Characteristics of F region dynamo currents deduced from CHAMP magnetic field measurements

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115,, doi: /2010ja015604, 2010 Characteristics of F region dynamo currents deduced from CHAMP magnetic field measurements Jaeheung Park, 1,2 Hermann Lühr, 1 and Kyoung Wook Min 2 Received 27 April 2010; revised 28 May 2010; accepted 8 June 2010; published 2 October [1] Using magnetic field observations of the CHAMP satellite we provide the first comprehensive study of F region dynamo currents as a function of season, local time, geographic longitude, and solar activity. From bipolar variations of the zonal magnetic field component the density of vertical current driven by the F region dynamo is deduced. The current strength is smallest around June solstice, which is attributed to a reduced F region Pedersen conductance caused by a lower electron density and neutral density at that season. During the hours around noon highest current densities are observed. They are flowing downward over the dip equator. A secondary peak of upward currents appears at dusk. The polarity switch occurs between15 and 16 (local time) independent of season. The noontime F region dynamo current peaks at longitudes connected to the South Atlantic Anomaly, which can be explained by the enhanced conductivity in the region of reduced B field. The F region current at dusk exhibits no peak in the longitude sector of the South Atlantic Anomaly. At noon, the F region dynamo currents exhibit a wave 4 longitudinal structure during equinoxes and June solstice. The wave 4 signature becomes weak during December solstice. At dusk the wave 4 signature of F region dynamo currents is much reduced in all seasons. This behavior can be explained by the DE3 tidal signature in the zonal wind at CHAMP altitude. F region dynamo currents increase linearly with the solar flux index, F 10.7, during both noon and dusk time sectors. The increase in current strength with increasing F 10.7 is slightly higher at dusk than at noon. Citation: Park, J., H. Lühr, and K. W. Min (2010), Characteristics of F region dynamo currents deduced from CHAMP magnetic field measurements, J. Geophys. Res., 115,, doi: /2010ja Introduction [2] Several decades ago Rishbeth [1971a, 1971b] suggested a wind induced dynamo in the low latitude ionospheric F region. Zonal wind blowing across the geomagnetic field lines can drive ions perpendicular to the B field in the meridional plane while electrons remain tied to the B field. The charge separation sets up an electric field. The strength of this polarization E field is dependent on the ratio between the conductances in the F and E regions. It can be written according to Kelley [2009]: E ¼ P E; N P P F P þ P E; S P þ P u B; F P 1 Helmholtz Centre Potsdam, GFZ, German Research Center for Geosciences, Potsdam, Germany. 2 Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon, Korea. Copyright 2010 by the American Geophysical Union /10/2010JA ð1þ where S P E,N and S P E,S are E region height integrated Pedersen conductivities at the northern and southern ionospheric footprints, S P F is the flux tube integrated conductivity in the F region, u is the wind velocity, and B is the ambient magnetic field. The resulting electric field in the meridional plane is mapped along field lines to the E region and drives a toroidal current system. Schematic illustrations of the current configuration have been presented by Heelis [2004] or Lühr and Maus [2006]. For further details of the electrodynamics involved, readers are referred to a review by Rishbeth [1997]. [3] There are only a few observational papers which investigated the F region dynamo. Maeda et al. [1982] was the first to interpret magnetic signatures of the F region dynamo current. In Magsat (altitude: km) data, bipolar latitudinal variations of the geomagnetic D (eastward) component were found. Their properties were as follows: (1) they were commonly observed at dusk (note that Magsat had a sun synchronous orbit near the dawn dusk meridian) irrespective of geomagnetic activity, (2) magnetic deflections peak at ±8 dip latitude, (3) the magnitude depends on longitude and altitude, and (4) no corresponding ground signature has been found. Maeda et al. [1982] attributed the deflections to a meridional current system. A cartoon in their Figure 5 shows a meridional current system in each hemisphere which exactly mirrors the other. Takeda and Maeda [1983] argued that the zonal wind (u) in the equatorial F region induces vertical currents (u B) at the equator, which result in field aligned currents (FACs) and E region closure currents. Using Magsat data Olsen [1997] confirmed 1of12

