3
-43.4
186.8
1.9
and lasts until sunrise hours. The maximum is reached at about 03-04 LT. The comparison with the corresponding high solar activity data from January and June 1980 at the bottom of Fig. 3a,b shows that the NWA vanishes at high solar activity conditions due to a strong increase of the night-time ionization in summer. Probably the mechanisms leading to the NWA effect are masked by the higher ionization level in summer night. It is interesting to note that the daytime anomaly phenomenon is well documented at high solar activity. The hypothesis that interhemispheric coupling may explain the NWA effect is underlined by close similarities in the annual variation of monthly values of the averaged night-time ionization obtained at nearly geomagnetically conjugated stations (Fig. 4). Following the idea of interhemispheric coupling and enhanced plasmaspheric fluxes during NWA winter nights, the topside ionosphere should be expected to react more sensitive to such fluxes than the lower parts. Indeed, as the Alouette 1 topside sounder profiles of the electron density n, indicate in Fig. 5, the increase of ionization during the northern winter night at the American longitude sector is much more pronounced at the topside ionosphere (cf. profiles 2 and 3). At the southern hemisphere the NWA is completely absent (cf. profiles 1 and 4). It is interesting to note that electron densities are nearly equal at both hemispheres at about 1000 km height during the December solstice (cf. profiles 3 and 4), providing a further argument for the existence of interhemispheric coupling processes.
3. Numerical model The physical-numerical computations of the present work are based on a coupled time dependent model describing the plasma behaviour along flux tubes. The flux tube which is derived from a centred geomagnetic dipole field tilted with regard to the geographic axis by about 11.4 degrees, comprises the local ionosphere above 120 km altitude, the plasmasphere, and the magnetically conjugated ionosphere down to 120 km altitude. An extended description of this model has been given in the paper of Fiirster and Jakowski (1988). In essence, the model solves the coupled, full time-dependent continuity and momentum equations (forming so-called diffusion equations) of the atomic ions H+, O+ and He+ as well as the electron and ion heat conduction equations along the selected flux tube and solves also the diffusion equations of the molecular ions O?+, NO+ and NT in the magnetically conjugated ionospheres The model, therefore, provides values of atomic ion concentrations for Of, H+ and He+, their field-aligned velocities, electron and ion temperatures along the whole flux tube as well as density and field-aligned velocities of the molecular ions 0:) NO+ and N: in the magnetically conjugated ionospheres. Further, to describe photoionization as ion particle and heat source in a more correct way the kinetic equation of the suprathermal electron component is also solved. This includes the photoionization and secondary ionization processes of the neutral gas, the generation of the suprathermal electrons, their motions, trapping, and thermalization in the plas-
N. Jakowski and M. Fiirster : NWA in the F-region of the ionosphere
605
Fig. 1. Schematic representation
of the geographic-geomagnetic relationship at the American and Asian sectors for a ray-path corresponding to Faraday rotation observations of a geostationary satellite from ground-based stations near L = 1.5 ; further the modelled flux tube is indicated
ionosphe-lc 0:3/04
2
~onlzatlon LT -
Summer
----
ulnter
/
Hav
foF2
/
Brl
PlHz
--
1974
foF2
.I976
.I978
;990
1982
'1994
'1986
YEftRS
86 model for neutral gas composition and neutral gas temperature (Hedin, 1987) has been used as background thermospheric model. It is very important for the ionized components as they are trace constituents at E- and Fregion heights whose physical behaviour are determined in a high degree by the neutral gas. Chemical reaction rates, photoionization and -absorption rates, solar EUV fluxes for low solar activity level, particle collision frequencies, cooling rates, thermal conductivities, etc., i.e. all this great amount of input parameters and coefficients which are an important part of concrete numerical simulation have been taken from recent review articles. For more details concerning the used parameters see the model description in the paper of Fdrster and Jakowski (1988). The altitude profile of the neutral air wind is assumed to vary in accordance with the neutral temperature profile T,(z), i.e. it increases steadily from zero value at the lower boundary at 120 km height up to an asymptotical amplitude in the F-region. The local time neutral wind variation in accordance with the global-scale dynamics of the thermosphere has been selected in an iterative manner for the northern and southern hemisphere and for both solstices separately as described in the next section.
Fig. 2. Solar cycle dependence of night-time averaged ionization (0344 LT) measured byfoE and the ionospheric electron content NF at the ionosonde stations Havana (Hav), Brisbane (Bri), and Mundaring (Mun). The NWA effect is indicated by shadowed areas. The solar activity level is indicated at the bottom by the solar radio flux E;0.7 (lo-” Wm-’ Hz-‘) maspheric flux tubes, i.e. the local and nonlocal heating processes of the thermal plasma provided by photoionization. No E x B drift of the flux tubes has been included so far, because for middle latitudes and during geomagnetically quiet conditions the only acting dynamoelectric &-fields (Wagner er al., 1980) are known to be small. For the model simulations carried out in this paper we assumed geomagnetically quiet conditions (Ap = 4) and a low level of solar activity representative for the years 1963-1964 and 1974.1976. We assumed an 81-day average of the solar radio flux parameter F,0.7 as input for the neutral gas and solar radiation models of F,,,, = 80 [1O-22 W me2 Hz-‘] and daily values of JirO., = 79 for December and F,,,, = 83 for June solstice. The MSIS-
4. Modelling and discussion The numerical model described in the previous section is a powerful tool to get more insight into the physical mechanism of the NWA effect. In a first step the windfree solution was compared with the corresponding observational data (hmF2, Nmf2, NF) to derive the structure of the meridional wind pattern at both hemispheres at the American longitude sector. We have probed a sequence of different wind patterns. There was also considered a self-consistent solution taking into account the equations of motion for neutral wind components (driving forces derived from MSIS-86 pressure gradients) together with the set of equations for the ionized components. The height of the F2-layer which is very sensitive to the meridional wind velocity in midlatitudes was used together with the measured NmF2 and NF values to optimize the meridional wind pattern in both hemispheres (Forster and Jakowski, 1986, 1988). So, a
N. Jakowski and M. Forster : NWA in the F-region of the ionosphere
606 7
:
I4
NmF2
12
Lo 0
10
” z
Ionosonde
monthly
6
Mundarinq
medians
8 -
6
z
summer
\
4
l’
\
\
I /
\ \
‘,
NmFZ
/
1’
PST
\
/
//
J
In
IO-
0
EJ-
E :
6-
1586
\
IRK
\
t-
\ \
2
‘-_
-_’
____--.
MUN 0
0
(‘1’1(1111’1”““1~~” 4
s
12
16
LOCFlL
(a)
20
24
TINE
60
JAN
NF ”
Havana monthly
:
E
---
L 20
MRR
APR
PtFlY JUN
JUL
RUG
SEP
OCT
NOL
DEC
(03-04 LT) represented by NmF2 and the ionospheric electron content NF observed at nearly geomagnetically conjugated stations at the American (upper panel) and Asian (bottom) longitude sectors
medians
40
0
z
FEB
Fig. 4. Seasonal variation of the averaged night-time ionization
summer winter
06/76 1200
Alouette
2 0
2
Electron 1000
1
1963
density
proflles Longitude
n,(h)
sector LT
: 64 - 82”W : 01:30
Dip: f53
(7 E
40
x 0 c IL 2
600
20 .
‘.
