- Email: [email protected]
Field-aligned plasma diffusive fluxes in the topside ionosphere from radio occultation measurements by CHAMP
ARTICLE IN PRESS Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Field-aligned plasma diffusive fluxes in the topside ionosphere from radio occultation measurements by CHAMP Guang-ming Chen a,b, Jiyao Xu a, Wenbin Wang c,, Jiuhou Lei d, Yue Deng c,1 a
State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100190, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China c High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307, USA d Department of Aerospace Engineering Sciences, University of Colorado, Boulder, CO 80309, USA b
a r t i c l e in fo
abstract
Article history: Received 12 June 2008 Received in revised form 27 February 2009 Accepted 25 March 2009 Available online 7 April 2009
O+ field-aligned diffusive velocities and fluxes in the topside ionosphere have been calculated from electron density profiles retrieved from CHAMP radio occultation (RO) measurements. The velocities and fluxes from January 2002 to December 2003 at low- and mid-latitudes have been statistically analyzed. The results show that daytime diffusive fluxes changed gradually from downward to upward as altitude increases. The largest values of the upward diffusive fluxes and velocities occurred at around 7251 geomagnetic latitude. During solstices the plasma fluxes in the winter hemisphere were larger than those in the summer hemisphere. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Topside ionosphere Diffusive flux Radio occultation Ionospheric dynamics
1. Introduction Field-aligned plasma flows between the topside ionosphere and the plasmasphere are of significant importance for these two regions. In the daytime, the plasma in the topside ionosphere diffuses upwards along the magnetic field lines into the plasmasphere, whereas at night downward plasma flows from the plasmasphere help to maintain the nighttime F2 layer (Rishbeth and Garriott, 1969). There have been some quantitative studies of plasma flow between these two regions. Evans (1971a, b, c, 1975) studied the O+ vertical flux using Millstone Hill incoherent scatter radar (ISR) observations during quiet periods. These studies showed a large upward flux at sunrise and a small downward flux at sunset. In the daytime, the flux was downward at altitudes around the F2 layer peak, but upward at altitudes above it. At night, the O+ vertical flux was usually downward with a magnitude of 3 107 cm2 s1 above 700 km. Ho and Moorcroft (1975) investigated daytime fluxes from the ISR at Arecibo in October 1967, which were an order of magnitude larger than those at Millstone Hill obtained by Evans (1975). Vickrey et al. (1979) also used the Arecibo ISR data to study these fluxes. They suggested that there was an inter-
Corresponding author.
E-mail address: [email protected] (W. Wang). Now at CIRES/University of Colorado and NOAA Space Weather Prediction Center, Boulder, CO 80305, USA. 1
1364-6826/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2009.03.027
hemispheric transport from the summer hemisphere to the winter hemisphere. They also found that O+ and H+ fluxes were sometimes oppositely directed. Greenspan et al. (1994) analyzed the field-aligned ion velocities from the F8 satellite of the Defense Meteorological Satellite Program (DMSP) at 840 km near the dawn and dusk meridional plane under low solar activity conditions. They found that there were field-aligned upward flows during the day and downward flows at night. Since direct measurements of plasma fluxes are very limited, these fluxes have been usually calculated by combining observed parameters and models. For instance, Buonsanto and Titheridge (1987) calculated the fluxes of ionization at 850 km height using neutral parameters from the mass spectrometer-incoherent scatter 83 (MSIS83) model (Hedin, 1983) and the F2 peak electron densities, NmF2, from ionosonde observations. Skoblin (1996) obtained ion fluxes at 1000 km altitude using ground-based ionosonde observations of electron density, satellite in situ measurements of electron concentration, temperature and mean ion mass, as well as a simple theoretical ionospheric model. These previous studies, however, were mainly focused on total ion velocities and fluxes that included the effect of neutral winds and electric fields. Investigations of ambipolar diffusive velocities and fluxes which cannot be observed directly have been very limited. The global distribution of plasma diffusive fluxes is not only important to the study of the dynamical and mass coupling between the ionosphere and plasmasphere/inner magnetosphere, but it also helps to elucidate the relative contributions of each physical process, including neutral wind, electric field, chemistry
ARTICLE IN PRESS 968
G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
and ambipolar diffusion, to variations of the global ionosphere during both storm and quite times. Topside diffusive fluxes are also one of the primary top boundary conditions for many global thermosphere/ionosphere models, such as the NCAR-TIEGCM and TING model (e.g., Wang et al., 1999). These models can selfconsistently calculate neutral winds and ionospheric electric fields, but they require time-dependent, global distributions of the plasma diffusive fluxes at their top boundaries to accurately solve the ion/plasma continuity equations. Although some of the flux tube models, e.g., SUPIM and SAMI2 (e.g., Bailey and Balan, 1996; Huba et al., 2000) do not require this boundary condition, the derived diffusive velocity and flux are still vary valuable in that they can be used to validate the outputs of these models. Lockwood (1983) calculated O+ diffusive fluxes from the Alouette 1 satellite observations between 1962 and 1968, and analyzed the diurnal variations of O+ diffusive fluxes from the F2 peak to 700 km altitude at mid-latitudes near sunspot minimum. Lockwood and Titheridge (1982) reported latitudinal variations of Fd/Fl, where Fd was O+ diffusive flux and Fl was the limiting value set by frictional drag. Nevertheless, altitudinal, seasonal and global distributions of Fd are still an open question, especially under high solar activity conditions. In this paper, we will use the new capability of the Global Positioning System (GPS) radio occultation (RO) measurements of the ionosphere, which provides global ionosphere electron density profiles, to obtain global distributions of the plasma diffusive fluxes and their variations with geophysical conditions. The GPS occultation technique for ionospheric sounding (e.g., Hajj and Romans, 1998; Jakowski et al., 2004; Lei et al., 2007) has been recently used to examine the global distribution of the ionospheric electron densities. This paper presents a method to calculate O+ ion field-aligned diffusive velocities and fluxes in the topside ionosphere from electron density profiles retrieved from ionospheric radio occultation measurements made by the German CHAllenging Minisatellite Payload (CHAMP) satellite. A statistical study of the diffusive velocities and fluxes in the low- and mid-latitudes is also carried out to investigate their altitude, latitudinal, seasonal and local time variations under geomagnetically quiet and solar maximum conditions.
2. Data The CHAMP satellite was successfully launched on 15 July 2000 into a near-polar, circular orbit at 454 km altitude (Reigber et al., 2002). It provided its first ionospheric radio occultation measurements on 11 April 2001. Since then, more than 100,000 ionospheric electron density profiles have been retrieved from these CHAMP radio occultaion data at the University Corporation for Atmospheric Research (UCAR) COSMIC Data Analysis and Archival Center (CDAAC) (http://cosmic-io.cosmic.ucar.edu/cdaac). The CHAMP radio occultation measurements provide eletron density profiles from the ionospheric E region to the topside ionosphere at around 420 km, where the satellite orbits, with an altitude resolution of about 1 km. The root mean square (RMS) difference of electron density was about 103 cm3 above 150 km for the horizontally stratified ionosphere (Schreiner et al., 2007). In addition, CHAMP uses a planar Langmuir probe (PLP) to perform in situ measurements of ionospheric electron density and temperature every 15 s. The accuracy of the PLP measurements is within 10% (Liu et al., 2007). A Chapman-a function (e.g., Rishbeth and Garriott, 1969) is applied to fit and smooth electron density profiles. The Chapman scale height is assumed to be linearly dependent on altitude (Lei et al., 2005, 2006). To ensure the validity of the electron density data (DRO) retrieved from radio occultation measurements, we compare DRO with electron densities obtained from the in situ measurements (DL) by the PLP at the same location. If 2/3oDRO/ DLo3/2, the data of RO are considered to be reasonable and reliable, otherwise, the RO retrieved electron density profiles are removed from the data set. About 10% of the profiles are removed. The measurements made during periods of KpX4 are also excluded. Finally, a total of 15,000 profiles between 651S and 651N geomagnetic latitude from January 2002 to December 2003 have been used in our statistical analysis, which is described below. Data distribution at an altitude of 50 km above the F2 peak height (F2+50 km) is shown in Fig. 1. In this paper, we use heights above the F2 peak, instead of constant heights for vertical coordinates. It has been shown that, at mid-latitudes, the F2
Geomagnetic latitude
March Equinox
September Equinox
60
70
60
70
40
60
40
60
50
20
40
0
20 0
30
-20
20
-40
10
-60
40 30
-20
20
-40
10
-60
1 3 5 7 9 11 13 15 17 19 21 23
1 3 5 7 9 11 13 15 17 19 21 23
June Solstice Geomagnetic latitude
50
December Solstice
60
70 60
70
40
60 40 50 20
60
40
40
20 0
30
-20
20
-40
10
-60 1 3 5 7 9 11 13 15 17 19 21 23 Local time
50
0
30
-20
20
-40
10
-60 1 3 5 7 9 11 13 15 17 19 21 23 Local time
Fig. 1. Data distribution as a function of geomagnetic latitude and local time in different seasons at F2+50 km.