2 the existence of upward vertical currents at the magnetic equator. He also showed that the F region dynamo variation has a lunar contribution and reported the existence of an interhemispheric current system; from winter to summer (summer to winter) hemisphere at dusk (dawn). Lühr and Maus [2006] also found the magnetic signatures of F region dynamo currents in CHAMP observations. Taking advantage of the satellite s precession they investigated the local time (LT) variation of the magnetic signatures. First, the magnetic signatures maximize with largest peak to peak amplitudes around noon and show a secondary peak at dusk. Second, the magnetic signatures at noon and dusk sectors have opposite polarities. The changeover occurs around 15:00 LT. This polarity switch was attributed to neutral wind forcing that changes direction from westward to eastward in the afternoon [Lühr and Maus, 2006]. [4] Though previous studies already reported certain aspects of the F region dynamo, there are still a number of open issues. First, Maeda et al. [1982] showed a longitude dependence of the F region dynamo current. It was based on a limited data set (18 days in December) and no explanation was given for the result. Second, Lühr and Maus [2006] found that the F region dynamo currents peak both at noon and at dusk. They averaged all the data irrespective of longitude, season, and years. In this paper we will give a climatological description of the F region dynamo currents as a function of seasonal longitudinal and solar cycle dependence at both LT maxima: noon and dusk. In section 2 the instruments and data processing scheme are described briefly. Climatological results are shown in section 3, which are discussed in detail in section 4. In the final section, we summarize all the results and draw conclusions. 2. Observation [5] The Challenging Minisatellite Payload (CHAMP) was launched on 15 July 2000 into a polar orbit at 450 km altitude. Its orbit precesses through all LT sectors in 131 days. Due to significant atmospheric drag the orbit altitude decreases slowly, and as of 2010 its altitude is below 300 km. CHAMP has a fluxgate magnetometer (FGM) on board to measure the geomagnetic field vectors, which is routinely cross calibrated by an on board Overhauser magnetometer. The measured vectors are transformed into the geographic coordinate system through the attitude information from two star cameras. CHAMP/FGM has been operating successfully for about 10 years, compiling a huge amount of high quality geomagnetic field data. [6] Pomme 6 is a geomagnetic field model derived from long term CHAMP observations. It is the most recent update of previous versions of the Potsdam Magnetic Model of the Earth 4 (POMME4; [Maus et al., 2006]). With the input of time, geographic location, solar activity, and geomagnetic conditions, the model returns a geomagnetic field vector considering the internal (Earth s core, crust, and mantle fields) and external (ring current, magnetopause, and magnetotail current) contributions. If we remove the model output (hereafter called mean field ) from the CHAMP/ FGM data, the remnant (hereafter called residual field ) reflects geomagnetic fields generated by ionospheric currents. This approach has been widely used in ionospheric studies (e.g., Lühr et al. [2004]; Ritter et al. [2004]; Stolle et al. [2006]; Park et al. [2010]) and will be used again in this work. The residual field is transformed into a mean field aligned (MFA) system defined by the Pomme 6 model. The z axis, parallel, is along the mean field; the y axis, zonal, is perpendicular to the magnetic meridian (positive eastward); and the x axis, meridional, completes the triad (positive outward to higher McIlwain L shells). Here we only use the y (zonal) component of the residual magnetic field to investigate meridional F region dynamo currents [Maeda et al., 1982; Lühr and Maus, 2006]. [7] Figure 1 shows an example of zonal residual fields around noon during equinoxes (March, April, September, and October). Residual fields within ±45 geomagnetic latitude (MLAT) are bin averaged within 24 geographic longitude (GLON), and subsequently the latitude dependence is linearly detrended. Throughout the paper we use the magnetic quasi dipolar coordinates at CHAMP altitude for MLAT, as defined by Richmond [1995]. In Figure 1a the resultant magnetic deflection (black solid lines) is shown for each of the longitude sectors. The magnetic field scale is chosen such that 2 in geographic longitude corresponds to 1 nt, as shown in Figure 1a. Using the discrete Fourier transform the detrended magnetic deflections are decomposed into cosine and sine terms: B y X4 i¼0 ½a i cosðiþþb i sinðiþš; ð2þ where b is geomagnetic latitude. Sums of the cosine and sine terms are shown in Figures 1b and 1c, respectively. To check the reliability of the decomposition, total sums of Figures 1b and 1c are overplotted in Figure 