‘\
400 0
4
(b)
a
12
LOCAL
16
TIME
20
w
24
south
(hours)
Fig. 3. Diurnal variations of ionospheric parameters NmF2 and
NF at the Asian longitude sector (a) and at the American longitude sector (b), respectively. Comparison is made between median values representative for winter and summer conditions under low (1975/76) and high (1980) solar activity conditions suited wind pattern was selected by an iteration cycle of trial and error. This meridional wind variation versus local time is shown for June (Fig. 6a) and December solstice (Fig. 6b) with dashed lines for the southern and full lines for the northern hemisphere. The highest amplitudes are about 200 m SK’ with generally larger amplitudes at the southern footpoint of the flux tube whereas during June solstice in the northern hemisphere the meridional wind near the subsolar point latitude is lowest. The wind pattern for northern December comprises the reversal of mer-
s
200
north
u north
‘\
\
\
5
soutt
1
’ 1E+5
1E+4
ELECTRON
lE+6
DENS1 TY (cm -3)
Fig. 5. Electron density profiles n,(h) obtained by the Alouette
1 topside sounder for geomagnetically conjugated regions at the American longitude sector on the days 156 (5 June 1963, curves 1 and 2) and 352 (18 December 1963, curves 3 and 4) at nighttime (01: 30 LT). The full lines correspond to the northern hemisphere, the dashed lines are related to southern hemisphere
idional wind for l-2 h after midnight caused by the secondary midnight pressure bulge as observed by AE-E (Spencer et al., 1979) and Fabry-Perot interferometer measurements above Arecibo (Friedman and Herrero, 1982). The availability of topside sounder profiles from Alouette 1 provides a good chance to test both the model
N. Jakowski ‘;‘
2oo z
: IOWA in the F-region
and M. Forster June
1963
(a)
z 3
100 r ,
2 E a z
s
f! F3 E
/’ -100
/’
,
,’
,. ’
_--.
‘\
‘\
‘\ \ \ \ \
,’
‘\
7”
-200-1
of the ionosphere
I
I
I.
I
‘.
..-
,
0
6
12 LOCAL, TIME (hours)
16
24
0
6
12 LOCAL TIME (hours)
16
24
601
Considering the FZlayer, the computed electron density profiles agree well with the measured electron densities for both days in the model assumptions so far. On the other hand, the upper ionosphere electron densities are somewhat overestimated indicating additional processes not considered. However, what is most important to explain the NWA effect, the essential higher densities in winter are well reproduced both for the upper ionosphere (Nr) as well as for the F2-layer (@IQ). Both model calculations for days 156 and 352 in 1963 may additionally be checked by TEC-measurements made in Havana by means of the Faraday rotation technique under similar geophysical conditions (Fig. 8). Since Faraday rotation data provide the ionospheric electron content up to about 2000 km height, the tube content was calculated by the model up to this height taking into account a simplified projection to the radio beacon ray path used for the measurements. The lower part of the flux tube up to an altitude of about 2000 km is approximately parallel to the beacon ray path used for the measurements obtained in Havana. So, the slant columnar content can be recalculated by a simple projection to the vertical electron content Nr. Although observations are related to different solar cycle minima, the model calculations are very similar to the TEC obser-
NF
Havana monthly
Fig. 6. Local time variation of the meridional neutral wind component at thermospheric height for June (upper panel a) and December (lower panel b) solstice assumed for model calculations in the northern (full lines) and southern hemisphere (dashed lines) ; positive values correspond to southward winds which are equatorward in the northern and poleward in the southern hemisphere
medians
‘z
as well as our hypothesis about the nature of the NWA effect. Topside sounder profiles obtained at individual days in June and December 1963 are compared with the corresponding numerical calculations in Fig. 7.
E t;
N,
= 20
MODEL
model
Alouette
calculations 1
ne(h)
related profiles,
to 1963
CALCULATIONS
0
4
8
12
LOCAl lo4 ELECTRON
DENSITY
lo5 (cm-‘)
Fig. 7. Comparison of calculated electron density profiles at 01: 30 h local time with Alouette 1 topside sounder data for June (day 153/63, full line and asterisks for model calculations and observations, respectively) and December (day 352/63, dashed line and crosses) solstices
16
20
TIME
(hours)
2’4
Fig. 8. Comparison of ionospheric electron content data NF observed at Havana in 1976 with corresponding numerical calculations at similar geophysical conditions. The upper panel shows monthly medians of NF obtained in June and December 1976 (F10.7= 70.6/76.7). The lower panel shows results of numerical calculations applied to individual days in June and December 1963 (F,0,7 = 83.5/78.4) selected for simulation studies (see Fig. 7)
608
N. Jakowski and M. Fiirster : NWA in the F-region of the ionosphere
vation at both seasons. So the assumed wind patterns (Fig. 6) at least for the northern hemisphere are thought to be justified. Since meridional winds are very effective in vertical transport at mid-latitudes (maximum at geomagnetic inclination I = 45”) significant upward plasma fluxes at the southern hemisphere may cause a high tube content in December/January. The fact that neutral winds may enhance the filling of the plasmasphere in mid-latitudes was already underlined by several authors (e.g. Sethia et al., 1983; Forster and Jakowski, 1988). Due to the southward displacement of the geomagnetic equator against geographic equator by about 11 degrees there results a seasonal asymmetry in filling conditions of the plasmasphere. So, the duration of the illumination at the conjugated southern hemisphere is additionally enlarged in December/January whereas in June/July it is additionally reduced. Such an argument was already suggested by Kersley and Klobuchar (1978) to explain the higher plasmaspheric electron content at night compared with daytime values at Hamilton in contradiction to Aberystwyth plasmaspheric data. As has been shown by various authors (e.g. Roble and Dickinson, 1974 ; Richards and Torr, 1986; Bailey et al., 1991) the tilted dipole of the Earth’s magnetic field leads to special features in the thermosphere and ionosphere structure. The NWA effect is probably one of these peculiarities. The asymmetry between both hemispheres is well documented in the model calculations along the flux tube L = 1.45 (Fig. 9) as already discussed above. The plasma distribution along the flux tube gives a good impression of the close coupling between both hemispheres leading to the seasonal anomaly at the northern winter night. The interhemispheric coupling may be studied in more detail by considering the field aligned plasma flow along the flux tube as shown in Fig. 10. This colour plot gives the diurnal variation of fieldaligned ion flux densities along the entire flux tube for the
,,s~
,
, MODYL CAFCULAYIONS /
I
120
406
, 21.5
I 18.5
, 32.8
I 30.8
1
/
~
I
I
1708 2907 1708 058 406 120 HEIGHT (km) I I I I / f 16.2 9.2 -11.3 -31.8 -38.8 -42.1 -44.0 GEOGRAPHIC LATITUDE (deg) I / I I I I 1 27.5 20.5 0.0 -20.5 -27.5 -30.8 -32.6 GEOMAGNETIC LATITUDE (deg) 856
Fig. 9. Electron density profiles along flux tube for 01: 30 h local time according to model calculations for day 156/63 (full line) and day 352163 (dashed line)