ARTICLE IN PRESS G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
peak at a given local time tends to lie at a fixed atmospheric pressure level regardless of the latitude, season and solar activity (Rishbeth and Edwards, 1990). This may also be true at low latitudes (Rishbeth and Edwards, 1989). Furthermore, in global thermospheric/ionospheric models, a pressure grid is usually used in the vertical direction instead of an altitude grid (e.g., Wang et al., 1999). In addition, the use of heights above the F2 peak reduces any complication that the movement of the F2 peak may cause in the interpretation of data. As shown in the Appendix A (Figs. A1 and A2), there were significant seasonal, latitudinal and local time variations in the observed F2 peak heights. Fig. 1 shows all observations made at an altitude of F2+50 km and within 90 days of either the solstices or equinoxes in 2-h local time and 101 geomagnetic latitude bins. Daytime is defined from 0600 to 1800 local solar time (LT), and nighttime from 1800 to 0600 LT. As shown in Fig. 1, there are sufficient data in most of the local time and magnetic latitude bins, especially in the daytime, to carry out a statistical analysis and to obtain statistically reliable results. Nevertheless, there are still some bins that do not have enough data points, such as those around the geomagnetic equator in the daytime. We are currently working on the new dataset from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) satellites, which have six satellites which are capable of obtaining more ionospheric radio occultation data and, thus, which have a better global coverage. The COSMIC measurements are for solar minimum conditions, whereas the CHAMP data we used for this study are for high solar activity conditions. In a follow-up study, we will compare diffusive fluxes from CHAMP with those from the COSMIC to see how these fluxes vary with solar activity.
3. Method In this study, the plasma equation of motion is used to calculate the plasma ambipolar diffusive velocity. On the assumption that the field-aligned electric field in the ionosphere is zero, the ion-inertial effects can be neglected, and the plasma diffuses along the magnetic field line, the diffusive velocity for O+ ions is given by k sin I ðT i þ T e Þ @ni @ðT i þ T e Þ mi g Vd ¼ þ þ (1) @h mi uin ni @h k and the O+ diffusive flux Fd is obtained by
Fd ¼ ni V d
969
Fig. 2. Ratio between CHAMP observed electron temperatures and IRI2007 predictions.
temperatures observed by CHAMP and those from IRI 2007 under geomagnetically quiet conditions at all local times from 2002 to 2003 are shown in Fig. 2. Electron temperatures from IRI 2007 were smaller than those observed in the equatorial regions, and there were also differences in other latitudes. It appears that there is a latitudinal change in the temperature ratio. It is unclear whether this bias is caused by IRI, CHAMP or both. However, the largest relative difference was only about 10%. Given that there have been no simultaneous global observations of Tn, Te and Ti height profiles, we use Tn, Te and Ti height profiles from the IRI model and scale the model electron temperatures using the CHAMP PLP in situ electron temperatures. Note that the change of the diffusive velocity can be about 5 ms1 at F2+100 km, when Ti+Te is changed by 10%. Therefore, this procedure would reduce the uncertainty induced by model electron temperature profiles and do not significantly affect the morphology and statistics of our results. The diffusive velocities and fluxes are averaged within 91-day periods centered on solstices or equinoxes. Parameters in different seasons are denoted as March equinox, September equinox, June solstice and December solstice, respectively. The regions from 251 to 651 and from 251 to 651 in geomagnetic latitudes are defined as mid-latitude regions.
(2)
where Vd is the plasma ambipolar diffusive velocity and ni the density of O+ ions. In the F2 region below about 600 km, most of the ions are O+ ions, thus ni equals the electron density, which is obtained from the radio occultation measurement; Ti and Te are the temperatures of O+ ions and electrons, respectively; mi the mass of O+ ions; I the geomagnetic dip angle; g the acceleration of gravity; k the Boltzmann constant; h the vertical height; uin, which is the momentum transfer collision frequency for O+ ions moving through the neutrals, is given by Pesnell et al. (1993). Neutral densities, n(O), n(O2) and n(N2), that are used in the calculation of uin, are obtained from the NRLMSISE00 model (Picone et al., 2002). The geomagnetic dip angle I is from IGRF-10 (Maus et al., 2005). The signs of diffusive fluxes and velocities are defined as positive if the O+ ions diffuse upwards along the magnetic field line, and negative if downward. Tn, Ti and Te are from the International Reference Ionosphere 2007 (IRI2007) model (Bilitza and Reinish, 2008). Furthermore, the values of Te from IRI2007 are scaled using the in situ electron temperatures from the PLP instrument onboard the CHAMP satellite. The latitudinal variation of the ratios between electron
4. Result and discussion In this section, we will carry out statistical analyses of the altitudinal, diurnal and latitudinal variations of the diffusive velocity and flux above the F2 peak height hmF2. 4.1. Altitude variations of the diffusive velocities and fluxes The altitude variations of the diffusive velocities and fluxes in the daytime around March equinox at low- and mid-latitudes are shown in Fig. 3. The averaged diffusive velocities are mainly between 15 and 0 ms1 at the F2 peak, and between 0 and 25 ms1 at F2+100 km and directed upward for latitudes from 101S to 651N. The diffusive velocities are comparable to the fieldaligned neutral wind speeds, the mean values of which are less than 40 ms1 (Hedin et al., 1988). In this paper, we are interested in the O+ ion diffusive velocities. The actual O+ velocities along the magnetic field lines in the topside ionosphere are the sum of diffusive velocities and the field-aligned component of the neutral winds, as neither diffusive velocities nor neutral winds can be
ARTICLE IN PRESS G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
h-hmF2 (km)
970
100
100
80
80 10°S-10°N 10°N-30°N 30°N-50°N 50°N-65°N
60 40
60 40
20
20
0
0 -15 -10
-5
0
5 10 Vd (m/s)
15
20
10°S-10°N 10°N-30°N 30°N-50°N 50°N-65°N
25
-10
-15
-5 0 5 Flux (108 cm-2s-1)
10
15
Vd (m/s)
Fig. 3. Altitude profiles of diffusive velocities (left panel) and fluxes (right panel) in the daytime near the March equinox at different latitudes.
March Equinox September Equinox
20 15 10 5 0 -5 -10 -15 -20 -25 -30 0
4
8
12 16 Local time
Summer Hemisphere Winter Hemisphere
20 15 10 5 0 -5 -10 -15 -20 -25 -30 20
24
0
4
8
12 16 Local time
20
24
Fig. 4. Local time variations of diffusive velocities (positive upward) for different seasons. The vertical lines are error bars.
neglected. The relative importance of the Vd and neutral wind effects in determining the total O+ velocities depends on the height of the observations. Similar characteristics of Vd and fluxes in different latitude regions, except around the magnetic equator (black lines), can be seen. Near the F2 peak, Vd is negative and the flows are downward. At about F2+40 km, a transition from downward flows to upward flows occurs, which is completed at about F2+70 km. At altitudes above the transition height, it appears that Vd increases much faster with altitude than it does below it. This is probably due to the exponential decrease of the neutral atmospheric densities, which causes the diffusion coefficients and diffusive velocities to increase with altitude. The altitudinal dependences of the diffusive fluxes are consistent with those from a numerical study by Balan and Bailey (1996). These altitude variations of the plasma diffusive velocities and fluxes have also been observed at Millstone Hill by Evans (1971c), Evans and Holt (1978). In Evans’ studies, however, the transition altitudes happened between 450 and 600 km. These are different from the transition altitudes obtained here which are lower than 400 km for the same latitude (see Fig. A2). The difference may be caused by the contributions from the neutral winds and electric field drift that were included in Evans’ results but not in ours. Other factors, such as solar activity conditions and electron temperature profiles, may also contribute to this difference. It is also interesting to note that diffusive velocities and fluxes around the magnetic equator (black lines: 101S–101N) are small and have different vertical variations from those in other latitude bands because of the small magnetic dip angles around the magnetic equator. The diffusive flux in the equatorial anomaly region (red line: 101N–301N) has a much larger absolute/downward value than in other regions between the F2 peak and F2+30 km, although diffusive velocities are smaller. This is caused by the higher electron densities that occur in the equatorial anomaly region.