1a as red dashed lines. The cosine terms in Figure 1b represent the symmetric part of the signal with respect to the dip equator. It may be related to the interhemispheric field aligned current (IHFAC) system. We know that F region dynamo currents induce bipolar deflections in the latitudinal profile of the magnetic field, and the peaks are expected within ±10 MLAT [Maeda et al., 1982; Lühr and Maus, 2006]. The sine terms in Figure 1c thus contain the magnetic signatures of the vertical F region dynamo currents. From the peak to peak amplitude of the zonal field sine terms we can calculate the F region dynamo current strength. The latitudinally integrated sheet current density, J F, of the vertical F region current can be estimated from the peak to peak deflection DB y within ±15 MLAT: J F ¼ 1 0 DB y ; Positive currents are flowing downward. If there is no extremum within ±15 MLAT, zero current density is assigned. If there are multiple extrema within ±15 MLAT, most equatorward peaks are chosen as inputs to equation (3). Using this approach we will investigate on a statistical basis how the F region dynamo current strength depends on season, longitude, LT, and solar activity. 3. Results [8] We investigate the 4 year interval from 2001 to 2004 for statistical studies. This time period of the CHAMP ð3þ 2of12

3 Figure 1. Zonal magnetic deflections caused by ionospheric current. (a) Averaged zonal residual field (black solid line) near noon during equinoxes (March, April, September, and October), separately for 15 longitudinal sectors. (b) Cosine and (c) sine components of the field residuals up to the fourth harmonics calculated by discrete Fourier transform. The sum of Figures 1b and 1c is overplotted in Figure 1a as red dashed lines for consistency check. mission has been selected because of its vicinity to the solar maximum. If daily averaged K p indices exceed 4, the whole data on the day are neglected. We also excluded data points that exhibit zonal residual fields larger than 50 nt in order to avoid spurious observations. First, in Figure 2, we show the LT dependence of the sine terms (which relate to F region dynamo currents) for (a) equinoxes (March, April, September, and October), (b) June solstice (May August), and (c) December solstice (November February), which have been averaged over all longitudes. Average solar flux levels (F 10.7 indices) for the seasons are given in the heading of each panel. The fluxes do not vary much among the seasons. In general we can confirm the LT variation of Lühr and Maus [2006]. The deflection amplitudes within ±15 MLAT peak at noon and dusk. Around noon the deflection of the zonal B field is eastward (westward) in the northern (southern) hemisphere. Near dusk, the opposite trend can be seen. Magnetic deflections are largely confined between 08:00 LT and 22:00 LT. The switch in polarity occurs around 15:00 LT at all seasons. Overall, the LT variation of the F region dynamo related deflections are similar for all seasons. The amplitudes of the deflections, however, are smallest around June solstice. Some LT sectors, for example, at LT in June solstice, show peaks beyond ±15 MLAT. These do not seem to reflect F region dynamo currents. We do not discuss them further. [9] Figure 3 shows sheet current density of F region dynamo currents that are calculated from Figure 2 by equation (3). Downward currents at the equator are denoted positive. We can see that (1) amplitudes of positive (negative) currents maximize around noon (dusk), and (2) the maximum amplitude depends on the season. June solstice exhibits very low current density with little difference in magnitude between noon and dusk. During equinox and December solstice the noontime current density is generally higher than that of dusk. [10] In Figure 4 the averaged strength of F region dynamo currents around noon is presented as a function of GLON separately for (a) equinoxes, (b) June solstice, and (c) December solstice. Average F 10.7 values for the seasons are given in the heading of each panel. In Figure 1a we can identify a wave 4 longitudinal structure with maxima (minima) at 36, 132, 228, and 300 E (84, 180, 252, and 324 E) GLON. In Figure 1b there is a similar but weaker four peak longi- 3of12