model calculations of day 156/63 in the upper panel and day 352/63 in the lower panel. The scales on the left and right hand side indicate height and distance along the flux tube from the northern footpoint, respectively, in nonuniform axes : the distances in the ionosphere are stretched and in the plasmasphere they are compressed. The abscissa refers to local time of the northern footpoint. The colour-coded flux intensities distinguish between positive fluxes with the blue-green scale which are directed from the northern to the southern hemisphere along the flux tube and negative Ruxes with the red-yellow scale for the opposite direction. Fluxes smaller than or equal to IO7 cm-2 s’ are indicated by a black area. The maxima of the ion flux intensities occur during sunrise and sunset at F-layer heights. The peak values in the lower part of the F-region during afternoon hours or near sunset are about lo9 cm-* s-’ in the summer hemispheres and about 2.5 x lo9 cmm2 s-’ in the winter hemispheres in both cases. Sunrise occurs in the F-layer around 04:45 LT in the northern and around 07 : 00 LT in the southern hemisphere in June (Fig. 10, upper panel) and around 06:OO LT and around 03: 15 LT, in December (Fig. 10, lower panel), respectively. During these times a rapid increase of the field-aligned plasma flux can be seen at all heights above about 200 km in the corresponding hemisphere which is directed into the plasmasphere. The heating of the plasmasphere and the high thermal conductivity lead to changes of the plasma flux in the magnetically conjugated ionosphere. This effect is reflected in a diminished or even reversed outflow from the plasmasphere before local sunrise (e.g. southern upper ionosphere at about 05:30 LT for the June case) or in an increased upward directed flux after local sunrise. Local sunset in the F-layer around 19: 15 LT in the northern and 17: 30 LT in the southern hemisphere for June and around 18 : 00 LT and 20 : 30 LT for the December case is accompanied by the thermal breakdown of the ionosphere with maxima1 fluxes from the plasmasphere into the ionosphere. Further obvious modulations of the field-aligned ion fluxes are caused by changes of the local meridional wind component as it can be seen, for example, during the wind reversal after midnight in the northern hemisphere of the December case (Fig. 10, lower panel). The night-time fluxes into the ionosphere lead to night-time enhancements of the ionospheric content as long as the local chemica1 loss processes are less than the influx of plasma. These fluxes are more intense for the December solstice winter hemisphere than for the June winter hemisphere and act in favour of the NWA effect in northern winter. It is well known that downward directed plasmaspheric fluxes play an important role in maintaining the night-time ionization in F2-layer heights at mid-latitudes (Park et al., 1978; Evans and Holt, 1978 ; Vickrey et al., 1979). In this simplified picture the plasmasphere then is refilled by upward directed plasma fluxes from the ionosphere at daytime. With increasing latitudes the plasmasphere acts more and more as a plasma reservoir with balanced up- and down-flows over a whole day (Jakowski et al., 1983). The computations at L = 1.45 have shown that interhemispheric fluxes lead to a significant imbalance of these fluxes particularly at the northern winter hemisphere generating the NWA effect. The net daily interhemispheric
N. Jakowski
and M. Forster : NWA in the F-region
FIELD -
609
of the ionosphere
ALIGNED FLUXES
:
JUNE
120
0.0
405
0.4
854
1.0
1687
2.2
2816
5.6
E3
1687
9.0
g s
854
10.3
* 5
405
10.9
s 5
120
11.3s
0
6
12
18
zi ki
24
DECEMBER
I
0.0
T
405
0.4
f$
854
1.0
E
1687
2.2
2816
5.6
1687
9.0
g s
854
10.3
d 5
405
10.9
3
120
11.3:
120
6
-9.0
-8.5 COLOR
-8.0 CODE
12 LOCAL TIME (hrs)
-7.5 k7.0 FOR LOG OF FLUX
18
8.0 7.5 8.5 INTENSITIES (cm%-‘)
24
9.0
Fig. 10. Colour plot of field-aligned plasma fluxes along a flux tub of L = 1.45 vs local time for June (upper panel) and December (lower panel) model calculations. The colour code for flux intensities is given at the bottom of the figure. Fluxes less than f 10’ cm-’ s-’ are indicated by a black area
2 5 ti
N. Jakowski and M. F6rster : NWA in the F-region of the ionosphere flow generally directed from the summer to the winter hemisphere (Vickrey et al., 1979) maximizes at the American longitude sector during the December solstice resulting in an increased plasmaspheric density compared with June solstice values. In accordance with this, the plasmaspheric tube content resulting from the calculations for day 352 exceeds that obtained for day 152 in 1963 as it is shown in Fig. 11. This result fully agrees with tube content data inferred from whistler measurements at the American longitude sector (Park et al., 1978). Beacon satellite observations using ATS-6 signals by Davies et al. (1976) in 1974-1975 revealed also the highest plasmaspheric electron content at Boulder station in December/January. It is evident that the high flux tube content leads to an enhanced plasma outflow at both hemispheres during night. If the downward fluxes are strong enough, loss processes may be overcompensated so that so-called nighttime enhancements of the ionospheric ionization may be observed. According to Jakowski et al. (1991) significant night-time enhancements observed in Havana under low solar activity conditions (19761) require downward fluxes of about 3 5 x 108 cm -2 s - ' . In agreement with observations and modelling results presented in that paper the occurrence probability PNE of night-time enhancements in Nv has a clear maximum in northern winter nights under low solar activity conditions (PNE ~ 100% for winter and PYE ~ 30 o/o for summer). It is interesting to note that the seasonal variation of the occurrence probability is opposite in Havana during years of high solar activity (PNF ~ 30-50% tbr winter and PNE ~ 80% for summer). This fact coincides very well 'with the vanishing of the NWA effect at an enhanced solar activity level (Fig. 2). We suppose that under high solar activity conditions the interhemispheric coupling is masked by local ionosphereplasmasphere coupling processes at both ends of the flux tube considered here, leading 1o a local equilibrium in a diurnal averaged sense. As Richards and Torr (1985) have shown theoretically, the limiting H + ion flux flowing from the ionosphere to the plasmasphere decreases with increasing solar activity. If so, interhemispheric coupling MODEL CALCULATIONS
611
via protonospheric fluxes is reduced with increasing solar activity. Thus the influence of interhemispheric fluxes diminishes with increasing solar activity in the same way as the effect of interhemispheric coupling reduces with growing latitudes under low solar activity conditions already discussed above.
5. Conclusions
Taking into account both the observations from different longitude sectors as well as the results of numerical modelling, the night-time winter anomaly effect (NWA) occurs : • at low solar activity level • at geomagnetic mid-latitudes • when geographic and geomagnetic latitudes are displaced by a certain degree. If the above-mentioned conditions are fulfilled, NWA is mainly explained by interhemispheric coupling via plasma fluxes along magnetic flux tubes. Geomagnetic-geographic asymmetry is an essential precondition for an effective filling of the plasmasphere at the summer hemisphere resulting in enhanced downward fluxes at the winter hemisphere up to 7 x 10~ c m -~ sduring sunset at 1000 km height in the winter hemisphere. As numerical simulation has shown, probably thermospheric meridional winds play an important role in driving interhemispheric fluxes. With increasing solar activity local plasma processes become more important versus the plasma reservoir aspect of the plasmasphere, so that interhemispheric coupling effects are masked leading to a vanishing of NWA at higher levels of solar activity. Acknowledgement. The authors are grateful to Mrs D. Adrian
for her assistance in data processing and in preparing the manuscript. This work was supported by the Deutsche Agentur for Raumfahrtangelegenheiten (DARA) GmbH under grant 50 QL 92060 and 50-YI9202. The authors thank the referees for valuable comments.