4.2. Diurnal variations of the diffusive velocities and fluxes Diurnal variations of the diffusive velocities and fluxes at F2+80 km altitude at mid-latitudes in different seasons are shown in Figs. 4 and 5, respectively. The summer hemisphere data include those from the northern hemisphere around the June solstice and the southern hemisphere around the December solstice. The winter hemisphere data are those from the northern hemisphere around the December solstice and the southern hemisphere around the June solstice, respectively. In the midlatitude regions diffusive velocities are upward in the daytime and downward at night. Vd often shows large negative values from 0200 to 0500 LT before sunrise in all seasons, and then changes rapidly to positive values after sunrise. The diffusive fluxes have maximum upward values around 1300 LT and then decrease gradually with local time, and turn to downward around sunset, as shown in Fig. 5. There are differences in the local time variations between Vd and flux. This is caused by local time variations of the electron densities. Lockwood (1983) reported the diurnal variations of diffusive fluxes, and found that the local time at which the ionosphere had maximum upward diffusive fluxes changed with seasons. The largest upward fluxes occurred following sunrise at equinox and in the afternoon during solstices. However, in our results, the largest upward fluxes take place around 1300 LT for both equinoxes and solstices. Theses differences may be related to the fact that Lockwood’s results represented conditions from the F2 peak to 700 km near sunspot minimum, whereas ours are for F2+80 km and for high solar activity. 4.3. Latitudinal variations of the diffusive velocities and fluxes Latitudinal variations of the diffusive velocities and fluxes in the daytime (0600–1800 LT) at three altitudes for different
ARTICLE IN PRESS
Flux (108cm-2s-1)
G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
14 12 10 8 6 4 2 0 -2 -4 -6
14 12 10 8 6 4 2 0 -2 -4 -6 -8
March Equinox September Equinox
4
0
8
12 16 Local time
20
24
971
Summer Hemisphere Winter Hemisphere
4
0
8
12 16 Local time
20
24
Fig. 5. Same as Fig. 4, but for diffusive fluxes.
March Equinox 25
Vd (m/s)
20
20 15
10
10
5
5
0
0
-5
-5
-10
-10 -40
-20
0
20
40
hmF2+50km hmF2+70km hmF2+90km
25
15
-60
60
-60
-40
hmF2+50km hmF2+70km hmF2+90km
20
25 15
10
10
5
5
0
0
-5
-5
-10
-10 -40
-20 0 20 M latitude
0
20
40
60
40
60
40
60
hmF2+50km hmF2+70km hmF2+90km
20
15
-60
-20
December Solstice
June Solstice
25
Vd (m/s)
September Equinox hmF2+50km hmF2+70km hmF2+90km
-60
-40
-20 0 20 M latitude
Fig. 6. Latitudinal variations of the diffusive velocities in different seasons.
seasons are shown in Figs. 6 and 7. Around the March equinox, both Vd and Fd are roughly symmetric around the magnetic equator. Around the September equinox, diffusive velocities have larger values in the southern hemisphere than they do in the northern hemisphere at F2+90 km, but at lower altitudes, diffusive velocities in the northern hemisphere are relatively larger. On the other hand, the largest diffusive fluxes in the northern hemisphere are larger than those in the southern hemisphere near the September equinox. During the solstices, both the diffusive fluxes and velocities in the winter hemisphere are consistently larger than those in the summer hemisphere. The summer hemisphere has a larger scale height than the winter hemisphere does, leading to smaller vertical gradients of electron density profiles in summer, and consequently smaller diffusive velocities. Lockwood and Titheridge (1982) reported the latitudinal variation of Fd/FL from hmF2 to 700 km between 1962 and 1968 over all local times. Their results indicated that the largest values of Fd/FL at the September equinox were also greater in the northern hemisphere than they were in the southern hemisphere. However, their values during solstices were larger in the summer
hemisphere than in the winter hemisphere at mid-latitudes. The difference may be caused by different solar conditions. The hemispheric asymmetric distribution of these fluxes and velocities during the solstices as well as at the September equinox is an interesting feature, and requires further investigations. The largest diffusive fluxes appear in the equatorial anomaly regions around 7251 geomagnetic latitudes in both hemispheres. The location of the maximum diffusive fluxes differs from that of the diffusive velocities, with the maximum displacement occurring in the equinoxes. This displacement is caused by the different latitudinal distribution of diffusive velocities compared with that of the electron densities. Flux depends both on the density of the flow and the velocity. The maxima will occur as a result of the convolution of these two effects rather than at the maxima of either individual effect. The diffusive fluxes around the geomagnetic equator (o751) are small, since the geomagnetic dip angle is small there. The diffusive fluxes at sub-polar latitudes are also smaller than those at other mid-latitude locations, because of smaller electron density and Vd at sub-polar latitudes. Lockwood and Titheridge (1982) found that the largest values of Fd/FL, which were normalized to 1000 km, appeared at less
ARTICLE IN PRESS 972
G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
March Equinox
Flux (109cm-2s-1)
1.5 1.0
0.5
0.0
0.0
-0.5
-0.5
1.5
-40
-20 0 20 June Solstice
40
60
-60
0.5
0.0
0.0
-0.5
-0.5 -40
-20 0 20 M latitude
40
60
September Equinox
-20 0 20 40 December Solstice
60
hmF2+50km hmF2+70km hmF2+90km
1.0
0.5
-60
-40
1.5
hmF2+50km hmF2+70km hmF2+90km
1.0
hmF2+50km hmF2+70km hmF2+90km
1.0
0.5
-60
Flux (109cm-2s-1)
1.5
hmF2+50km hmF2+70km hmF2+90km
-60
-40
-20 0 20 M latitude
40
60
Fig. 7. Same as Fig. 6, but for diffusive fluxes.
than 7251 geomagnetic latitudes. If the diffusive fluxes in this paper are projected to 1000 km along magnetic field lines, the largest diffusive fluxes will appear at less than 7201 geomagnetic latitude, which are close to the peak locations found in the study by Lockwood and Titheridge (1982). It should be pointed out, however, that the absolute values of our diffusive fluxes are different from Lockwood and Titheridge’s. Although their results represented conditions over a wide altitude range (from F2 peak to 700 km) and in low solar activity and ours are for a particular altitude and high solar activity conditions, the largest flux values for both cases occurred near the equatorial anomaly regions. The causes of this agreement are complicated, since F2 peak heights change with latitude. From Eq. (1), we can see that Vd is related to the vertical scale height (ni@h/@ni) of the electron density profiles. Liu et al. (2008) showed the scale height has a maximum value near the equatorial anomaly regions. The latitudinal variations of the vertical scale height of electron density in the equatorial anomaly regions should be an important factor in determining the peak locations. Furthermore, the maximum electron densities occurring in the equatorial anomaly regions should contribute significantly to the diffusive flux maximum occurring there. Chao et al. (2004) showed the global distributions of total field-aligned ion velocities at lower latitudes observed by the ROCSAT satellite. Their results showed that there was an interhemispheric plasma transport from the summer hemisphere to the winter hemisphere. Similar results were obtained using a numerical model by Bailey et al. (1975). Their ion velocities, however, represented total ion drift velocities that included the neutral wind effect and other processes. Our results are diffusive velocities and fluxes that result only from ambipolar diffusion.
5. Conclusions In this paper, a method of calculating O+ ion field-aligned diffusive velocities and fluxes in the topside ionosphere using the CHAMP radio occultation measurements and empirical models is described. Statistical analysis is carried out to study the altitudinal, local time, seasonal and latitudinal variations of these velocities and fluxes from January 2002 to December 2003. The
altitudinal, seasonal and latitudinal variations of diffusive velocities and fluxes are reported for the first time. Our conclusions are: (1) At mid-latitudes, diffusive velocity Vd is downward near the F2 peak and the corresponding diffusive fluxes are also downward. At altitudes between F2+40 km and F2+70 km, there is a transition from a downward to an upward direction in both the velocities and fluxes. At altitudes above the transition height, Vd increases with height much faster than it does below the transition height. (2) At F2+80 km in the mid-latitude regions, the diffusive velocities and fluxes are upward in the daytime and downward at night. The maximum values of the daytime upward fluxes occur around 1300 LT. (3) There are significant latitudinal variations in the diffusive fluxes and velocities. Both diffusive fluxes and velocities are very small near the geomagnetic equator, and have maximum values around 7251 geomagnetic latitudes. (4) Near the March equinox, diffusive fluxes and velocities show roughly hemispheric symmetry around the geomagnetic equator. Near the September equinox, however, there is a hemispheric asymmetry in both the diffusive velocities and fluxes. During solstices, the diffusive fluxes in the winter hemisphere are larger than those in the summer hemisphere.
