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Dynamic model and observation of the equatorial ionosphere
Pergamon
A&. SpaceRexVol. IS, No. 2.pp.(2)109-(2)112.1995 copyxighiol~““&y RiJlWdiJiGreatBhh.AU 02?+117719 ‘tt” $7.00+ 0.00
DYNAMIC MODEL AND OBSERVATION OF THE EQUATORIAL IONOSPHERE S. Watanabe*and K-I. Oyama** *Hokkaido Institute of Information Technology, 45 Nishinopporv Betsy Hokkaido, 069 Japan * * The Institute of Space and Astronautical Science, Sagamihara 229 Japan
ABSTRACT The Hinotori satellite observed the electron density and temperature around the equatorial anomaly region at -600 km altitude during high solar activity. The observed electron temperature in the equatorial F region ionosphere was compared with the International Reference Ionosphere (IRI) model. The results showed the large difference between observation and model. Time dependent three dimensional computer simulations of the equatorial ionospheric F region have been carried out to investigate the electron temperature anomaly. These simulations coincide with the equatorial electron density and temperature structure observed by the Hinotori satellite. INTRODUCTION In the equatorial ionospheric F region, plasma production and loss as well as and diffusion are important for the structure of plasma density and temperature. A time dependent continuity equation with an equatorial plasma drift model qualitatively explains the daily variation of the equatorial anomaly /l/. This model including a neutral wind showed the latitudinal positions of the crests to depend only on an E x B drift velocity and not on the neutral wind velocity /2, 3/. Simulations with a realistic geomagnetic field model were carried out to investigate the longitudinal differences in the equatorial ionosphere /4, 5/. The time dependent continuity equation with the energy equations of electron and ions was also solved for the mid-latitude ionospheric F region /6,7/.
transport
The Hinotori satellite was launched on February 1981 from Kagoshima Space Flight Center of the Institute of Space and Astronautical Science in Japan for investigations of the solar flares and the Earths ionosphere during solar maximum. The satellite was positioned at -600 km altitude with an inclination of 30’. The ionospheric plasma was observed with an impedance probe /8/ and an electron temperature probe on board /9, lo/. MODEL AND EQUATION The ion densities and the electron and ion temperatures in the equatorial ionosphere are simulated by the following differential equations, a~i I 1 aANiVi --=pi-Li, (1) dt A ds
S. Watanabe and
(2)llO
K.-I. Oyama
dV. dV,+v.,= at ’ as k ~N.T. +gsinI__--?-_--Nimi ds
where Ni is density, Vi is field-aligned velocity, Pi is production rate, Li is loss rate, A is cross-sectional area of magnetic flux tube, Ti is temperature, Qi is heating and cooling rate, ~~is thermal conductivity, k is the Boltzmann constant, mi is ion mass, g is gravitational acceleration, vij is the collision frequencies between particles i and j, (I, is neutral wind velocity, I is magnetic dip angle, t is time and s is arc length along magnetic field line. i represents electron, O+ and H+. and n represents oxygen atom, hydrogen atom, oxygen molecule and nitrogen molecule. For the electron velocity V, we assume that there is no field-aligned current,
N,V, = No+V,+ + NM+ V,+.
(4)
Equations (l)-(3) are numerically solved-for’b+, H+ along the magnetic field line with an E x B drift and a neutral wind; E is the electric field and B is the magnetic field. Equations (2) and (4) are also used for electrons. At the lower boundary of 120 km altitude we assume the ion densities to be in chemical equilibrium. As for the ion and electron temperatures, these were taken as these temperatures to be equal to the neutral temperature at the lower boundary. The neutral atmosphere is calculated with Jacchia’s model. We use his neutral atmospheres with a daytime T, between 1200 and 1560 K. For equinox conditions the upward drift velocity induced by the ionospheric electric field is taken from Radar observations in Jicamarca /l l/. We use the eastward plasma drift of Coley and Heelis [1989], the zonal neutral wind of Wharton et al. [1984] and the meridional neutral wind of Herrero at al. [1988]. COMPARISON BETWEEN OBSERVATION AND MODEL Figure 1 shows the average electron temperature at -600 km altitude as a function of local time and geomagnetic latitude during high solar flux and equinox conditions (F10.7>200). The data are, however, averaged for longitude. An electron temperature enhancement occurs in the morning (-6, LT) with the peak at the magnetic equator. A temperature enhancement occurs also in the mid-latitude in the evening (-18LT). In the pre-midnight period there occur small electron temperature enhancements around the equatorial density anomaly [see Oyarna et al., 1993 for detailed results]. The electron temperature at 600km. for F10.7 = 200 has been calculated by the IRI model for equinox condition and is shown in figure 2. The temperature increases with increasing magnetic latitude in the daytime. However, we can not see temperature enhancements in the morning and evening while the observed temperature (see Figure 1) increases clearly in those periods. Figure 3 shows the electron temperature structure obtained by the computer simulation. The electron temperature increases in the topside equatorial F region; enhancement in the morning with a peak at the magnetic equator, enhancement in the evening with a minimum at the magnetic equator and enhancement around the equatorial density anomaly region.