References Bailey, G. J., Sellek, R. and Balan, N., The effect of inter-
8
oZ ~
/
4
"
..352/63
~ ~ -
×
L= 0
6
12 18 LOCAL TIME ( h o u r s )
1.45
24
Fig. ll. Total flux tube content vs local time for June (day 156/63, full line) and December (day 352/63, dashed line) solstices according to the model calculations presented in this paper (a cross-sectional area of 1 cm2 refers to the 120 km altitude level)
hemispheric coupling on nighttime enhancements in ionospheric total electron content during winter at solar minimum. Ann. Geophys. 9, 738-747, 1991. Davies, K., Fritz, R. B. and Gray, T. B., Measurements of columnar electron contents of the ionosphere and plasmasphere. J. geophys. Res. 81, 2825-2834, 1976. Evans, J. V. and Holt, J. M., Nighttime proton fluxes at Millstone Hill. Planet. Space Sci. 36, 727-744, 1978. F6rster, M. and Jakowski, N., Interhemispheric ionospheric coupling at the American sector during low solar activity, II. Modelling, Gerlands Beitr. Geophys. 95, 301-314, 1986. F6rster, M. and Jakowski, N., The nighttime winter anomaly (NWA) effect in the American sector as a consequence of interhemispheric ionospheric coupling. PAGEOPH 127, 447 471, 1988. Friedman, J. F. and Herrero, F. A., Fabry-Perot interferometer measurements of thermospheric neutral wind gradients and reversals at Arecibo. Geophys. Res. Lett. 9, 785-788, 1982.
612
N. Jakowski and M. F6rster : NWA in the F-region of the ionosphere
Hedin, A. E., MSIS-86 thermospheric model. J. geophys. Res. 92, 46494662, 1987.
Jakowski, N., Bettac, H. D., Lazo, B. and Lois, L., Seasonal variations of the columnar electron content of the ionosphere observed in Havana from July 1974 to April 1975. J. atmos. terr. Phys. 43, 7-11, 1981.
Jakowski, N., Bettac, H. D., Felske, D., Lazo, B. and Lois, L., Observations of the total electron content of the ionosphere in Havana during a period of low solar activity 1974-1976. Gerlands Beitr. Geophys. 92, 151-161, 1983. Jakowski, N., F6rster, M., Lazo, B. and Lois, L., Interhemispheric ionospheric coupling at the American sector during low solar activity, I. Observation. Gerlands Beitr. Geophys. 95, 219-227, 1988. Jakowski, N., Landrock, R. and Jungstand, A., The Nighttime Winter Anomaly (NWA) effect at the Asian longitude sector. Gerlands Beitr. Geophys. 99, 163-168, 1990. Jakowski, N., Jungstand, A., Lois, L. and Lazo, B., Night-time enhancement on the F2-1ayer ionization over H a v a n a - Cuba. J. atmos, terr. Phys. 53, 113-118, 1991. Kersley, L. and Klobuchar, J. A., Comparison of protonospheric electron content measurements from the American and European sectors. Geophys. Res. Lett. 5, 123-126, 1978. Park, C. G., Some features of plasma distributions in the plasmasphere deduced from Antarctic Whistlers. J. geophys. Res. 79, 169-173, 1974. Park, C. G., Carpenter, D. L. and Wiggin, D. B., Electron density
in the plasmasphere: Whistler data on solar cycle, annual. and diurnal variations. J. geophys. Res. 83, 3137+ 3144, 1978. Richards, P. G. and Torr, D. G., Seasonal, diurnal and solar cyclical variations of the limiting H + flux in the Earth's topside ionosphere. J. geophys. Res. 90, 5261 5268. 1985. Richards, P. G. and Torr, D. G., Thermal coupling of conjugate ionospheres and the tilt of the Earth's magnetic field. J. geophys. Res. 91, 9017-9021, 1986. Rohle, R. G. and Dickinson, R. E., The effect of displaced geomagnetic poles on the thermospheric neutral winds. Planet. Space Sci. 22, 623-631, 1974.
Sethia, G. C., Bailey, G. J., Moffett, R. J. and Hargreaves, J. K., The effects of neutral air winds on the electron content of the mid-latitude ionosphere and protonosphere in summer. Planet. Space Sci. 31, 377-387, 1983.
Spencer, N. W., Carignan, G. R., Mayr, H. G., Niemann, H. B., Theis, R. F. and Wharton, L. E., The nighttime temperature maximum in the earth's equatorial thermosphere. Geophys. Res. Lett. 6, 444~46, 1979. Vickrey, J. F., Swartz, W. E. and Farley, D. T., Ion transport in the topside ionosphere at Arecibo. J. geophys. Res. 84, 7307 +7314, 1979.
Wagner, C.-U., M6hlmann, D., Schifer, K., Mishin, V. M. and Matveev, M. I., Large-scale electric fields and currents and related geomagnetic variations in the quiet plasmasphere. Space Sci. Rev. 26, 391-446, 1980.
1995 Elsevier Science Ltd Printed in Great Britain 0032-0633/95 $9.50+0.00
Pergamon
0032-0633(94)00115-4
About the nature abfthe Night-time Winter Anomaly effect (NWA) in the F-region of the ionosphere N. Jakowski’
and M. F6rster’
’ Deutsche Forschungsanstalt fiir Luft- und Raumfahrt e.V., Fernerkundungsstation Neustrelitz, Kalkhorstweg 53, 17235 Neustrelitz, Germany 2 Max-Planck-Institut fur extraterrestrische Physik, AuBenstelle Berlin, Rudower Chaussee 5, 12489 Berlin-Adlershof, Germany Received 15 March 1994 ; revised 11 May 1994 ; accepted 11 May 1994
sector during the low solar activity years 1974-1975. In analogy to the well-known seasonal or winter anomaly effect of the daytime F2-layer the NWA is characterized by a higher night-time F2-layer ionization level during winter months than during summer. Since the daytime winter anomaly effect was not observed during years when NWA occurred and vice versa, both phenomena are decoupled and subjected to different physical mechanisms. In a first attempt to interpret the NWA effect, Jakowski and co-workers (1981, 1983, 1988) assumed that the considerable asymmetry in geomagnetic-geographic relationships at the American longitude sector provide good conditions for an effective interhemispheric plasma transport from the southern (summer) hemisphere to the northern (winter) hemisphere. Due to this asymmetry which is well documented by the displaced geomagnetic and geographic equators, the NWA is not observed at the southern hemisphere winter in June/July. This hypothesis was supported by apparent similarities in the behaviour of the night-time ionization at nearly geomagnetically conjugated regions (Havana, Port Stanley) over 11 months and unusual high slab thickness values at the northern winter night indicating incoming plasmaspheric fluxes. To check the influence of the geomagnetic-geographic relationship, the nocturnal ionospheric behaviour at the Asian sector around 1 = 110”E has been studied by comparing ionosonde data from Irkutsk (L = 2.0) in the northern hemisphere and from Mundaring (L = 1.9) in the southern hemisphere (Jakowski et al., 1990). Although less pronounced than in Havana, the NWA effect was observed in Mundaring too. In analogy to the stations Havana and Port Stanley there were found apparent similarities in the night-time behaviour at the nearly conjugated stations Irkutsk and Mundaring. To study the effect in more detail an adequate physicalnumerical model has been applied to the coupled system ionosphere-plasmasphere-ionosphere along flux tubes (Forster and Jakowski, 1986,1988). Modelling the plasma
N. Jakowski and M. F(irster : NWA in the F-region of the ionosphere
604
along the L = 1.5 flux tube at the American sector there have been found indications that the NWA effect is closely related to interhemispheric coupling processes. The aim of this paper is to provide further evidence that the NWA effect is a regular phenomenon at special geomagnetic-geographic relationships under low solar activity conditions. Numerical modelling is used to get more insight into the physical mechanism of this phenomenon.