Acknowledgements This work is supported by the National Natural Science Foundation of China (40621003, 40674088, 40523006) and the National Basic Research Program (2006CB806306). The authors are grateful to the University Corporation for Atmospheric Research for providing electron density profiles and to GeoForschungsZentrum (GFZ) Informations Systems and Data Center (ISDC) for providing the data of PLP soundings from CHAMP. We are also grateful to National Space Science Data Center (NSSDC) for providing the models of IRI2007, NRLMSISE00 and IGRF-10. This work is also supported in part by the Center for Integrated Space Weather Modeling (CISM) which is funded by the STC
ARTICLE IN PRESS G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
330 320
hmF2 (km)
330
March Equinox September Equinox
310
300
300
290
290
280
280
270
270
260
260
250
250
240
0
4
8
12
Summer Hemisphere Winter Hemisphere
320
310
16
20
24
240
973
0
4
8
Local time
12
16
20
24
Local time
Fig. A1. Local time variation of hmF2 at mid-latitudes for different seasons.
hmF2 (km)
360 340
340
320
320
300
300
280
280
260
260
240
240 -60
-40
-20
June Solstice December Solstice
360
March Equinox September Equinox
0
20
40
60
M latitude
-60
-40
-20
0 20 M latitude
40
60
Fig. A2. Geomagnetic latitudinal variations of hmF2 in the daytime for different seasons.
program under agreement number ATM-0120950. The National Center for Atmospheric Research is sponsored by the National Science Foundation. Yue Deng’s effort was supported by the NSF through grants ATM-0823689 and the University of Colorado through CIRES fellowship. We thank Dr. Alan Burns for very helpful comments.
Appendix A The diurnal variations of hmF2 in the mid-latitude regions and different seasons are shown in Fig. A1. hmF2 varies significantly with local time, being higher at night and lower in the daytime, especially in the morning. The latitudinal variations of hmF2 in the daytime are shown in Fig. A2. In the daytime, hmF2 in the equatorial regions is higher than that in the other latitude regions and the hmF2 in the summer hemisphere is higher than that in the winter hemisphere. References Bailey, G.J., Moffett, R.J., Swartz, W.E., 1975. Effects of photoelectron heating and interhemisphere transport on day-time plasma temperatures at low latitudes. Planetary and Space Science 23, 599–610. Bailey, G.J., Balan, N., 1996. A low-latitude ionosphere–plasmasphere model. In: Schunk, R.W. (Ed.), STEP Hand Book. pp. 173–206. Balan, N., Bailey, G.J., 1996. Modeling studies of equatorial plasma fountain and equatorial anomaly. Advances in Space Research 18 (3), 107–116. Bilitza, D., Reinish, B.W., 2008. International reference ionosphere 2007: improvement and new parameters. Advances in Space Research 42 (4), 599–609.
Buonsanto, M.J., Titheridge, J.E., 1987. Diurnal variations in the flux of ionosation above the F2 peak in the northern and southern hemisphere. Journal of Atmospheric and Solar-Terrestrial Physics 49, 1093–1105. Chao, C.K., Su, S.-Y., Yeh, H.C., 2004. Ion temperature crests and troughs in the morning sector of the low-latitude and midlatitude topside ionosphere. Journal of Geophysical Research 109, A11303. Evans, J.V., 1971a. Observations of F-region vertical velocities at Millstone Hill, 3, determination of altitude distribution of H+. Radio Science 6, 855–861. Evans, J.V., 1971b. Observations of F region vertical velocities at Millstone Hill, 1, evidence for drifts due to expansion, contraction, and winds. Radio Science 6, 609–626. Evans, J.V., 1971c. Observations of F region vertical velocities at Millstone Hill, 2, evidence for fluxes into and out of the protonsphere. Radio Science 6, 843–854. Evans, J.V., 1975. A study of F2 region daytime vertical ionization fluxes at Millstone Hill during 1969. Planetary and Space Science 23, 1461–1482. Evans, J.V., Holt, J.M., 1978. Night proton fluxes at Millstone Hill. Planetary and Space Science 26, 727–744. Greenspan, M.E., Burke, W.J., Richs, F.J., Hughes, W.J., Heelis, R.A., 1994. DMSP F8 observations of the mid-latitude and low-latitude topside ionosphere near solar minimum. Journal of Geophysical Research 99 (A3), 3817–3826. Hajj, G.A., Romans, L.J., 1998. Ionospheric electron density profiles obtained with the global positioning system: results from the GPS/MET experiment. Radio Science 33, 175–190. Hedin, A.E., 1983. A revised thermospheric model based on mass spectrometer and incoherent scatter data-MSIS-83. Journal of Geophysical Research 88, 10170–10188. Hedin, A.E., Spencer, N.W., Killeen, T.L., 1988. Empirical global model of upper thermosphere winds based on atmosphere and dynamics explorer satellite data. Journal of Geophysical Research 93 (A9), 9959–9978. Ho, M.C., Moorcroft, D.R., 1975. Composition and temperatures of the topside ionosphere over Arecibo. Planetary and Space Science 23, 315–322. Huba, J.D., Joyce, G., Fedder, J.A., 2000. The formation of an electron hole in the topside equatorial ionosphere. Geophysical Research Letters 27, 181–184. Jakowski, N., Heise, S., Wehrenpfennig, A., Tsybulya, K., 2004. Ionospheric radio occultation measurements and space weather. In: Kirchengast, G., Foelsche, U., Steiner, A. (Eds.), Occultations for Probing Atmosphere and Climate. Springer, Berlin, pp. 441–446.
ARTICLE IN PRESS 974
G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
Lei, J., Liu, L., Wan, W., Zhang, S.-R., 2005. Variations of electron density based on long-term incoherent scatter radar and ionosonde measurements over Millstone Hill. Radio Science 40, RS2008. Lei, J., Liu, L., Wan, W., Zhang, S.-R., van Eyken, A.P., 2006. Comparison of the first long-duration IS experiment measurements over Millstone Hill and EISCAT Svalbard radar with IRI2001. Advances in Space Research 37, 1102–1107. Lei, J., Syndergaard, S., Burns, A.G., Solomon, S.C., Wang, W., Zeng, Z., Roble, R.G., Wu, Q., Kuo, Y.-H., Holt, J.M., Zhang, S.-R., Hysell, D.L., Rodrigues, F.S., Lin, C.H., 2007. Comparison of COSMIC ionospheric measurements with ground-based observations and model predictions: preliminary results. Journal of Geophysical Research 112, A07308. Liu, H., Stolle, C., Watanabe, S., 2007. Evaluation of the IRI model using CHAMP observations in polar and equatorial regions. Advances in Space Research 39, 904–909. Liu, L., He, M., Wan, W., Zhang, M.-L., 2008. Topside ionospheric scale heights retrieved from constellation observing system for meteorology, ionosphere, and climate radio occultation measurements. Journal of Geophysical Research 113, A10304. Lockwood, M., Titheridge, J.E., 1982. Departures from diffusive equilibrium in the topside F-layer from satellite soundings. Journal of Atmospheric and SolarTerrestrial Physics 44, 425–440. Lockwood, M., 1983. Field-aligned plasma flow in the quiet, mid-latitude ionosphere deduce from topside soundings. Journal of Atmospheric and Solar-Terrestrial Physics 45, 1–14. Maus, S., Macmillan, S., Chernova, T., 2005. The 10th generation international geomagnetic reference field. Physics of the Earth and Planetary Interiors 151, 320–322.
Pesnell, W.D., Omidvar, K., Hoegy, W.R., 1993. Momentum transfer collision frequency of O+-O. Geophysical Research Letters 20, 1343–1346. Picone, J.M., Hedin, A.E., Drob, D.P., Aikin, A.C., 2002. NRLMSISE-00 empirical model of the atmosphere: statistical comparisons and scientific issues. Journal of Geophysical Research 107 (A12), 1468. Reigber, C., Lu¨hr, H., Schwintzer, P., 2002. First CHAMP Mission Results for Gravity. Magnetic and Atmospheric Studies. Springer, Berlin, Heidelberg, New York. Rishbeth, H., Garriott, O.K., 1969. Introduction to Ionospheric Physics. International Geophysical Series. Academic Press, New York. Rishbeth, H., Edwards, R., 1989. The isobaric F2-layer. Journal of Atmospheric and Terrestrial Physics 51, 321–338. Rishbeth, H., Edwards, R., 1990. Modeling the F2 layer peak height in terms of atmospheric pressure. Radio Science 25, 757–769. Schreiner, W., Rocken, C., Sokolovskiy, S., Syndergaard, S., Hunt, D., 2007. Estimates of the precision of GPS radio occultations from the COSMIC/FORMOSAT-3 mission. Geophysical Research Letter 34, L04808. Skoblin, M.G., 1996. Ionosphere-protonosphere thermal plasma fluxes estimated from Magion2 and ionosonde data in August-September 1990. Journal of Geophysical Research 101 (A12), 26971–26980. Vickrey, J.F., Swartz, W.E., Farley, D.T., 1979. Ion transport in the topside ionosphere at Arecibo. Journal of Geophysical Research 84, 7307–7314. Wang, W., Killeen, T.L., Burns, A.G., Roble, R.G., 1999. A high resolution, three-dimensional, time dependent, nested grid model of the coupled thermosphere-ionosphere. Journal of Atmospheric and Solar-Terrestrial Physics 61, 385–397.
