Seasonal and longitudinal effects on plasmaspheric tube content

Adv. Space Res. Vol.12, No.6, pp. (6)179—(6)182, 1992 Printed in Great Britain. All rights reaerved.

0273-1177/92 $15.00 Copyright © 1991 COSPAR

SEASONAL AND LONGiTUDINAL EFFECTS ON PLASMASPHERIC TUBE CONTENT Y. Rippeth, R. J. Moffett and 0. J. Bailey Department ofApplied and Computational Mathematics, University of Sheffiel4 Sheffield, 510 2TN, U.K

ABSTRACT A mathematical model of the ionosphere and plasmasphere that includes an eccentric dipole geomagnetic field has been constructed to model, in the first instance, whistler results at L = 2.5. Observations show that there is a pronounced annual variation in plasmaspheric tube content at mid-latitudes and that this effect is modulated by longitude. The model results indicate that the geometry of the magnetic field (in terms of geographic coordinates) is a major factor in determining the magnitude of the annual variation, with neutral air winds and the degree of plasmaspheric refilling also important. A global variation in neutral atomic hydrogen abundance appears to have little influence. INTRODUCTION Whistler data has, for many years, indicated the existence of an annual variation in plasmaspheric electron density, with a maximum in December and a minimum in June /1-4/. Clilverd /5/ recently investigated the origin of the annual variation in plasmaspheric electron density for geomagnetically quiet conditions using 1986 data from a British Antarctic Survey experiment at Faraday, located at 64°W, 65°S /6/. The average L shell of propagation for the data considered was L = 2.5 with no seasonal or diurnal variation. Clilverd found that the diurnally averaged group delay had a maximum in December and a minimum in June (Figure 1) with a December to June ratio of 1.8 (+ 0.1, -0.1). Clilverd also considered the annual variation of electron densities in the F2-region. The monthly medians of foF2 taken from ionosonde data from Argentine Islands (ie, close to Faraday) and Wallops Island (approximately conjugate to Faraday) were combined as a root mean square /7/. This showed a maximum in December and a minimum in June/July with a characteristic ratio of about 1.7. Using the group delay and foF2 data, Clilverd suggested that the variations in plasmaspheric electron density are primarily due to the variations of electron density in the F2region of the ionosphere and that contributions come from both ends of the field line. Having found a correlation between F-region and plasmaspheric electron densities, Clilverd next predicted equatorial electron densities from foF2 values by assuming that, during periods of quiet magnetic activity, diffusive equilibrium is maintained from the F-region to the plasmasphere. The results were similar to those obtained from the measures of group delay. The longitudinal effects on the annual variation of plasmaspheric electron density were then investigated using foF2 values, from two other pairs of stations, to deduce the equatorial electron densities. Little annual variation was observed at either longitude. Clilverd’s results suggest that the annual variation can be explained in terms of the magnetic field configuration. We note that there is a large difference in geographic latitude between the two ends of the field line passing through the Faraday longitude and that at the other two longitudes (47°E, 1700 W) there is no significant difference in geographic latitudes between the end points (Figure 2). JASR 12:6-L (6)179

(6)180

Y. Rippeth etaL

FARADAY VLF DOPPLER EXPERIMENT 900 800

+ cx

300 200

-

100

-

0 0

50

I

I

I

I

I

I

100

150

200

250

300

350

DAY OF YEAR Fig 1. Group delays measured in 1986 using observations of whistler signals at Faraday / 5/.

90 L

~:

2~5

:OIGRF ECCENTRIC

DIPOLE

-J U

=

-180

-120

-60 0 60 LONGITUDE (DEG)

120

180

Fig 2. Geographic latitudes of the L = 2.5 magnetic field line in the northern and southern ionospheres, as predictedby the IGRF and by the eccentric dipole model used in the present calculations.

Seasonal and Longitudinal Effects on Plasmaspheric Tube Content

(6)181

MODEL OF IONOSPHERE AND PLASMASPHERE AND RESULTS The model is described in more detail elsewhere /8,9/. In summary, in the model are calculated, as a function of time and arc-length along the flux tube, the concentrations, field-aligned velocities and temperatures of the 0+, H + and He + ions and the electrons in the ionosphere and plasmasphere. The magnetic field is represented by an eccentric dipole /10/. The input neutral atmosphere is provided by the MSIS-86 empirical model /11/ with, for these calculations, low solar activity and geomagnetically quiet conditions. The neutral air winds are based on results from the UCL thermospheric model /12/. For the present calculations, the E x B drift velocity was set equal to zero. The calculations were carried out for an atmospheric period of 2 1/2 days, for both June and December conditions. The results quoted are usually for the final 24 hours of this period but results for a later period (giving a longer refilling time) are also mentioned. We have chosen to discuss model values of Neq~the H + concentration in the equatorial plane on the L = 2.5 field line, and of NmF2, the F2-region peak electron concentrations on the same fieldline in the two hemispheres. It should be recalled that plasmaspheric tube content is approximately proportional to Nea at a given L-value and that the experimentally m,çasured group delay is roughly proportionaf toaTNeq; the experimental foF2 is proportional toj! NmF2. Thus (in model terms) the Clilverd /5/ 1986 resultsfor the Faraday field line give a Dec/June NmF2 ratio of about 2.9 and Dec/June Neq ratio of about 3.2 (+ 0.4, -0.3). Faraday Model Results on the Annual Variation To compare with experiment, a diurnal average is made of the NmF2 results for the fmal 24 hours of calculation for each hemisphere. Then the values from the two hemispheres are averaged for the June set of results and for the December set of results For the L = 2.5 field line at the longitude of Faraday (64°W), the resulting Dec/June NmF2 ratio is 3.0. A diurnal average is also performed on the calculated Neq values (which are, of course, common to both hemispheres) and the Dec/June Neq ratio is 2.1. The model NmF2 ratio is in good agreement with experiment whereas the Neq ratio is not. .