2. Observations In order to probe the influence of the geomagnetic-geographic relationship upon the occurrence of the NWA effect, ionospheric data both from the American as well as from the Asian longitude sector are analysed. The stations from which data have been taken are listed in Table 1. With respect to the geomagnetic-geographic relationship the American sector is apparently symmetric to the Asian longitude sector (A = 1lO”E) as it is illustrated in Fig. 1. Thus NWA observations at the American northern hemisphere should correspond with indications of NWA at the southern hemisphere at the Asian longitude sector. Using vertical sounding (@2) and total electron content (&) data the mean ionization level of the night-time ionosphere (03/04 LT) obtained at different stations at both longitude sectors are compared in Fig. 2. The ionospheric electron content NF has been derived from Faraday rotation measurements at linearly polarized VHF signals received in Havana from different geostationary satellites such as ATS-6, SMS-1, and GOES between 1974 and 1983. As Fig. 2 shows, the NWA effect is a persistent feature during years of low solar activity both at the American longitude sector (northern hemisphere) as well as at the Asian longitude sector (southern hemisphere). To study the local time dependence of the NWA effect at the Mundaring ionosonde station, the diurnal behaviour of NmF2 is plotted for January and June 1975 in Fig. 3a. Whereas at the Havana station (L = 1.5) the NWA effect starts already at pre-midnight and ends at sunrise (Fig. 3b), the NWA effect observed in Mundaring (L = 1.9) which is somewhat less pronounced, starts after midnight
Table 1. Table of geographic and geomagnetic coordinates
and corresponding L-values of stations from which data have been used in this paper
Station Havana
Geographic Lat. Long. 23.1
277.5
Geomagnetic Lat. Long. 34.2
L-value
346.0
1.5
9.5
1.6
WAV)
Port Stanley (PST) Irkutsk (IRK) Brisbane (BBI) Mundaring (MUN)
-51.7 52.5
302.2
-40.4
104.0
41.1
174.8
2.0
-27.5
152.9
- 35.7
227.4
1.5
- 32.0
116.3
-43.4
186.8
1.9
and lasts until sunrise hours. The maximum is reached at about 03-04 LT. The comparison with the corresponding high solar activity data from January and June 1980 at the bottom of Fig. 3a,b shows that the NWA vanishes at high solar activity conditions due to a strong increase of the night-time ionization in summer. Probably the mechanisms leading to the NWA effect are masked by the higher ionization level in summer night. It is interesting to note that the daytime anomaly phenomenon is well documented at high solar activity. The hypothesis that interhemispheric coupling may explain the NWA effect is underlined by close similarities in the annual variation of monthly values of the averaged night-time ionization obtained at nearly geomagnetically conjugated stations (Fig. 4). Following the idea of interhemispheric coupling and enhanced plasmaspheric fluxes during NWA winter nights, the topside ionosphere should be expected to react more sensitive to such fluxes than the lower parts. Indeed, as the Alouette 1 topside sounder profiles of the electron density n, indicate in Fig. 5, the increase of ionization during the northern winter night at the American longitude sector is much more pronounced at the topside ionosphere (cf. profiles 2 and 3). At the southern hemisphere the NWA is completely absent (cf. profiles 1 and 4). It is interesting to note that electron densities are nearly equal at both hemispheres at about 1000 km height during the December solstice (cf. profiles 3 and 4), providing a further argument for the existence of interhemispheric coupling processes.
3. Numerical model The physical-numerical computations of the present work are based on a coupled time dependent model describing the plasma behaviour along flux tubes. The flux tube which is derived from a centred geomagnetic dipole field tilted with regard to the geographic axis by about 11.4 degrees, comprises the local ionosphere above 120 km altitude, the plasmasphere, and the magnetically conjugated ionosphere down to 120 km altitude. An extended description of this model has been given in the paper of Fiirster and Jakowski (1988). In essence, the model solves the coupled, full time-dependent continuity and momentum equations (forming so-called diffusion equations) of the atomic ions H+, O+ and He+ as well as the electron and ion heat conduction equations along the selected flux tube and solves also the diffusion equations of the molecular ions O?+, NO+ and NT in the magnetically conjugated ionospheres The model, therefore, provides values of atomic ion concentrations for Of, H+ and He+, their field-aligned velocities, electron and ion temperatures along the whole flux tube as well as density and field-aligned velocities of the molecular ions 0:) NO+ and N: in the magnetically conjugated ionospheres. Further, to describe photoionization as ion particle and heat source in a more correct way the kinetic equation of the suprathermal electron component is also solved. This includes the photoionization and secondary ionization processes of the neutral gas, the generation of the suprathermal electrons, their motions, trapping, and thermalization in the plas-
N. Jakowski and M. Fiirster : NWA in the F-region of the ionosphere
605
Fig. 1. Schematic representation
of the geographic-geomagnetic relationship at the American and Asian sectors for a ray-path corresponding to Faraday rotation observations of a geostationary satellite from ground-based stations near L = 1.5 ; further the modelled flux tube is indicated
ionosphe-lc 0:3/04
2
~onlzatlon LT -
Summer
----
ulnter
/
Hav
foF2
/
Brl
PlHz
--
1974
foF2
.I976
.I978
;990
1982
'1994
'1986
YEftRS
86 model for neutral gas composition and neutral gas temperature (Hedin, 1987) has been used as background thermospheric model. It is very important for the ionized components as they are trace constituents at E- and Fregion heights whose physical behaviour are determined in a high degree by the neutral gas. Chemical reaction rates, photoionization and -absorption rates, solar EUV fluxes for low solar activity level, particle collision frequencies, cooling rates, thermal conductivities, etc., i.e. all this great amount of input parameters and coefficients which are an important part of concrete numerical simulation have been taken from recent review articles. For more details concerning the used parameters see the model description in the paper of Fdrster and Jakowski (1988). The altitude profile of the neutral air wind is assumed to vary in accordance with the neutral temperature profile T,(z), i.e. it increases steadily from zero value at the lower boundary at 120 km height up to an asymptotical amplitude in the F-region. The local time neutral wind variation in accordance with the global-scale dynamics of the thermosphere has been selected in an iterative manner for the northern and southern hemisphere and for both solstices separately as described in the next section.