4 Figure 2. Globally averaged LT dependence of the hemispherically antisymmetric zonal B field deflections for (a) equinoxes (March, April, September, and October), (b) June solstice (May August), and (c) December solstice (November February).Average solar flux levels (F 10.7 index) for each season are given in the heading of each panel. tudinal structure: instead of the peak at 228 E in Figure 1a two adjacent peaks appear at 204 and 252 E. Figure 1c shows no indication of a wave 4 signature. An outstanding feature at all seasons is the high current density around 300 E GLON. This longitude sector coincides well with the region of weakest geomagnetic field at the dip equator (e.g., see Lühr et al. [2004], Figure 11) related to the South Atlantic Anomaly (SAA). [11] After having investigated the characteristics of the F region dynamo around noon we turn now to the second activity maximum of the dynamo in the evening sector. Figure 5 has the same format as Figure 4 but shows dusktime results. As the F region dynamo currents have opposite directions between noon and dusk, most currents in Figure 5 are negative. Note that average F 10.7 values are in a reverse order to those in Figure 4: equinox < June < December. We can first see the absence of prominent peaks at the SAA in contrast to Figure 4. The panels show smaller longitudinal variations than in Figure 4. Generally speaking, the longitudinal distribution is rather flat. During June solstice currents from 120 to 216 E are generally large in amplitude (note the currents in this LT sector are mainly negative). Points with zero current signify that no peak of zonal B field was identified within ±15 MLAT, as stated in section 2. During December solstice currents are generally large in amplitude from 216 to 24 E. [12] Lühr and Maus [2006] have shown in their equation (6) that the vertical F region current strength is to first order proportional to the field line integrated Pedersen conductance in the F region during daytime. The F region conductance is dependent on the solar extreme ultraviolet (EUV) flux, which is commonly approximated by the F 10.7 index. In Figure 6a, the longitudinally averaged strength of the F region dynamo current around noon is shown as a function of solar F 10.7 index. We can see that the F region dynamo current density shows a high correlation with the solar activity. A similar relationship is found at dusk, as shown in Figure 6b. The linear regression reveals the quantitative relation between current density and solar flux for the two time sectors. At low solar fluxes the noontime 4of12

5 Figure 3. F region dynamo currents calculated from Figure 2. Downward currents at the equator are denoted positive. Average solar flux levels (F 10.7 index) for the seasons are given in the heading of each panel. Data points between 15:00 and 17:00 LT are omitted because their correspondences to F region dynamo currents are questionable. current densities are clearly larger than at dusk. However, this difference decreases toward larger solar fluxes and vanishes at F 10.7 = Discussion [13] In this study detailed investigations of the vertical currents in the F region driven by a zonal wind dynamo are presented. Five years of data from the CHAMP satellite cruising around 400 km altitude have been utilized. The large amount of samples (more than 56,000 equator crossings) allowed for achieving statistically significant results even when sorting the data according to many different aspects. We thus provide the most comprehensive picture of the F region dynamo so far Local Time and Seasonal Variation [14] The intensity and direction of the vertical F region current are deduced from the deflection of the zonal magnetic field component. This component is highly variable due to many other processes (e.g., pulsation and field aligned currents). It thus requires a large number of equator crossings for isolating the vertical current signature of a few nt reliably. The latitude profiles of the magnetic field shown in Figure 2 confirm that we observed consistent bipolar variations at the magnetic equator at least during the LT hours 10:00 to 22:00 for all seasons. The polarity and LT distribution of the signature are in agreement with the results of Lühr and Maus [2006]. At the time around the polarity switch, 14:00 17:00 LT, spurious variations sometimes appear that we do not consider in our analysis. [15] The zonal magnetic deflections have been interpreted in terms of vertical current density. In Figure 3 we see the LT variation. It is interesting to note that the diurnal variation is practically the same in all seasons. There is, however, a clear seasonal dependence of the current density. The lower current density during June solstice months is attributed to reduced electron density [Liu et al., 2007] and neutral density (e.g., Müller et al. [2009]) during this season. Both these quantities influence directly the F region conductance. This June depression of the F region dynamo has never been reported before. 5of12