Neutral atomic hydrogen plays a key part in the production of H + via the 0 + H charge exchange reaction. It was noted that the MSIS-86 empirical model gives greater overall n(H) in June than in December, which would tend to depress the Dec/June Neq ratio. Model runs were carried out using the MSIS-86 December n(H) distribution for June also. The resulting Dec/June Neq ratio is 2.3, a slight improvement relative to the experimental value. -

A futher factor, the degree of refilling of the plasmaspheric flux tube, was considered for the Faraday model-experiment comparison. Calculations were carried out for a further 2 days and again the results for the final 24 hour period averaged. A further increase in the Dec/June Neq ratio is achieved: the value on this occaision is 2.5. Longitudinal Variations Calculations were performed with the L = 2.5 field line having equatorial crossing point at 47°E longitude and at 170°W longitude. At these longitudes the MSIS-86 n(H) distribution is close to that at the Faraday longitude (640 W); the same neutral air wind pattern is used. The only significant change is the magnetic field configuration relative to the geographic frame. Figure 2 shows that the IGRF predicts that, at 47°E and 170°W, the geographic latitudes of the field line in the ionosphere are approximately the same, while the model eccentric dipole field gives similar results. The data from ionosondes located near the ends of the L = 2.5 field lines give, in both cases, a Dec/June NmF2 ratio of about 1. No experimental data are available at these longitudes for the Dec/June Neq ratio.

Y. Rippeth eta!.

(6)182

The model results are shown in Table 1. It should be noted that at 47°E the NmF2 ratio agrees with experiment and the Neq ratio appears to be correlated with NmF2 when comparison is made with the 64°W (Faraday) results. On the other hand, the 170t1 W results for NmF2 appear contrary to the longitudinal trend. When the UCL model neutral air wind patterns for 640 W (used in all the above calculations), 47° E and 170°W were examined, it was found that the 170°W June winds are similar to those at 64° W whereas the December winds blow more strongly poleward. Thus another set of calculations were carried out in which the actual 170°W winds from the UCL model were used for 170°W calculations. The results in Table 1 show a very marked decrease in the Dec/June NmF2 ratio while the Neq ratio remains unchanged. TABLE 1 Model Dec/June NmF2 and Neq Ratios Longitude

Variation in Conditions*

NmF2 Ratio

Neq Ratio

64°W

none

3.0

2.1

64°W

June n(H)

3.0

2.3

64°W

2 more days refilling

3.0

2.5

47°E

geog lat same in 2 hems

1.0

0.9

170°W

geog lat same in 2 hems

1.9

0.8

170°W

fmodel winds for 170°W

1.2

0.8

*

=

Dec n(H)

1. geog lat same in 2 hems standard conditions are given in the MODEL section

In conclusion, the geometry of the geomagnetic field clearly has a major influence on the annual variation in the values of Neq. It has been suggested /13/ that this arises from the variation in ionization rate due to changing solar zenith angle at a particular location. The degree of correlation between NmF2 ratios and Neq ratios appears to depend also on the neutral air wind pattern and on the time during which the tube of plasma has been allowed to refill. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

R. A. Helliwell, Whistlers and related ionospheric phenomena. Stanford University Press, Stanford, Calif., 1965. M. Bouriot, M. Tixior and Y. Corcuff,Ann. Geophys, 24,5 (1967). C. G. Park,J. Geophvs. Res, 79, 169 (1974). C. G. Park, D. L. Carpenter and D. B. Wiggin, J. Geophvs. Res. 83,3137 (1978). M. A. Clilverd, PhD Thesis. University of Sheffield, 1990. A. J. Smith, K. H. Yearby, K. Bullough, J. M. Saxton, H. J. Strangeways and N. R. Thomson, Nat. Inst. Polar.Res. 183, 1987. C. G. Park, Stanford report number 3454, 1972. Y. Rippeth, R. J. Moffett and G. J. Bailey, J. Atmos. Terr. Phvs. in press (1990a). Y. Rippeth, R. J. Moffett and G. J. Bailey, Planet. Space. Sci. submitted (1990b). A. C. Fraser-Smith, Rev. Geophvs. Space Phys. 25, 1 (1987). A. E. Hedin, J. Gepnhvs. Res, 92, 4649 (1987). T. J. Fuller-Rowell and D. Rees, J. Atmos. Sci, 37, 2545 (1980). R. W. Schunk, private communication, 1990.