Fig. 2. Solar cycle dependence of night-time averaged ionization (0344 LT) measured byfoE and the ionospheric electron content NF at the ionosonde stations Havana (Hav), Brisbane (Bri), and Mundaring (Mun). The NWA effect is indicated by shadowed areas. The solar activity level is indicated at the bottom by the solar radio flux E;0.7 (lo-” Wm-’ Hz-‘) maspheric flux tubes, i.e. the local and nonlocal heating processes of the thermal plasma provided by photoionization. No E x B drift of the flux tubes has been included so far, because for middle latitudes and during geomagnetically quiet conditions the only acting dynamoelectric &-fields (Wagner er al., 1980) are known to be small. For the model simulations carried out in this paper we assumed geomagnetically quiet conditions (Ap = 4) and a low level of solar activity representative for the years 1963-1964 and 1974.1976. We assumed an 81-day average of the solar radio flux parameter F,0.7 as input for the neutral gas and solar radiation models of F,,,, = 80 [1O-22 W me2 Hz-‘] and daily values of JirO., = 79 for December and F,,,, = 83 for June solstice. The MSIS-
4. Modelling and discussion The numerical model described in the previous section is a powerful tool to get more insight into the physical mechanism of the NWA effect. In a first step the windfree solution was compared with the corresponding observational data (hmF2, Nmf2, NF) to derive the structure of the meridional wind pattern at both hemispheres at the American longitude sector. We have probed a sequence of different wind patterns. There was also considered a self-consistent solution taking into account the equations of motion for neutral wind components (driving forces derived from MSIS-86 pressure gradients) together with the set of equations for the ionized components. The height of the F2-layer which is very sensitive to the meridional wind velocity in midlatitudes was used together with the measured NmF2 and NF values to optimize the meridional wind pattern in both hemispheres (Forster and Jakowski, 1986, 1988). So, a
N. Jakowski and M. Forster : NWA in the F-region of the ionosphere
606 7
:
I4
NmF2
12
Lo 0
10
” z
Ionosonde
monthly
6
Mundarinq
medians
8 -
6
z
summer
\
4
l’
\
\
I /
\ \
‘,
NmFZ
/
1’
PST
\
/
//
J
In
IO-
0
EJ-
E :
6-
1586
\
IRK
\
t-
\ \
2
‘-_
-_’
____--.
MUN 0
0
(‘1’1(1111’1”““1~~” 4
s
12
16
LOCFlL
(a)
20
24
TINE
60
JAN
NF ”
Havana monthly
:
E
---
L 20
MRR
APR
PtFlY JUN
JUL
RUG
SEP
OCT
NOL
DEC
(03-04 LT) represented by NmF2 and the ionospheric electron content NF observed at nearly geomagnetically conjugated stations at the American (upper panel) and Asian (bottom) longitude sectors
medians
40
0
z
FEB
Fig. 4. Seasonal variation of the averaged night-time ionization
summer winter
06/76 1200
Alouette
2 0
2
Electron 1000
1
1963
density
proflles Longitude
n,(h)
sector LT
: 64 - 82”W : 01:30
Dip: f53
(7 E
40
x 0 c IL 2
600
20 .
‘.
‘\
400 0
4
(b)
a
12
LOCAL
16
TIME
20
w
24
south
(hours)
Fig. 3. Diurnal variations of ionospheric parameters NmF2 and
NF at the Asian longitude sector (a) and at the American longitude sector (b), respectively. Comparison is made between median values representative for winter and summer conditions under low (1975/76) and high (1980) solar activity conditions suited wind pattern was selected by an iteration cycle of trial and error. This meridional wind variation versus local time is shown for June (Fig. 6a) and December solstice (Fig. 6b) with dashed lines for the southern and full lines for the northern hemisphere. The highest amplitudes are about 200 m SK’ with generally larger amplitudes at the southern footpoint of the flux tube whereas during June solstice in the northern hemisphere the meridional wind near the subsolar point latitude is lowest. The wind pattern for northern December comprises the reversal of mer-
s
200
north
u north
‘\
\
\
5
soutt
1
’ 1E+5
1E+4
ELECTRON
lE+6
DENS1 TY (cm -3)
Fig. 5. Electron density profiles n,(h) obtained by the Alouette
1 topside sounder for geomagnetically conjugated regions at the American longitude sector on the days 156 (5 June 1963, curves 1 and 2) and 352 (18 December 1963, curves 3 and 4) at nighttime (01: 30 LT). The full lines correspond to the northern hemisphere, the dashed lines are related to southern hemisphere
idional wind for l-2 h after midnight caused by the secondary midnight pressure bulge as observed by AE-E (Spencer et al., 1979) and Fabry-Perot interferometer measurements above Arecibo (Friedman and Herrero, 1982). The availability of topside sounder profiles from Alouette 1 provides a good chance to test both the model
N. Jakowski ‘;‘
2oo z
: IOWA in the F-region
and M. Forster June
1963
(a)
z 3
100 r ,
2 E a z
s
f! F3 E
/’ -100
/’
,
,’
,. ’
_--.
‘\
‘\
‘\ \ \ \ \
,’
‘\
7”
-200-1
of the ionosphere
I
I
I.
I
‘.
..-
,
0
6
12 LOCAL, TIME (hours)
16
24
0
6
12 LOCAL TIME (hours)
16
24
601
Considering the FZlayer, the computed electron density profiles agree well with the measured electron densities for both days in the model assumptions so far. On the other hand, the upper ionosphere electron densities are somewhat overestimated indicating additional processes not considered. However, what is most important to explain the NWA effect, the essential higher densities in winter are well reproduced both for the upper ionosphere (Nr) as well as for the F2-layer (@IQ). Both model calculations for days 156 and 352 in 1963 may additionally be checked by TEC-measurements made in Havana by means of the Faraday rotation technique under similar geophysical conditions (Fig. 8). Since Faraday rotation data provide the ionospheric electron content up to about 2000 km height, the tube content was calculated by the model up to this height taking into account a simplified projection to the radio beacon ray path used for the measurements. The lower part of the flux tube up to an altitude of about 2000 km is approximately parallel to the beacon ray path used for the measurements obtained in Havana. So, the slant columnar content can be recalculated by a simple projection to the vertical electron content Nr. Although observations are related to different solar cycle minima, the model calculations are very similar to the TEC obser-
NF
Havana monthly
Fig. 6. Local time variation of the meridional neutral wind component at thermospheric height for June (upper panel a) and December (lower panel b) solstice assumed for model calculations in the northern (full lines) and southern hemisphere (dashed lines) ; positive values correspond to southward winds which are equatorward in the northern and poleward in the southern hemisphere
medians
‘z
as well as our hypothesis about the nature of the NWA effect. Topside sounder profiles obtained at individual days in June and December 1963 are compared with the corresponding numerical calculations in Fig. 7.