Contents lists available at ScienceDirect
Journal of Atmospheric and Solar-Terrestrial Physics journal homepage: www.elsevier.com/locate/jastp
Field-aligned plasma diffusive fluxes in the topside ionosphere from radio occultation measurements by CHAMP Guang-ming Chen a,b, Jiyao Xu a, Wenbin Wang c,, Jiuhou Lei d, Yue Deng c,1 a
State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100190, China Graduate University of Chinese Academy of Sciences, Beijing 100049, China c High Altitude Observatory, National Center for Atmospheric Research, Boulder, CO 80307, USA d Department of Aerospace Engineering Sciences, University of Colorado, Boulder, CO 80309, USA b
a r t i c l e in fo
abstract
Article history: Received 12 June 2008 Received in revised form 27 February 2009 Accepted 25 March 2009 Available online 7 April 2009
O+ field-aligned diffusive velocities and fluxes in the topside ionosphere have been calculated from electron density profiles retrieved from CHAMP radio occultation (RO) measurements. The velocities and fluxes from January 2002 to December 2003 at low- and mid-latitudes have been statistically analyzed. The results show that daytime diffusive fluxes changed gradually from downward to upward as altitude increases. The largest values of the upward diffusive fluxes and velocities occurred at around 7251 geomagnetic latitude. During solstices the plasma fluxes in the winter hemisphere were larger than those in the summer hemisphere. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Topside ionosphere Diffusive flux Radio occultation Ionospheric dynamics
1. Introduction Field-aligned plasma flows between the topside ionosphere and the plasmasphere are of significant importance for these two regions. In the daytime, the plasma in the topside ionosphere diffuses upwards along the magnetic field lines into the plasmasphere, whereas at night downward plasma flows from the plasmasphere help to maintain the nighttime F2 layer (Rishbeth and Garriott, 1969). There have been some quantitative studies of plasma flow between these two regions. Evans (1971a, b, c, 1975) studied the O+ vertical flux using Millstone Hill incoherent scatter radar (ISR) observations during quiet periods. These studies showed a large upward flux at sunrise and a small downward flux at sunset. In the daytime, the flux was downward at altitudes around the F2 layer peak, but upward at altitudes above it. At night, the O+ vertical flux was usually downward with a magnitude of 3 107 cm2 s1 above 700 km. Ho and Moorcroft (1975) investigated daytime fluxes from the ISR at Arecibo in October 1967, which were an order of magnitude larger than those at Millstone Hill obtained by Evans (1975). Vickrey et al. (1979) also used the Arecibo ISR data to study these fluxes. They suggested that there was an inter-
Corresponding author.
E-mail address: [email protected] (W. Wang). Now at CIRES/University of Colorado and NOAA Space Weather Prediction Center, Boulder, CO 80305, USA. 1
1364-6826/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jastp.2009.03.027
hemispheric transport from the summer hemisphere to the winter hemisphere. They also found that O+ and H+ fluxes were sometimes oppositely directed. Greenspan et al. (1994) analyzed the field-aligned ion velocities from the F8 satellite of the Defense Meteorological Satellite Program (DMSP) at 840 km near the dawn and dusk meridional plane under low solar activity conditions. They found that there were field-aligned upward flows during the day and downward flows at night. Since direct measurements of plasma fluxes are very limited, these fluxes have been usually calculated by combining observed parameters and models. For instance, Buonsanto and Titheridge (1987) calculated the fluxes of ionization at 850 km height using neutral parameters from the mass spectrometer-incoherent scatter 83 (MSIS83) model (Hedin, 1983) and the F2 peak electron densities, NmF2, from ionosonde observations. Skoblin (1996) obtained ion fluxes at 1000 km altitude using ground-based ionosonde observations of electron density, satellite in situ measurements of electron concentration, temperature and mean ion mass, as well as a simple theoretical ionospheric model. These previous studies, however, were mainly focused on total ion velocities and fluxes that included the effect of neutral winds and electric fields. Investigations of ambipolar diffusive velocities and fluxes which cannot be observed directly have been very limited. The global distribution of plasma diffusive fluxes is not only important to the study of the dynamical and mass coupling between the ionosphere and plasmasphere/inner magnetosphere, but it also helps to elucidate the relative contributions of each physical process, including neutral wind, electric field, chemistry
ARTICLE IN PRESS 968
G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
and ambipolar diffusion, to variations of the global ionosphere during both storm and quite times. Topside diffusive fluxes are also one of the primary top boundary conditions for many global thermosphere/ionosphere models, such as the NCAR-TIEGCM and TING model (e.g., Wang et al., 1999). These models can selfconsistently calculate neutral winds and ionospheric electric fields, but they require time-dependent, global distributions of the plasma diffusive fluxes at their top boundaries to accurately solve the ion/plasma continuity equations. Although some of the flux tube models, e.g., SUPIM and SAMI2 (e.g., Bailey and Balan, 1996; Huba et al., 2000) do not require this boundary condition, the derived diffusive velocity and flux are still vary valuable in that they can be used to validate the outputs of these models. Lockwood (1983) calculated O+ diffusive fluxes from the Alouette 1 satellite observations between 1962 and 1968, and analyzed the diurnal variations of O+ diffusive fluxes from the F2 peak to 700 km altitude at mid-latitudes near sunspot minimum. Lockwood and Titheridge (1982) reported latitudinal variations of Fd/Fl, where Fd was O+ diffusive flux and Fl was the limiting value set by frictional drag. Nevertheless, altitudinal, seasonal and global distributions of Fd are still an open question, especially under high solar activity conditions. In this paper, we will use the new capability of the Global Positioning System (GPS) radio occultation (RO) measurements of the ionosphere, which provides global ionosphere electron density profiles, to obtain global distributions of the plasma diffusive fluxes and their variations with geophysical conditions. The GPS occultation technique for ionospheric sounding (e.g., Hajj and Romans, 1998; Jakowski et al., 2004; Lei et al., 2007) has been recently used to examine the global distribution of the ionospheric electron densities. This paper presents a method to calculate O+ ion field-aligned diffusive velocities and fluxes in the topside ionosphere from electron density profiles retrieved from ionospheric radio occultation measurements made by the German CHAllenging Minisatellite Payload (CHAMP) satellite. A statistical study of the diffusive velocities and fluxes in the low- and mid-latitudes is also carried out to investigate their altitude, latitudinal, seasonal and local time variations under geomagnetically quiet and solar maximum conditions.
2. Data The CHAMP satellite was successfully launched on 15 July 2000 into a near-polar, circular orbit at 454 km altitude (Reigber et al., 2002). It provided its first ionospheric radio occultation measurements on 11 April 2001. Since then, more than 100,000 ionospheric electron density profiles have been retrieved from these CHAMP radio occultaion data at the University Corporation for Atmospheric Research (UCAR) COSMIC Data Analysis and Archival Center (CDAAC) (http://cosmic-io.cosmic.ucar.edu/cdaac). The CHAMP radio occultation measurements provide eletron density profiles from the ionospheric E region to the topside ionosphere at around 420 km, where the satellite orbits, with an altitude resolution of about 1 km. The root mean square (RMS) difference of electron density was about 103 cm3 above 150 km for the horizontally stratified ionosphere (Schreiner et al., 2007). In addition, CHAMP uses a planar Langmuir probe (PLP) to perform in situ measurements of ionospheric electron density and temperature every 15 s. The accuracy of the PLP measurements is within 10% (Liu et al., 2007). A Chapman-a function (e.g., Rishbeth and Garriott, 1969) is applied to fit and smooth electron density profiles. The Chapman scale height is assumed to be linearly dependent on altitude (Lei et al., 2005, 2006). To ensure the validity of the electron density data (DRO) retrieved from radio occultation measurements, we compare DRO with electron densities obtained from the in situ measurements (DL) by the PLP at the same location. If 2/3oDRO/ DLo3/2, the data of RO are considered to be reasonable and reliable, otherwise, the RO retrieved electron density profiles are removed from the data set. About 10% of the profiles are removed. The measurements made during periods of KpX4 are also excluded. Finally, a total of 15,000 profiles between 651S and 651N geomagnetic latitude from January 2002 to December 2003 have been used in our statistical analysis, which is described below. Data distribution at an altitude of 50 km above the F2 peak height (F2+50 km) is shown in Fig. 1. In this paper, we use heights above the F2 peak, instead of constant heights for vertical coordinates. It has been shown that, at mid-latitudes, the F2
Geomagnetic latitude
March Equinox
September Equinox
60
70
60
70
40
60
40
60
50
20
40
0
20 0
30
-20
20
-40
10
-60
40 30
-20
20
-40
10
-60
1 3 5 7 9 11 13 15 17 19 21 23
1 3 5 7 9 11 13 15 17 19 21 23
June Solstice Geomagnetic latitude
50
December Solstice
60
70 60
70
40
60 40 50 20
60
40
40
20 0
30
-20
20
-40
10
-60 1 3 5 7 9 11 13 15 17 19 21 23 Local time
50
0
30
-20
20
-40
10
-60 1 3 5 7 9 11 13 15 17 19 21 23 Local time
Fig. 1. Data distribution as a function of geomagnetic latitude and local time in different seasons at F2+50 km.