(2)lll
ThcI?@amidIo~
CONCLUSION The Hinotori satellite observed the electron density and temperature around the equatorial anomaly region at -600 km altitude during high solar activity. The observed electron temperature in the equatorial ionospheric F region was compared with the International Reference Ionosphere (IRI). The results show a large difference between observation and model. The observations show that the electron temperature increases in the topside equatorial F region with (1)electron temperature enhancement at -6LT with a peak at the magnetic equator, (2)temperature enhancement at -18LT with a minimum at the magnetic equator and (3)temperature enhancement around the equatorial density anomaly region. Our computer simulations of the equatorial ionosphere coincide with the equatorial electron density and temperatures structure observed by the Hinotori satellite. The results suggest that the ionospheric dynamics influences the thermal as well as the density structure in the equatorial ionosphere. Electron Temperature (kl
3000
2000
1000
0
Fig. 1. Observed electron temperature in the local time - geomagnetic latitude space at -400 km altitude.
Electron Temperature (k)
20
gnetic Latitude (deg)
Fig. 2. Electron temperature at 600 km altitude calculated by the IRI model .
s. wataoalx andK-1. oyama
(2)112
Electron Temperature (k)
.atitude
Fig. 3. Simulated electron temperature in the local time - geomagnetic space at 600 km altitude.
REFERENCES 1. R.G. Baxter, and P.C. Kendall, A theoretical technique for evaluating the time-dependent effects of general electrodynamic drifts in the F2 layer of the ionosphere, Proc. Roy. Sot. A, 304, 171-185, (1968). 2. M.F.K. Abur-Robb, and D.W. Windle, On the day and night reversal in NmF2 north-south asymmetry, Planet. Space Sci., 17.97, (1969). 3. D.L. Sterling, W.B. Hanson, R.J. Moffett. and R.G. Baxter, Influence on electromagnetic drifts and neutral air winds on some features of the F2 region, Radio Sci., 4. 1005- 1023, (1969). 4. D.N. Anderson, A theoretical study of the ionospheric F region equatorial anomaly - I. theory, Planer. Space Sci., 21.409419, (1973). 5. D.N. Anderson, A theoretical study of the ionospheric F region equatorial anomaly - II. results in the American and Asian sectors, Planet. Space Sci., 21,421-442, (1973). 6. G.J. Bailey, The topside ionosphere above Arecibo at equinox during sunspot maximum, Planet. Space Sci., 28,47-59, (1980). 7. G.J. Bailey, The effect of a meridional ExB arift on the thermal plasma at L=l.4. Planet. Space Sci., 31, 389-409, (1983). 8. H. Oya. T. Takahashi, and S. Watanabe, Observation of low latitude ionosphere by the impedance probe on board the Hinotori satellite, J. Geomug. Geoelectr, 38, 11 l-123, (1986). 9. K-I. Oyama, K. Schlegel, and S. Watanabe, Temperature structure of plasma bubbles in the low latitude ionosphere around 600 km altitude, Planer. Space Sci., 36,553-567, (1988). 10. K-I, Oyama, M.A. Abdu, T. Takahashi, E.R. de Paula, I.S. Batista, S. Watanabe, H. Oya, An electron temperature anomaly in the equatorial ionosphere, submitted to Geophys. Res. Lea, (1993). 11. B.G. Fejer, The equatorial ionospheric electric fields. A review, J. Atmos. Terr. Phys., 43, 377-386, (1981). 12. W.R. Coley, and R.A. Heelis, Low-latitude zonal and vertical ion drifts seen by DE-2, J. Geophys. Res., 94,6751-6761, (1989). 13. L.E. Wharton, N.W. Spencer, and H.G. Mayr, The Earth’s thermospheric superrotation from Dynamic Explorer 2, Geophys. Res. Len., 11.531-533, (1984). 14. F.A. Hetrero, H.G. Mayr, and N.W. Spencer, Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data, J. Afmos. Terr. Phys., 50, lOOl1006, (1988).