E t;
N,
= 20
MODEL
model
Alouette
calculations 1
ne(h)
related profiles,
to 1963
CALCULATIONS
0
4
8
12
LOCAl lo4 ELECTRON
DENSITY
lo5 (cm-‘)
Fig. 7. Comparison of calculated electron density profiles at 01: 30 h local time with Alouette 1 topside sounder data for June (day 153/63, full line and asterisks for model calculations and observations, respectively) and December (day 352/63, dashed line and crosses) solstices
16
20
TIME
(hours)
2’4
Fig. 8. Comparison of ionospheric electron content data NF observed at Havana in 1976 with corresponding numerical calculations at similar geophysical conditions. The upper panel shows monthly medians of NF obtained in June and December 1976 (F10.7= 70.6/76.7). The lower panel shows results of numerical calculations applied to individual days in June and December 1963 (F,0,7 = 83.5/78.4) selected for simulation studies (see Fig. 7)
608
N. Jakowski and M. Fiirster : NWA in the F-region of the ionosphere
vation at both seasons. So the assumed wind patterns (Fig. 6) at least for the northern hemisphere are thought to be justified. Since meridional winds are very effective in vertical transport at mid-latitudes (maximum at geomagnetic inclination I = 45”) significant upward plasma fluxes at the southern hemisphere may cause a high tube content in December/January. The fact that neutral winds may enhance the filling of the plasmasphere in mid-latitudes was already underlined by several authors (e.g. Sethia et al., 1983; Forster and Jakowski, 1988). Due to the southward displacement of the geomagnetic equator against geographic equator by about 11 degrees there results a seasonal asymmetry in filling conditions of the plasmasphere. So, the duration of the illumination at the conjugated southern hemisphere is additionally enlarged in December/January whereas in June/July it is additionally reduced. Such an argument was already suggested by Kersley and Klobuchar (1978) to explain the higher plasmaspheric electron content at night compared with daytime values at Hamilton in contradiction to Aberystwyth plasmaspheric data. As has been shown by various authors (e.g. Roble and Dickinson, 1974 ; Richards and Torr, 1986; Bailey et al., 1991) the tilted dipole of the Earth’s magnetic field leads to special features in the thermosphere and ionosphere structure. The NWA effect is probably one of these peculiarities. The asymmetry between both hemispheres is well documented in the model calculations along the flux tube L = 1.45 (Fig. 9) as already discussed above. The plasma distribution along the flux tube gives a good impression of the close coupling between both hemispheres leading to the seasonal anomaly at the northern winter night. The interhemispheric coupling may be studied in more detail by considering the field aligned plasma flow along the flux tube as shown in Fig. 10. This colour plot gives the diurnal variation of fieldaligned ion flux densities along the entire flux tube for the
,,s~
,
, MODYL CAFCULAYIONS /
I
120
406
, 21.5
I 18.5
, 32.8
I 30.8
1
/
~
I
I
1708 2907 1708 058 406 120 HEIGHT (km) I I I I / f 16.2 9.2 -11.3 -31.8 -38.8 -42.1 -44.0 GEOGRAPHIC LATITUDE (deg) I / I I I I 1 27.5 20.5 0.0 -20.5 -27.5 -30.8 -32.6 GEOMAGNETIC LATITUDE (deg) 856
Fig. 9. Electron density profiles along flux tube for 01: 30 h local time according to model calculations for day 156/63 (full line) and day 352163 (dashed line)
model calculations of day 156/63 in the upper panel and day 352/63 in the lower panel. The scales on the left and right hand side indicate height and distance along the flux tube from the northern footpoint, respectively, in nonuniform axes : the distances in the ionosphere are stretched and in the plasmasphere they are compressed. The abscissa refers to local time of the northern footpoint. The colour-coded flux intensities distinguish between positive fluxes with the blue-green scale which are directed from the northern to the southern hemisphere along the flux tube and negative Ruxes with the red-yellow scale for the opposite direction. Fluxes smaller than or equal to IO7 cm-2 s’ are indicated by a black area. The maxima of the ion flux intensities occur during sunrise and sunset at F-layer heights. The peak values in the lower part of the F-region during afternoon hours or near sunset are about lo9 cm-* s-’ in the summer hemispheres and about 2.5 x lo9 cmm2 s-’ in the winter hemispheres in both cases. Sunrise occurs in the F-layer around 04:45 LT in the northern and around 07 : 00 LT in the southern hemisphere in June (Fig. 10, upper panel) and around 06:OO LT and around 03: 15 LT, in December (Fig. 10, lower panel), respectively. During these times a rapid increase of the field-aligned plasma flux can be seen at all heights above about 200 km in the corresponding hemisphere which is directed into the plasmasphere. The heating of the plasmasphere and the high thermal conductivity lead to changes of the plasma flux in the magnetically conjugated ionosphere. This effect is reflected in a diminished or even reversed outflow from the plasmasphere before local sunrise (e.g. southern upper ionosphere at about 05:30 LT for the June case) or in an increased upward directed flux after local sunrise. Local sunset in the F-layer around 19: 15 LT in the northern and 17: 30 LT in the southern hemisphere for June and around 18 : 00 LT and 20 : 30 LT for the December case is accompanied by the thermal breakdown of the ionosphere with maxima1 fluxes from the plasmasphere into the ionosphere. Further obvious modulations of the field-aligned ion fluxes are caused by changes of the local meridional wind component as it can be seen, for example, during the wind reversal after midnight in the northern hemisphere of the December case (Fig. 10, lower panel). The night-time fluxes into the ionosphere lead to night-time enhancements of the ionospheric content as long as the local chemica1 loss processes are less than the influx of plasma. These fluxes are more intense for the December solstice winter hemisphere than for the June winter hemisphere and act in favour of the NWA effect in northern winter. It is well known that downward directed plasmaspheric fluxes play an important role in maintaining the night-time ionization in F2-layer heights at mid-latitudes (Park et al., 1978; Evans and Holt, 1978 ; Vickrey et al., 1979). In this simplified picture the plasmasphere then is refilled by upward directed plasma fluxes from the ionosphere at daytime. With increasing latitudes the plasmasphere acts more and more as a plasma reservoir with balanced up- and down-flows over a whole day (Jakowski et al., 1983). The computations at L = 1.45 have shown that interhemispheric fluxes lead to a significant imbalance of these fluxes particularly at the northern winter hemisphere generating the NWA effect. The net daily interhemispheric
N. Jakowski
and M. Forster : NWA in the F-region
FIELD -
609
of the ionosphere
ALIGNED FLUXES
:
JUNE
120
0.0
405
0.4
854
1.0
1687
2.2
2816
5.6
E3
1687
9.0
g s
854
10.3
* 5
405
10.9
s 5
120
11.3s
0
6
12
18
zi ki
24
DECEMBER
I
0.0
T
405
0.4
f$
854
1.0
E
1687
2.2
2816
5.6
1687
9.0
g s
854
10.3
d 5
405
10.9
3
120
11.3:
120
6
-9.0
-8.5 COLOR
-8.0 CODE
12 LOCAL TIME (hrs)
-7.5 k7.0 FOR LOG OF FLUX
18
8.0 7.5 8.5 INTENSITIES (cm%-‘)
24
9.0
Fig. 10. Colour plot of field-aligned plasma fluxes along a flux tub of L = 1.45 vs local time for June (upper panel) and December (lower panel) model calculations. The colour code for flux intensities is given at the bottom of the figure. Fluxes less than f 10’ cm-’ s-’ are indicated by a black area
2 5 ti
N. Jakowski and M. F6rster : NWA in the F-region of the ionosphere flow generally directed from the summer to the winter hemisphere (Vickrey et al., 1979) maximizes at the American longitude sector during the December solstice resulting in an increased plasmaspheric density compared with June solstice values. In accordance with this, the plasmaspheric tube content resulting from the calculations for day 352 exceeds that obtained for day 152 in 1963 as it is shown in Fig. 11. This result fully agrees with tube content data inferred from whistler measurements at the American longitude sector (Park et al., 1978). Beacon satellite observations using ATS-6 signals by Davies et al. (1976) in 1974-1975 revealed also the highest plasmaspheric electron content at Boulder station in December/January. It is evident that the high flux tube content leads to an enhanced plasma outflow at both hemispheres during night. If the downward fluxes are strong enough, loss processes may be overcompensated so that so-called nighttime enhancements of the ionospheric ionization may be observed. According to Jakowski et al. (1991) significant night-time enhancements observed in Havana under low solar activity conditions (19761) require downward fluxes of about 3 5 x 108 cm -2 s - ' . In agreement with observations and modelling results presented in that paper the occurrence probability PNE of night-time enhancements in Nv has a clear maximum in northern winter nights under low solar activity conditions (PNE ~ 100% for winter and PYE ~ 30 o/o for summer). It is interesting to note that the seasonal variation of the occurrence probability is opposite in Havana during years of high solar activity (PNF ~ 30-50% tbr winter and PNE ~ 80% for summer). This fact coincides very well 'with the vanishing of the NWA effect at an enhanced solar activity level (Fig. 2). We suppose that under high solar activity conditions the interhemispheric coupling is masked by local ionosphereplasmasphere coupling processes at both ends of the flux tube considered here, leading 1o a local equilibrium in a diurnal averaged sense. As Richards and Torr (1985) have shown theoretically, the limiting H + ion flux flowing from the ionosphere to the plasmasphere decreases with increasing solar activity. If so, interhemispheric coupling MODEL CALCULATIONS
611
via protonospheric fluxes is reduced with increasing solar activity. Thus the influence of interhemispheric fluxes diminishes with increasing solar activity in the same way as the effect of interhemispheric coupling reduces with growing latitudes under low solar activity conditions already discussed above.