6 Figure 4. F region dynamo current density around noon given as function of geographic longitude during (a) equinoxes, (b) June solstice, and (c) December solstice. Downward currents at the equator are denoted positive Longitudinal Variation of the Current Density [16] According to Lühr and Maus [2006] the vertical current density, J F, is given by J F ¼ 2 P E P F P 2 P E P þ P F ub; P where S P E (S P F ) is the field line integrated conductivity in the E (F) region, u is the zonal wind velocity across magnetic meridians, and B is the horizontal strength of the ambient geomagnetic field. All the quantities are known to show distinct longitudinal variations. Best known is the distribution of the B field. The field strength along the dip equator at CHAMP height can, for example, be read from Figure 11 of Lühr et al. [2004]. Largest values ( 40,000 nt) are encountered at 110 E GLON and smallest ( 26,000 nt) at 320 E GLON. At dip equator latitudes longitudinal structures of the zonal wind across geographic meridians were presented by Häusler et al. [2007]. This wind component, however, does not strictly correspond to u in equation (4). The distribution of ionospheric conductance is less well documented. ð4þ [17] When looking at the longitudinal variation of J F presented for the LT sectors around noon and dusk in Figures 4 and 5, respectively, we do not find a clear correlation of the current density with the B field strength. The effect of this parameter seems to be overridden by other influences. Interestingly, obvious current density peaks occur in the vicinity of SAA around 300 E GLON in the noon sector. Opposed to that no such enhancements are observed around dusk. [18] The prominent peak in F region dynamo currents at SAA longitudes is as expected from the theoretical work of Takeda [1996]. According to his paper a reduced B field at noon shifts the dynamo region upward and enhances the field aligned Pedersen conductivity. The effect is much larger than that of the reduced dynamo E field (u B), leading to a significant enhancement of the F region dynamo current. His Figure 3 (top and middle) shows that the effect at equatorial regions is strongest at noon and much reduced around dusk. Our observations in Figures 4 and 5 confirm the prediction of Takeda [1996] in F region dynamics. [19] During June solstice months we find negative current densities in some of the longitude bins around noon 6of12

7 Figure 5. Same format as shown in Figure 4 but for times around dusk. (cf. Figure 4). This polarity reversal is partly the reason for the low net current density presented in Figure 3 when averaging over all longitudes. The polarity reversals during this season seem to occur at longitudes where local eastward winds overtake the weak westward background wind. [20] In order to obtain a more quantitative impression of the longitudinal variation we performed a discrete Fourier transform. Results from the noon sector are shown in Figure 7. In case of equinox the amplitude spectrum is increasing up to the fourth harmonic and decreases thereafter. Particularly weak is the first harmonic. The amplitudes of this harmonic are larger for the solstice seasons. In the right column of Figures 7b and 7c the peak current densities of this harmonic are separated by 245 E in longitude (June: 0 E GLON; December: 245 E GLON). Häusler et al. [2007] reported that westward winds at 09:00 12:00 LT are enhanced at longitudes of about ( ) E GLON during June (December) solstice months for high solar activity (see their Figure 2). During equinox months such a large scale longitudinal feature of the wind is less conspicuous. The consistency between the two independent kinds of observations (zonal wind in Häusler et al. [2007] and F region dynamo current in this study) provides supporting evidence that the large scale zonal wind variation is responsible for the wave 1 longitudinal structure of current density around noon. The other outstanding spectral feature is the fourth harmonic, in particular during June and equinox months. This topic will be discussed in more details in the next section. [21] Spectral features of the dusktime current densities are shown in Figure 8. Still, the average current (zeroth harmonic) is smallest during June solstice. The fourth harmonic is small at all seasons. Quite prominent is the first harmonic. Note that overall spectra of zonal delta winds are also dominated by the first rather than the fourth harmonic around dusk [Häusler et al., 2007]. It again implies close relationship between the zonal wind and F region dynamo. An outstanding feature is the third harmonic during December solstice Wave 4 Longitudinal Structure [22] A prominent longitudinal variation of the noontime F region dynamo is the wave 4 structure (see Figures 4 and 7). This feature has been observed in many ionospheric and thermospheric quantities, for example, in F region plasma density (e.g., Immel et al. [2006]; Lin et al. [2007]), vertical plasma drift [Hartman and Heelis, 2007; Kil et al., 2007; Fejer et al., 2008], equatorial electrojet (e.g., England et al. 7of12