A&. SpaceRexVol. IS, No. 2.pp.(2)109-(2)112.1995 copyxighiol~““&y RiJlWdiJiGreatBhh.AU 02?+117719 ‘tt” $7.00+ 0.00
DYNAMIC MODEL AND OBSERVATION OF THE EQUATORIAL IONOSPHERE S. Watanabe*and K-I. Oyama** *Hokkaido Institute of Information Technology, 45 Nishinopporv Betsy Hokkaido, 069 Japan * * The Institute of Space and Astronautical Science, Sagamihara 229 Japan
ABSTRACT The Hinotori satellite observed the electron density and temperature around the equatorial anomaly region at -600 km altitude during high solar activity. The observed electron temperature in the equatorial F region ionosphere was compared with the International Reference Ionosphere (IRI) model. The results showed the large difference between observation and model. Time dependent three dimensional computer simulations of the equatorial ionospheric F region have been carried out to investigate the electron temperature anomaly. These simulations coincide with the equatorial electron density and temperature structure observed by the Hinotori satellite. INTRODUCTION In the equatorial ionospheric F region, plasma production and loss as well as and diffusion are important for the structure of plasma density and temperature. A time dependent continuity equation with an equatorial plasma drift model qualitatively explains the daily variation of the equatorial anomaly /l/. This model including a neutral wind showed the latitudinal positions of the crests to depend only on an E x B drift velocity and not on the neutral wind velocity /2, 3/. Simulations with a realistic geomagnetic field model were carried out to investigate the longitudinal differences in the equatorial ionosphere /4, 5/. The time dependent continuity equation with the energy equations of electron and ions was also solved for the mid-latitude ionospheric F region /6,7/.
transport
The Hinotori satellite was launched on February 1981 from Kagoshima Space Flight Center of the Institute of Space and Astronautical Science in Japan for investigations of the solar flares and the Earths ionosphere during solar maximum. The satellite was positioned at -600 km altitude with an inclination of 30’. The ionospheric plasma was observed with an impedance probe /8/ and an electron temperature probe on board /9, lo/. MODEL AND EQUATION The ion densities and the electron and ion temperatures in the equatorial ionosphere are simulated by the following differential equations, a~i I 1 aANiVi --=pi-Li, (1) dt A ds
S. Watanabe and
(2)llO
K.-I. Oyama
dV. dV,+v.,= at ’ as k ~N.T. +gsinI__--?-_--Nimi ds
where Ni is density, Vi is field-aligned velocity, Pi is production rate, Li is loss rate, A is cross-sectional area of magnetic flux tube, Ti is temperature, Qi is heating and cooling rate, ~~is thermal conductivity, k is the Boltzmann constant, mi is ion mass, g is gravitational acceleration, vij is the collision frequencies between particles i and j, (I, is neutral wind velocity, I is magnetic dip angle, t is time and s is arc length along magnetic field line. i represents electron, O+ and H+. and n represents oxygen atom, hydrogen atom, oxygen molecule and nitrogen molecule. For the electron velocity V, we assume that there is no field-aligned current,
N,V, = No+V,+ + NM+ V,+.
(4)
Equations (l)-(3) are numerically solved-for’b+, H+ along the magnetic field line with an E x B drift and a neutral wind; E is the electric field and B is the magnetic field. Equations (2) and (4) are also used for electrons. At the lower boundary of 120 km altitude we assume the ion densities to be in chemical equilibrium. As for the ion and electron temperatures, these were taken as these temperatures to be equal to the neutral temperature at the lower boundary. The neutral atmosphere is calculated with Jacchia’s model. We use his neutral atmospheres with a daytime T, between 1200 and 1560 K. For equinox conditions the upward drift velocity induced by the ionospheric electric field is taken from Radar observations in Jicamarca /l l/. We use the eastward plasma drift of Coley and Heelis [1989], the zonal neutral wind of Wharton et al. [1984] and the meridional neutral wind of Herrero at al. [1988]. COMPARISON BETWEEN OBSERVATION AND MODEL Figure 1 shows the average electron temperature at -600 km altitude as a function of local time and geomagnetic latitude during high solar flux and equinox conditions (F10.7>200). The data are, however, averaged for longitude. An electron temperature enhancement occurs in the morning (-6, LT) with the peak at the magnetic equator. A temperature enhancement occurs also in the mid-latitude in the evening (-18LT). In the pre-midnight period there occur small electron temperature enhancements around the equatorial density anomaly [see Oyarna et al., 1993 for detailed results]. The electron temperature at 600km. for F10.7 = 200 has been calculated by the IRI model for equinox condition and is shown in figure 2. The temperature increases with increasing magnetic latitude in the daytime. However, we can not see temperature enhancements in the morning and evening while the observed temperature (see Figure 1) increases clearly in those periods. Figure 3 shows the electron temperature structure obtained by the computer simulation. The electron temperature increases in the topside equatorial F region; enhancement in the morning with a peak at the magnetic equator, enhancement in the evening with a minimum at the magnetic equator and enhancement around the equatorial density anomaly region.