5. Conclusions
Taking into account both the observations from different longitude sectors as well as the results of numerical modelling, the night-time winter anomaly effect (NWA) occurs : • at low solar activity level • at geomagnetic mid-latitudes • when geographic and geomagnetic latitudes are displaced by a certain degree. If the above-mentioned conditions are fulfilled, NWA is mainly explained by interhemispheric coupling via plasma fluxes along magnetic flux tubes. Geomagnetic-geographic asymmetry is an essential precondition for an effective filling of the plasmasphere at the summer hemisphere resulting in enhanced downward fluxes at the winter hemisphere up to 7 x 10~ c m -~ sduring sunset at 1000 km height in the winter hemisphere. As numerical simulation has shown, probably thermospheric meridional winds play an important role in driving interhemispheric fluxes. With increasing solar activity local plasma processes become more important versus the plasma reservoir aspect of the plasmasphere, so that interhemispheric coupling effects are masked leading to a vanishing of NWA at higher levels of solar activity. Acknowledgement. The authors are grateful to Mrs D. Adrian
for her assistance in data processing and in preparing the manuscript. This work was supported by the Deutsche Agentur for Raumfahrtangelegenheiten (DARA) GmbH under grant 50 QL 92060 and 50-YI9202. The authors thank the referees for valuable comments.
References Bailey, G. J., Sellek, R. and Balan, N., The effect of inter-
8
oZ ~
/
4
"
..352/63
~ ~ -
×
L= 0
6
12 18 LOCAL TIME ( h o u r s )
1.45
24
Fig. ll. Total flux tube content vs local time for June (day 156/63, full line) and December (day 352/63, dashed line) solstices according to the model calculations presented in this paper (a cross-sectional area of 1 cm2 refers to the 120 km altitude level)
hemispheric coupling on nighttime enhancements in ionospheric total electron content during winter at solar minimum. Ann. Geophys. 9, 738-747, 1991. Davies, K., Fritz, R. B. and Gray, T. B., Measurements of columnar electron contents of the ionosphere and plasmasphere. J. geophys. Res. 81, 2825-2834, 1976. Evans, J. V. and Holt, J. M., Nighttime proton fluxes at Millstone Hill. Planet. Space Sci. 36, 727-744, 1978. F6rster, M. and Jakowski, N., Interhemispheric ionospheric coupling at the American sector during low solar activity, II. Modelling, Gerlands Beitr. Geophys. 95, 301-314, 1986. F6rster, M. and Jakowski, N., The nighttime winter anomaly (NWA) effect in the American sector as a consequence of interhemispheric ionospheric coupling. PAGEOPH 127, 447 471, 1988. Friedman, J. F. and Herrero, F. A., Fabry-Perot interferometer measurements of thermospheric neutral wind gradients and reversals at Arecibo. Geophys. Res. Lett. 9, 785-788, 1982.
612
N. Jakowski and M. F6rster : NWA in the F-region of the ionosphere
Hedin, A. E., MSIS-86 thermospheric model. J. geophys. Res. 92, 46494662, 1987.
Jakowski, N., Bettac, H. D., Lazo, B. and Lois, L., Seasonal variations of the columnar electron content of the ionosphere observed in Havana from July 1974 to April 1975. J. atmos. terr. Phys. 43, 7-11, 1981.
Jakowski, N., Bettac, H. D., Felske, D., Lazo, B. and Lois, L., Observations of the total electron content of the ionosphere in Havana during a period of low solar activity 1974-1976. Gerlands Beitr. Geophys. 92, 151-161, 1983. Jakowski, N., F6rster, M., Lazo, B. and Lois, L., Interhemispheric ionospheric coupling at the American sector during low solar activity, I. Observation. Gerlands Beitr. Geophys. 95, 219-227, 1988. Jakowski, N., Landrock, R. and Jungstand, A., The Nighttime Winter Anomaly (NWA) effect at the Asian longitude sector. Gerlands Beitr. Geophys. 99, 163-168, 1990. Jakowski, N., Jungstand, A., Lois, L. and Lazo, B., Night-time enhancement on the F2-1ayer ionization over H a v a n a - Cuba. J. atmos, terr. Phys. 53, 113-118, 1991. Kersley, L. and Klobuchar, J. A., Comparison of protonospheric electron content measurements from the American and European sectors. Geophys. Res. Lett. 5, 123-126, 1978. Park, C. G., Some features of plasma distributions in the plasmasphere deduced from Antarctic Whistlers. J. geophys. Res. 79, 169-173, 1974. Park, C. G., Carpenter, D. L. and Wiggin, D. B., Electron density
in the plasmasphere: Whistler data on solar cycle, annual. and diurnal variations. J. geophys. Res. 83, 3137+ 3144, 1978. Richards, P. G. and Torr, D. G., Seasonal, diurnal and solar cyclical variations of the limiting H + flux in the Earth's topside ionosphere. J. geophys. Res. 90, 5261 5268. 1985. Richards, P. G. and Torr, D. G., Thermal coupling of conjugate ionospheres and the tilt of the Earth's magnetic field. J. geophys. Res. 91, 9017-9021, 1986. Rohle, R. G. and Dickinson, R. E., The effect of displaced geomagnetic poles on the thermospheric neutral winds. Planet. Space Sci. 22, 623-631, 1974.
Sethia, G. C., Bailey, G. J., Moffett, R. J. and Hargreaves, J. K., The effects of neutral air winds on the electron content of the mid-latitude ionosphere and protonosphere in summer. Planet. Space Sci. 31, 377-387, 1983.
Spencer, N. W., Carignan, G. R., Mayr, H. G., Niemann, H. B., Theis, R. F. and Wharton, L. E., The nighttime temperature maximum in the earth's equatorial thermosphere. Geophys. Res. Lett. 6, 444~46, 1979. Vickrey, J. F., Swartz, W. E. and Farley, D. T., Ion transport in the topside ionosphere at Arecibo. J. geophys. Res. 84, 7307 +7314, 1979.
Wagner, C.-U., M6hlmann, D., Schifer, K., Mishin, V. M. and Matveev, M. I., Large-scale electric fields and currents and related geomagnetic variations in the quiet plasmasphere. Space Sci. Rev. 26, 391-446, 1980.