8 Figure 6. Dependence of F region dynamo current density around (a) noon and (b) dusk on solar flux index (F 10.7 ). Figure 7. (left) Spectrum amplitude and (right) phase of current density from Figure 4 for (a) equinoxes, (b) June solstice, and (c) December solstice. The phase reflects the longitude of the first peak for each harmonics. 8of12

9 Figure 8. Same format as shown in Figure 7 but for times around dusk. [2006]; Lühr et al. [2008]), zonal wind (e.g., Oberheide et al. [2006]; Häusler and Lühr [2009]), or thermospheric mass density [Liu et al., 2009]. Lühr et al. [2007] predicted that such a longitudinal structure should also exist in the F region dynamo current. Generally, the wave 4 signature in satellite observations is related to the eastward propagating nonmigrating diurnal tidal mode with wave 3, abbreviated DE3. This relation, however, is not unique (see Forbes et al. [2006]; Häusler and Lühr [2009]). It is known that the forcing of the DE3 tide is strongest around the months July September (see Forbes et al. [2003] and references therein). For that reason we have resampled the current density distribution over longitude for these three months for a dedicated study of the wave 4 signature. Figure 9 displays the longitudinal variation and its spectral characteristic. The spectrum shows a clear peak at the fourth harmonic. From the phase spectrum we deduce current density maxima at 38, 128, 218, and 308 E GLON. There is obviously a superposition of the SAA peak with a wave 4 maximum. Since the wave 4 signature does not persist in the current density over many LT hours, a self consistent determination of the related tidal mode (i.e., checking phase shifts with LT) is not possible. We thus try to find a plausible explanation by comparison with other observations. From equation (4) we know that the vertical current density depends on three quantities. The longitudinal structure of the horizontal magnetic field strength cannot explain the generated wave 4 signature. A viable candidate is the zonal wind variation. Häusler and Lühr [2009] have given a detailed overview of the tidal signals in the thermospheric wind at CHAMP altitude. We will have a look at the phase relation between the two quantities for assessing the connection. According to Table 2 of Häusler and Lühr [2009] the DE3 eastward wind crest passes during August the Greenwich meridian at 20:00 LT. This means the maximum westward wind has passed it 12 h earlier at 08:00 LT. Note that the eastward propagation speed of the wave 4 structure is 3.75 GLON per hour [Häusler and Lühr, 2009]. Our F region currents are averaged from 10:00 to 14:00 LT. At 12:00 LT the westward wind maximum is 15 E GLON. This can partly explain why the current density peak is around 38 E GLON. [23] Another quantity that can induce the four peaked longitudinal structure in the current strength is the F region Pedersen conductance. We are, however, not aware of any report on wave 4 Pedersen conductivity modulation around noontime. According to Liu et al. [2009] the wave 4 sig- 9of12

10 Figure 9. (a) F region dynamo current density around noon as a function of geographic longitude for July through September, (b) its spectral amplitude, and (c) phase. nature of neutral density is not in phase with that of plasma density while both contribute to the Pedersen conductance. In their Figure 2, showing an average over the time period 14:00 18:00 LT, an ion (neutral) density peak is located at 0 ( 30 ) E GLON near equatorial ionization anomaly (EIA) latitudes. Longitudinal structures of zonal wind and neutral density also manifest phase difference [Liu et al., 2009]. Hence, phase differences among zonal wind, neutral, and plasma density can also contribute to the phase difference between the longitudinal structures of zonal wind and F region dynamo currents. In order to confirm our suggestions more coordinated observations of wind speed, neutral/plasma density/temperature, and B field are needed Dependence on Solar Flux [24] The intensity of the F region dynamo currents is expected to increase when the solar EUV flux gets stronger. Our observations confirm that suggestion for both the noon and dusk time sectors. The characteristics of the current increases, however, differ somewhat at the two local times. Around noon we detect ionospheric currents down to low F 10.7 values (<80 solar flux unit (sfu)). For higher solar flux levels Figure 6 shows a linear increase. The prime reason for the stronger current is the increase in F region conductance. According to Liu et al. [2006] the zonal wind velocity shows little dependence on F 10.7 around noon. One factor for enhancing the conductivity is the electron density. Stolle et al. [2008] reported in their Table 2 an increase of the electron density in the EIA crest by 43% when the solar flux