(2)lll
ThcI?@amidIo~
CONCLUSION The Hinotori satellite observed the electron density and temperature around the equatorial anomaly region at -600 km altitude during high solar activity. The observed electron temperature in the equatorial ionospheric F region was compared with the International Reference Ionosphere (IRI). The results show a large difference between observation and model. The observations show that the electron temperature increases in the topside equatorial F region with (1)electron temperature enhancement at -6LT with a peak at the magnetic equator, (2)temperature enhancement at -18LT with a minimum at the magnetic equator and (3)temperature enhancement around the equatorial density anomaly region. Our computer simulations of the equatorial ionosphere coincide with the equatorial electron density and temperatures structure observed by the Hinotori satellite. The results suggest that the ionospheric dynamics influences the thermal as well as the density structure in the equatorial ionosphere. Electron Temperature (kl
3000
2000
1000
0
Fig. 1. Observed electron temperature in the local time - geomagnetic latitude space at -400 km altitude.
Electron Temperature (k)
20
gnetic Latitude (deg)
Fig. 2. Electron temperature at 600 km altitude calculated by the IRI model .
s. wataoalx andK-1. oyama
(2)112
Electron Temperature (k)
.atitude
Fig. 3. Simulated electron temperature in the local time - geomagnetic space at 600 km altitude.
REFERENCES 1. R.G. Baxter, and P.C. Kendall, A theoretical technique for evaluating the time-dependent effects of general electrodynamic drifts in the F2 layer of the ionosphere, Proc. Roy. Sot. A, 304, 171-185, (1968). 2. M.F.K. Abur-Robb, and D.W. Windle, On the day and night reversal in NmF2 north-south asymmetry, Planet. Space Sci., 17.97, (1969). 3. D.L. Sterling, W.B. Hanson, R.J. Moffett. and R.G. Baxter, Influence on electromagnetic drifts and neutral air winds on some features of the F2 region, Radio Sci., 4. 1005- 1023, (1969). 4. D.N. Anderson, A theoretical study of the ionospheric F region equatorial anomaly - I. theory, Planer. Space Sci., 21.409419, (1973). 5. D.N. Anderson, A theoretical study of the ionospheric F region equatorial anomaly - II. results in the American and Asian sectors, Planet. Space Sci., 21,421-442, (1973). 6. G.J. Bailey, The topside ionosphere above Arecibo at equinox during sunspot maximum, Planet. Space Sci., 28,47-59, (1980). 7. G.J. Bailey, The effect of a meridional ExB arift on the thermal plasma at L=l.4. Planet. Space Sci., 31, 389-409, (1983). 8. H. Oya. T. Takahashi, and S. Watanabe, Observation of low latitude ionosphere by the impedance probe on board the Hinotori satellite, J. Geomug. Geoelectr, 38, 11 l-123, (1986). 9. K-I. Oyama, K. Schlegel, and S. Watanabe, Temperature structure of plasma bubbles in the low latitude ionosphere around 600 km altitude, Planer. Space Sci., 36,553-567, (1988). 10. K-I, Oyama, M.A. Abdu, T. Takahashi, E.R. de Paula, I.S. Batista, S. Watanabe, H. Oya, An electron temperature anomaly in the equatorial ionosphere, submitted to Geophys. Res. Lea, (1993). 11. B.G. Fejer, The equatorial ionospheric electric fields. A review, J. Atmos. Terr. Phys., 43, 377-386, (1981). 12. W.R. Coley, and R.A. Heelis, Low-latitude zonal and vertical ion drifts seen by DE-2, J. Geophys. Res., 94,6751-6761, (1989). 13. L.E. Wharton, N.W. Spencer, and H.G. Mayr, The Earth’s thermospheric superrotation from Dynamic Explorer 2, Geophys. Res. Len., 11.531-533, (1984). 14. F.A. Hetrero, H.G. Mayr, and N.W. Spencer, Low latitude thermospheric meridional winds between 250 and 450 km altitude: AE-E satellite data, J. Afmos. Terr. Phys., 50, lOOl1006, (1988).