ARTICLE IN PRESS G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
peak at a given local time tends to lie at a fixed atmospheric pressure level regardless of the latitude, season and solar activity (Rishbeth and Edwards, 1990). This may also be true at low latitudes (Rishbeth and Edwards, 1989). Furthermore, in global thermospheric/ionospheric models, a pressure grid is usually used in the vertical direction instead of an altitude grid (e.g., Wang et al., 1999). In addition, the use of heights above the F2 peak reduces any complication that the movement of the F2 peak may cause in the interpretation of data. As shown in the Appendix A (Figs. A1 and A2), there were significant seasonal, latitudinal and local time variations in the observed F2 peak heights. Fig. 1 shows all observations made at an altitude of F2+50 km and within 90 days of either the solstices or equinoxes in 2-h local time and 101 geomagnetic latitude bins. Daytime is defined from 0600 to 1800 local solar time (LT), and nighttime from 1800 to 0600 LT. As shown in Fig. 1, there are sufficient data in most of the local time and magnetic latitude bins, especially in the daytime, to carry out a statistical analysis and to obtain statistically reliable results. Nevertheless, there are still some bins that do not have enough data points, such as those around the geomagnetic equator in the daytime. We are currently working on the new dataset from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) satellites, which have six satellites which are capable of obtaining more ionospheric radio occultation data and, thus, which have a better global coverage. The COSMIC measurements are for solar minimum conditions, whereas the CHAMP data we used for this study are for high solar activity conditions. In a follow-up study, we will compare diffusive fluxes from CHAMP with those from the COSMIC to see how these fluxes vary with solar activity.
3. Method In this study, the plasma equation of motion is used to calculate the plasma ambipolar diffusive velocity. On the assumption that the field-aligned electric field in the ionosphere is zero, the ion-inertial effects can be neglected, and the plasma diffuses along the magnetic field line, the diffusive velocity for O+ ions is given by k sin I ðT i þ T e Þ @ni @ðT i þ T e Þ mi g Vd ¼ þ þ (1) @h mi uin ni @h k and the O+ diffusive flux Fd is obtained by
Fd ¼ ni V d
969
Fig. 2. Ratio between CHAMP observed electron temperatures and IRI2007 predictions.
temperatures observed by CHAMP and those from IRI 2007 under geomagnetically quiet conditions at all local times from 2002 to 2003 are shown in Fig. 2. Electron temperatures from IRI 2007 were smaller than those observed in the equatorial regions, and there were also differences in other latitudes. It appears that there is a latitudinal change in the temperature ratio. It is unclear whether this bias is caused by IRI, CHAMP or both. However, the largest relative difference was only about 10%. Given that there have been no simultaneous global observations of Tn, Te and Ti height profiles, we use Tn, Te and Ti height profiles from the IRI model and scale the model electron temperatures using the CHAMP PLP in situ electron temperatures. Note that the change of the diffusive velocity can be about 5 ms1 at F2+100 km, when Ti+Te is changed by 10%. Therefore, this procedure would reduce the uncertainty induced by model electron temperature profiles and do not significantly affect the morphology and statistics of our results. The diffusive velocities and fluxes are averaged within 91-day periods centered on solstices or equinoxes. Parameters in different seasons are denoted as March equinox, September equinox, June solstice and December solstice, respectively. The regions from 251 to 651 and from 251 to 651 in geomagnetic latitudes are defined as mid-latitude regions.
(2)
where Vd is the plasma ambipolar diffusive velocity and ni the density of O+ ions. In the F2 region below about 600 km, most of the ions are O+ ions, thus ni equals the electron density, which is obtained from the radio occultation measurement; Ti and Te are the temperatures of O+ ions and electrons, respectively; mi the mass of O+ ions; I the geomagnetic dip angle; g the acceleration of gravity; k the Boltzmann constant; h the vertical height; uin, which is the momentum transfer collision frequency for O+ ions moving through the neutrals, is given by Pesnell et al. (1993). Neutral densities, n(O), n(O2) and n(N2), that are used in the calculation of uin, are obtained from the NRLMSISE00 model (Picone et al., 2002). The geomagnetic dip angle I is from IGRF-10 (Maus et al., 2005). The signs of diffusive fluxes and velocities are defined as positive if the O+ ions diffuse upwards along the magnetic field line, and negative if downward. Tn, Ti and Te are from the International Reference Ionosphere 2007 (IRI2007) model (Bilitza and Reinish, 2008). Furthermore, the values of Te from IRI2007 are scaled using the in situ electron temperatures from the PLP instrument onboard the CHAMP satellite. The latitudinal variation of the ratios between electron
4. Result and discussion In this section, we will carry out statistical analyses of the altitudinal, diurnal and latitudinal variations of the diffusive velocity and flux above the F2 peak height hmF2. 4.1. Altitude variations of the diffusive velocities and fluxes The altitude variations of the diffusive velocities and fluxes in the daytime around March equinox at low- and mid-latitudes are shown in Fig. 3. The averaged diffusive velocities are mainly between 15 and 0 ms1 at the F2 peak, and between 0 and 25 ms1 at F2+100 km and directed upward for latitudes from 101S to 651N. The diffusive velocities are comparable to the fieldaligned neutral wind speeds, the mean values of which are less than 40 ms1 (Hedin et al., 1988). In this paper, we are interested in the O+ ion diffusive velocities. The actual O+ velocities along the magnetic field lines in the topside ionosphere are the sum of diffusive velocities and the field-aligned component of the neutral winds, as neither diffusive velocities nor neutral winds can be
ARTICLE IN PRESS G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
h-hmF2 (km)
970
100
100
80
80 10°S-10°N 10°N-30°N 30°N-50°N 50°N-65°N
60 40
60 40
20
20
0
0 -15 -10
-5
0
5 10 Vd (m/s)
15
20
10°S-10°N 10°N-30°N 30°N-50°N 50°N-65°N
25
-10
-15
-5 0 5 Flux (108 cm-2s-1)
10
15
Vd (m/s)
Fig. 3. Altitude profiles of diffusive velocities (left panel) and fluxes (right panel) in the daytime near the March equinox at different latitudes.
March Equinox September Equinox
20 15 10 5 0 -5 -10 -15 -20 -25 -30 0
4
8
12 16 Local time
Summer Hemisphere Winter Hemisphere
20 15 10 5 0 -5 -10 -15 -20 -25 -30 20
24
0
4
8
12 16 Local time
20
24
Fig. 4. Local time variations of diffusive velocities (positive upward) for different seasons. The vertical lines are error bars.
neglected. The relative importance of the Vd and neutral wind effects in determining the total O+ velocities depends on the height of the observations. Similar characteristics of Vd and fluxes in different latitude regions, except around the magnetic equator (black lines), can be seen. Near the F2 peak, Vd is negative and the flows are downward. At about F2+40 km, a transition from downward flows to upward flows occurs, which is completed at about F2+70 km. At altitudes above the transition height, it appears that Vd increases much faster with altitude than it does below it. This is probably due to the exponential decrease of the neutral atmospheric densities, which causes the diffusion coefficients and diffusive velocities to increase with altitude. The altitudinal dependences of the diffusive fluxes are consistent with those from a numerical study by Balan and Bailey (1996). These altitude variations of the plasma diffusive velocities and fluxes have also been observed at Millstone Hill by Evans (1971c), Evans and Holt (1978). In Evans’ studies, however, the transition altitudes happened between 450 and 600 km. These are different from the transition altitudes obtained here which are lower than 400 km for the same latitude (see Fig. A2). The difference may be caused by the contributions from the neutral winds and electric field drift that were included in Evans’ results but not in ours. Other factors, such as solar activity conditions and electron temperature profiles, may also contribute to this difference. It is also interesting to note that diffusive velocities and fluxes around the magnetic equator (black lines: 101S–101N) are small and have different vertical variations from those in other latitude bands because of the small magnetic dip angles around the magnetic equator. The diffusive flux in the equatorial anomaly region (red line: 101N–301N) has a much larger absolute/downward value than in other regions between the F2 peak and F2+30 km, although diffusive velocities are smaller. This is caused by the higher electron densities that occur in the equatorial anomaly region.