level, F 10.7, goes up from 100 to 150. From our Figure 6a we read almost a doubling of the current density for that amount of solar flux increase. Another factor is the collision frequency. This is also influenced by the neutral density. Müller et al. [2009] reported an increase of mass density at equatorial latitudes around noon by 127% for a solar flux increase from 100 to 150 sfu. Combining both the ion and neutral particle increases seems to be more than sufficient to explain the enhancement in current density. [25] In cases of the time around dusk F region currents are observed only for flux levels of F 10.7 > 100. For higher fluxes the current intensity increases faster than around noon. We suggest that besides the increase in neutral and ion density in this time sector also the uplift of the F region plays a role. It is known that the prereversal vertical plasma drift velocity depends significantly on the solar flux level (e.g., Fejer and Scherliess [2001]). In case of an uplifted 10 of 12

11 F region, CHAMP passes through a larger latitudinal part with vertical currents and thus the integrated current density increases. There are of course also other possible reasons for the steep increase of current density. The characteristics of the dusktime F region dynamo warrant further dedicated studies because of their importance for the postsunset electrodynamics in the F region. This will be the topic of a follow up paper. 5. Summary [26] Using geomagnetic field observations of the CHAMP satellite we investigated general climatological features of the F region dynamo current strength as a function of season, longitude, local time, and solar activity. Major findings are summarized as follows: [27] 1. The current strength is smallest around June solstice, which is attributed to a reduced F region Pedersen conductance caused by a lower electron density and neutral density at that season. [28] 2. Magnetic deflections associated with F region dynamo currents are largely confined to times between 08:00 LT and 22:00 LT. During the hours around noon (dusk) the highest downward (upward) current densities are observed. The switch in polarity occurs around 15:00 LT at all seasons. Overall, LT variations of the magnetic deflections related with the F region dynamo are similar for all seasons, showing that the results in Lühr and Maus [2006] are valid in individual seasons. [29] 3. The noontime F region dynamo currents maximize at SAA longitudes while this trend disappears around dusk. It can be explained by the effect of reduced B field as predicted by Takeda [1996]. [30] 4. Large scale (wave 1) variations of the F region dynamo current density are generally compatible with longitudinal structures of the zonal wind [Häusler et al., 2007]. [31] 5. The noontime F region dynamo currents exhibit wave 4 longitudinal patterns during equinoxes and June solstice. The wave 4 signature becomes weak during December solstice. [32] 6. From July to September, when the DE3 tidal component is known to be strongest, the first current density peak of the wave 4 signature appears around 38 E GLON at noontime. It is not far from the DE3 related peak in westward wind at 15 E GLON around 12:00 LT [Häusler and Lühr, 2009]. [33] 7. At dusk the wave 4 signature in F region dynamo currents is much reduced, which might be related to the reduced coupling between E and F regions after sunset. [34] 8. The F region dynamo current increases with solar F 10.7 index during both noon and dusk. It can be explained by higher neutral/plasma density during solar maximum years. The slope of current increase is slightly higher at dusk, implying a non negligible contribution of the prereversal upward plasma drift that depends also on solar activity (e.g., Fejer and Scherliess [2001]). [35] Acknowledgments. The CHAMP mission is sponsored by the Space Agency of the German Aerospace Center (DLR) through funds of the Federal Ministry of Economics and Technology, following a decision of the German Federal Parliament (grant code 50EE0944). The data retrieval and operation of the CHAMP satellite by the German Space Operations Center (GSOC) is acknowledged. [36] Robert Lysak thanks Kazuo Shiokawa and another reviewer for their assistance in evaluating this paper. References England, S. L., S. Maus, T. J. Immel, and S. B. Mende (2006), Longitudinal variation of the E region electric fields caused by atmospheric tides, Geophys. Res. Lett., 33, L21105, doi: /2006gl Fejer, B. G., and L. Scherliess (2001), On the variability of equatorial F region vertical plasma drifts, J. Atmos. Sol. Terr. Phys., 63(9), Fejer, B. G., J. W. Jensen, and S. Y. Su (2008), Quiet time equatorial F region vertical plasma drift model derived from ROCSAT 1 observations, J. Geophys. 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