4.2. Diurnal variations of the diffusive velocities and fluxes Diurnal variations of the diffusive velocities and fluxes at F2+80 km altitude at mid-latitudes in different seasons are shown in Figs. 4 and 5, respectively. The summer hemisphere data include those from the northern hemisphere around the June solstice and the southern hemisphere around the December solstice. The winter hemisphere data are those from the northern hemisphere around the December solstice and the southern hemisphere around the June solstice, respectively. In the midlatitude regions diffusive velocities are upward in the daytime and downward at night. Vd often shows large negative values from 0200 to 0500 LT before sunrise in all seasons, and then changes rapidly to positive values after sunrise. The diffusive fluxes have maximum upward values around 1300 LT and then decrease gradually with local time, and turn to downward around sunset, as shown in Fig. 5. There are differences in the local time variations between Vd and flux. This is caused by local time variations of the electron densities. Lockwood (1983) reported the diurnal variations of diffusive fluxes, and found that the local time at which the ionosphere had maximum upward diffusive fluxes changed with seasons. The largest upward fluxes occurred following sunrise at equinox and in the afternoon during solstices. However, in our results, the largest upward fluxes take place around 1300 LT for both equinoxes and solstices. Theses differences may be related to the fact that Lockwood’s results represented conditions from the F2 peak to 700 km near sunspot minimum, whereas ours are for F2+80 km and for high solar activity. 4.3. Latitudinal variations of the diffusive velocities and fluxes Latitudinal variations of the diffusive velocities and fluxes in the daytime (0600–1800 LT) at three altitudes for different
ARTICLE IN PRESS
Flux (108cm-2s-1)
G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
14 12 10 8 6 4 2 0 -2 -4 -6
14 12 10 8 6 4 2 0 -2 -4 -6 -8
March Equinox September Equinox
4
0
8
12 16 Local time
20
24
971
Summer Hemisphere Winter Hemisphere
4
0
8
12 16 Local time
20
24
Fig. 5. Same as Fig. 4, but for diffusive fluxes.
March Equinox 25
Vd (m/s)
20
20 15
10
10
5
5
0
0
-5
-5
-10
-10 -40
-20
0
20
40
hmF2+50km hmF2+70km hmF2+90km
25
15
-60
60
-60
-40
hmF2+50km hmF2+70km hmF2+90km
20
25 15
10
10
5
5
0
0
-5
-5
-10
-10 -40
-20 0 20 M latitude
0
20
40
60
40
60
40
60
hmF2+50km hmF2+70km hmF2+90km
20
15
-60
-20
December Solstice
June Solstice
25
Vd (m/s)
September Equinox hmF2+50km hmF2+70km hmF2+90km
-60
-40
-20 0 20 M latitude
Fig. 6. Latitudinal variations of the diffusive velocities in different seasons.
seasons are shown in Figs. 6 and 7. Around the March equinox, both Vd and Fd are roughly symmetric around the magnetic equator. Around the September equinox, diffusive velocities have larger values in the southern hemisphere than they do in the northern hemisphere at F2+90 km, but at lower altitudes, diffusive velocities in the northern hemisphere are relatively larger. On the other hand, the largest diffusive fluxes in the northern hemisphere are larger than those in the southern hemisphere near the September equinox. During the solstices, both the diffusive fluxes and velocities in the winter hemisphere are consistently larger than those in the summer hemisphere. The summer hemisphere has a larger scale height than the winter hemisphere does, leading to smaller vertical gradients of electron density profiles in summer, and consequently smaller diffusive velocities. Lockwood and Titheridge (1982) reported the latitudinal variation of Fd/FL from hmF2 to 700 km between 1962 and 1968 over all local times. Their results indicated that the largest values of Fd/FL at the September equinox were also greater in the northern hemisphere than they were in the southern hemisphere. However, their values during solstices were larger in the summer
hemisphere than in the winter hemisphere at mid-latitudes. The difference may be caused by different solar conditions. The hemispheric asymmetric distribution of these fluxes and velocities during the solstices as well as at the September equinox is an interesting feature, and requires further investigations. The largest diffusive fluxes appear in the equatorial anomaly regions around 7251 geomagnetic latitudes in both hemispheres. The location of the maximum diffusive fluxes differs from that of the diffusive velocities, with the maximum displacement occurring in the equinoxes. This displacement is caused by the different latitudinal distribution of diffusive velocities compared with that of the electron densities. Flux depends both on the density of the flow and the velocity. The maxima will occur as a result of the convolution of these two effects rather than at the maxima of either individual effect. The diffusive fluxes around the geomagnetic equator (o751) are small, since the geomagnetic dip angle is small there. The diffusive fluxes at sub-polar latitudes are also smaller than those at other mid-latitude locations, because of smaller electron density and Vd at sub-polar latitudes. Lockwood and Titheridge (1982) found that the largest values of Fd/FL, which were normalized to 1000 km, appeared at less
ARTICLE IN PRESS 972
G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
March Equinox
Flux (109cm-2s-1)
1.5 1.0
0.5
0.0
0.0
-0.5
-0.5
1.5
-40
-20 0 20 June Solstice
40
60
-60
0.5
0.0
0.0
-0.5
-0.5 -40
-20 0 20 M latitude
40
60
September Equinox
-20 0 20 40 December Solstice
60
hmF2+50km hmF2+70km hmF2+90km
1.0
0.5
-60
-40
1.5
hmF2+50km hmF2+70km hmF2+90km
1.0
hmF2+50km hmF2+70km hmF2+90km
1.0
0.5
-60
Flux (109cm-2s-1)
1.5
hmF2+50km hmF2+70km hmF2+90km
-60
-40
-20 0 20 M latitude
40
60
Fig. 7. Same as Fig. 6, but for diffusive fluxes.
than 7251 geomagnetic latitudes. If the diffusive fluxes in this paper are projected to 1000 km along magnetic field lines, the largest diffusive fluxes will appear at less than 7201 geomagnetic latitude, which are close to the peak locations found in the study by Lockwood and Titheridge (1982). It should be pointed out, however, that the absolute values of our diffusive fluxes are different from Lockwood and Titheridge’s. Although their results represented conditions over a wide altitude range (from F2 peak to 700 km) and in low solar activity and ours are for a particular altitude and high solar activity conditions, the largest flux values for both cases occurred near the equatorial anomaly regions. The causes of this agreement are complicated, since F2 peak heights change with latitude. From Eq. (1), we can see that Vd is related to the vertical scale height (ni@h/@ni) of the electron density profiles. Liu et al. (2008) showed the scale height has a maximum value near the equatorial anomaly regions. The latitudinal variations of the vertical scale height of electron density in the equatorial anomaly regions should be an important factor in determining the peak locations. Furthermore, the maximum electron densities occurring in the equatorial anomaly regions should contribute significantly to the diffusive flux maximum occurring there. Chao et al. (2004) showed the global distributions of total field-aligned ion velocities at lower latitudes observed by the ROCSAT satellite. Their results showed that there was an interhemispheric plasma transport from the summer hemisphere to the winter hemisphere. Similar results were obtained using a numerical model by Bailey et al. (1975). Their ion velocities, however, represented total ion drift velocities that included the neutral wind effect and other processes. Our results are diffusive velocities and fluxes that result only from ambipolar diffusion.
5. Conclusions In this paper, a method of calculating O+ ion field-aligned diffusive velocities and fluxes in the topside ionosphere using the CHAMP radio occultation measurements and empirical models is described. Statistical analysis is carried out to study the altitudinal, local time, seasonal and latitudinal variations of these velocities and fluxes from January 2002 to December 2003. The
altitudinal, seasonal and latitudinal variations of diffusive velocities and fluxes are reported for the first time. Our conclusions are: (1) At mid-latitudes, diffusive velocity Vd is downward near the F2 peak and the corresponding diffusive fluxes are also downward. At altitudes between F2+40 km and F2+70 km, there is a transition from a downward to an upward direction in both the velocities and fluxes. At altitudes above the transition height, Vd increases with height much faster than it does below the transition height. (2) At F2+80 km in the mid-latitude regions, the diffusive velocities and fluxes are upward in the daytime and downward at night. The maximum values of the daytime upward fluxes occur around 1300 LT. (3) There are significant latitudinal variations in the diffusive fluxes and velocities. Both diffusive fluxes and velocities are very small near the geomagnetic equator, and have maximum values around 7251 geomagnetic latitudes. (4) Near the March equinox, diffusive fluxes and velocities show roughly hemispheric symmetry around the geomagnetic equator. Near the September equinox, however, there is a hemispheric asymmetry in both the diffusive velocities and fluxes. During solstices, the diffusive fluxes in the winter hemisphere are larger than those in the summer hemisphere.
Acknowledgements This work is supported by the National Natural Science Foundation of China (40621003, 40674088, 40523006) and the National Basic Research Program (2006CB806306). The authors are grateful to the University Corporation for Atmospheric Research for providing electron density profiles and to GeoForschungsZentrum (GFZ) Informations Systems and Data Center (ISDC) for providing the data of PLP soundings from CHAMP. We are also grateful to National Space Science Data Center (NSSDC) for providing the models of IRI2007, NRLMSISE00 and IGRF-10. This work is also supported in part by the Center for Integrated Space Weather Modeling (CISM) which is funded by the STC
ARTICLE IN PRESS G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
330 320
hmF2 (km)
330
March Equinox September Equinox
310
300
300
290
290
280
280
270
270
260
260
250
250
240
0
4
8
12
Summer Hemisphere Winter Hemisphere
320
310
16
20
24
240
973
0
4
8
Local time
12
16
20
24
Local time
Fig. A1. Local time variation of hmF2 at mid-latitudes for different seasons.
hmF2 (km)
360 340
340
320
320
300
300
280
280
260
260
240
240 -60
-40
-20
June Solstice December Solstice
360
March Equinox September Equinox
0
20
40
60
M latitude
-60
-40
-20
0 20 M latitude
40
60
Fig. A2. Geomagnetic latitudinal variations of hmF2 in the daytime for different seasons.
program under agreement number ATM-0120950. The National Center for Atmospheric Research is sponsored by the National Science Foundation. Yue Deng’s effort was supported by the NSF through grants ATM-0823689 and the University of Colorado through CIRES fellowship. We thank Dr. Alan Burns for very helpful comments.
Appendix A The diurnal variations of hmF2 in the mid-latitude regions and different seasons are shown in Fig. A1. hmF2 varies significantly with local time, being higher at night and lower in the daytime, especially in the morning. The latitudinal variations of hmF2 in the daytime are shown in Fig. A2. In the daytime, hmF2 in the equatorial regions is higher than that in the other latitude regions and the hmF2 in the summer hemisphere is higher than that in the winter hemisphere. References Bailey, G.J., Moffett, R.J., Swartz, W.E., 1975. Effects of photoelectron heating and interhemisphere transport on day-time plasma temperatures at low latitudes. Planetary and Space Science 23, 599–610. Bailey, G.J., Balan, N., 1996. A low-latitude ionosphere–plasmasphere model. In: Schunk, R.W. (Ed.), STEP Hand Book. pp. 173–206. Balan, N., Bailey, G.J., 1996. Modeling studies of equatorial plasma fountain and equatorial anomaly. Advances in Space Research 18 (3), 107–116. Bilitza, D., Reinish, B.W., 2008. International reference ionosphere 2007: improvement and new parameters. Advances in Space Research 42 (4), 599–609.
Buonsanto, M.J., Titheridge, J.E., 1987. Diurnal variations in the flux of ionosation above the F2 peak in the northern and southern hemisphere. Journal of Atmospheric and Solar-Terrestrial Physics 49, 1093–1105. Chao, C.K., Su, S.-Y., Yeh, H.C., 2004. Ion temperature crests and troughs in the morning sector of the low-latitude and midlatitude topside ionosphere. Journal of Geophysical Research 109, A11303. Evans, J.V., 1971a. Observations of F-region vertical velocities at Millstone Hill, 3, determination of altitude distribution of H+. Radio Science 6, 855–861. Evans, J.V., 1971b. Observations of F region vertical velocities at Millstone Hill, 1, evidence for drifts due to expansion, contraction, and winds. Radio Science 6, 609–626. Evans, J.V., 1971c. Observations of F region vertical velocities at Millstone Hill, 2, evidence for fluxes into and out of the protonsphere. Radio Science 6, 843–854. Evans, J.V., 1975. A study of F2 region daytime vertical ionization fluxes at Millstone Hill during 1969. Planetary and Space Science 23, 1461–1482. Evans, J.V., Holt, J.M., 1978. Night proton fluxes at Millstone Hill. Planetary and Space Science 26, 727–744. Greenspan, M.E., Burke, W.J., Richs, F.J., Hughes, W.J., Heelis, R.A., 1994. DMSP F8 observations of the mid-latitude and low-latitude topside ionosphere near solar minimum. Journal of Geophysical Research 99 (A3), 3817–3826. Hajj, G.A., Romans, L.J., 1998. Ionospheric electron density profiles obtained with the global positioning system: results from the GPS/MET experiment. Radio Science 33, 175–190. Hedin, A.E., 1983. A revised thermospheric model based on mass spectrometer and incoherent scatter data-MSIS-83. Journal of Geophysical Research 88, 10170–10188. Hedin, A.E., Spencer, N.W., Killeen, T.L., 1988. Empirical global model of upper thermosphere winds based on atmosphere and dynamics explorer satellite data. Journal of Geophysical Research 93 (A9), 9959–9978. Ho, M.C., Moorcroft, D.R., 1975. Composition and temperatures of the topside ionosphere over Arecibo. Planetary and Space Science 23, 315–322. Huba, J.D., Joyce, G., Fedder, J.A., 2000. The formation of an electron hole in the topside equatorial ionosphere. Geophysical Research Letters 27, 181–184. Jakowski, N., Heise, S., Wehrenpfennig, A., Tsybulya, K., 2004. Ionospheric radio occultation measurements and space weather. In: Kirchengast, G., Foelsche, U., Steiner, A. (Eds.), Occultations for Probing Atmosphere and Climate. Springer, Berlin, pp. 441–446.
ARTICLE IN PRESS 974
G.-m. Chen et al. / Journal of Atmospheric and Solar-Terrestrial Physics 71 (2009) 967–974
Lei, J., Liu, L., Wan, W., Zhang, S.-R., 2005. Variations of electron density based on long-term incoherent scatter radar and ionosonde measurements over Millstone Hill. Radio Science 40, RS2008. Lei, J., Liu, L., Wan, W., Zhang, S.-R., van Eyken, A.P., 2006. Comparison of the first long-duration IS experiment measurements over Millstone Hill and EISCAT Svalbard radar with IRI2001. Advances in Space Research 37, 1102–1107. Lei, J., Syndergaard, S., Burns, A.G., Solomon, S.C., Wang, W., Zeng, Z., Roble, R.G., Wu, Q., Kuo, Y.-H., Holt, J.M., Zhang, S.-R., Hysell, D.L., Rodrigues, F.S., Lin, C.H., 2007. Comparison of COSMIC ionospheric measurements with ground-based observations and model predictions: preliminary results. Journal of Geophysical Research 112, A07308. Liu, H., Stolle, C., Watanabe, S., 2007. Evaluation of the IRI model using CHAMP observations in polar and equatorial regions. Advances in Space Research 39, 904–909. Liu, L., He, M., Wan, W., Zhang, M.-L., 2008. Topside ionospheric scale heights retrieved from constellation observing system for meteorology, ionosphere, and climate radio occultation measurements. Journal of Geophysical Research 113, A10304. Lockwood, M., Titheridge, J.E., 1982. Departures from diffusive equilibrium in the topside F-layer from satellite soundings. Journal of Atmospheric and SolarTerrestrial Physics 44, 425–440. Lockwood, M., 1983. Field-aligned plasma flow in the quiet, mid-latitude ionosphere deduce from topside soundings. Journal of Atmospheric and Solar-Terrestrial Physics 45, 1–14. Maus, S., Macmillan, S., Chernova, T., 2005. The 10th generation international geomagnetic reference field. Physics of the Earth and Planetary Interiors 151, 320–322.
Pesnell, W.D., Omidvar, K., Hoegy, W.R., 1993. Momentum transfer collision frequency of O+-O. Geophysical Research Letters 20, 1343–1346. Picone, J.M., Hedin, A.E., Drob, D.P., Aikin, A.C., 2002. NRLMSISE-00 empirical model of the atmosphere: statistical comparisons and scientific issues. Journal of Geophysical Research 107 (A12), 1468. Reigber, C., Lu¨hr, H., Schwintzer, P., 2002. First CHAMP Mission Results for Gravity. Magnetic and Atmospheric Studies. Springer, Berlin, Heidelberg, New York. Rishbeth, H., Garriott, O.K., 1969. Introduction to Ionospheric Physics. International Geophysical Series. Academic Press, New York. Rishbeth, H., Edwards, R., 1989. The isobaric F2-layer. Journal of Atmospheric and Terrestrial Physics 51, 321–338. Rishbeth, H., Edwards, R., 1990. Modeling the F2 layer peak height in terms of atmospheric pressure. Radio Science 25, 757–769. Schreiner, W., Rocken, C., Sokolovskiy, S., Syndergaard, S., Hunt, D., 2007. Estimates of the precision of GPS radio occultations from the COSMIC/FORMOSAT-3 mission. Geophysical Research Letter 34, L04808. Skoblin, M.G., 1996. Ionosphere-protonosphere thermal plasma fluxes estimated from Magion2 and ionosonde data in August-September 1990. Journal of Geophysical Research 101 (A12), 26971–26980. Vickrey, J.F., Swartz, W.E., Farley, D.T., 1979. Ion transport in the topside ionosphere at Arecibo. Journal of Geophysical Research 84, 7307–7314. Wang, W., Killeen, T.L., Burns, A.G., Roble, R.G., 1999. A high resolution, three-dimensional, time dependent, nested grid model of the coupled thermosphere-ionosphere. Journal of Atmospheric and Solar-Terrestrial Physics 61, 385–397.