Dayside ionospheric response to recurrent geomagnetic activity during the extreme solar minimum of 2008

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1 Click Here for Full Article GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L02101, doi: /2009gl041038, 2010 Dayside ionospheric response to recurrent geomagnetic activity during the extreme solar minimum of 2008 S. Tulasi Ram, 1 J. Lei, 2 S.-Y. Su, 3 C. H. Liu, 4 C. H. Lin, 5 and W. S. Chen 3 Received 18 September 2009; revised 3 December 2009; accepted 10 December 2009; published 16 January [1] Global observations of electron density profiles from the COSMIC satellites are used to investigate, for the first time, the altitudinal dependence of the ionospheric response to the recurrent geomagnetic activity at different latitudinal regions during the extreme solar minimum period of Our results show that the 9-day oscillations in N m F2 are out of phase with those in Kp at high latitudes, whereas they are in phase at low-middle latitudes. This is consistent with changes in neutral composition associated with the recurrent geomagnetic activity. Meanwhile, the 9-day perturbations in the h m F2 and the thickness parameter (H T ) exhibit good correspondence with the perturbations in Kp from pole to pole, suggesting that the ionospheric response is global and undergoes periodic expansion/contraction. Further, the ionospheric response to the recurrent geomagnetic activity strongly depends on altitude. The density perturbations are generally in phase with Kp above the F2 peak, while they are out of phase around the F2 peak at high latitudes. These changes in electron density at different altitudes are explained by different physical processes, such as photoionization-chemistry, particle precipitation, and dynamic and diffusion transport. Citation: Tulasi Ram, S., J. Lei, S.-Y. Su, C. H. Liu, C. H. Lin, and W. S. Chen (2010), Dayside ionospheric response to recurrent geomagnetic activity during the extreme solar minimum of 2008, Geophys. Res. Lett., 37, L02101, doi: /2009gl Introduction [2] Oscillations at multi-day periods (near 5.5, 7 and 9 day subharmonics of solar rotation) associated with recurrent high speed solar wind streams were recently discovered in the thermosphere and ionospheric total electron content (TEC) [Lei et al., 2008a, 2008b, 2008c; Mlynczak et al., 2008; Thayer et al., 2008]. A new solarterrestrial connection has been established between the corotating solar coronal holes and the variations in the Earth s thermosphere/ionosphere vis-à-vis the high-speed solar wind streams and the subsequent recurrent geomagnetic activity variations. 1 Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan. 2 Aerospace Engineering Sciences, University of Colorado, Boulder, Colorado, USA. 3 Institute of Space Science, National Central University, Chung-Li, Taiwan. 4 Academia Sinica, Taipei, Taiwan. 5 Plasma and Space Science Center, National Cheng Kung University, Tainan, Taiwan. Copyright 2010 by the American Geophysical Union /10/2009GL041038$05.00 [3] Crowley et al. [2008] have also reported 9-day periodic oscillations in the P O/N2 ratio measured by Global Ultraviolet Imager (GUVI) on the TIMED satellite and demonstrated that the P O/N2 ratio response is opposite at high and low latitudes due to upwelling and downwelling winds. Since daytime electron density in the F-region is proportional to O/N 2, the changes in O/N 2 associated with the recurrent geomagnetic forcing shown by Crowley et al. [2008] are expected to result in similar changes in F-region electron densities. Lei et al. [2008a, 2008b] and Thayer et al. [2008] have shown that the thermospheric mass density response is global and varies coherently with the recurrent geomagnetic activity, although the response is slightly larger at high latitudes. The expected changes in neutral temperature, which may be responsible for the observed global oscillations in mass density, can change the electron density as well by altering ion-neutral chemistry reaction rates and the plasma scale height. Therefore, the ionospheric response to the periodic geomagnetic forcing should exhibit significant latitudinal and altitudinal dependence due to these competing processes. [4] The objective of this paper is thus to investigate, for the first time, the altitudinal variation in the ionospheric response to recurrent geomagnetic activity at different latitudinal regions by using the global electron density observations from the Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) satellites during the extreme solar minimum period of Data [5] FORMOSAT-3/COSMIC (in short F3/C) is a constellation of six micro satellites orbiting around 800 km altitude, 72 inclination angle, and 30 separation in longitude. Each F3/C satellite has a GPS occultation experiment payload, performing the radio occultation observations in the ionosphere. F3/C satellites provide 2000 vertical electron density profiles per day which are uniformly distributed all over the globe. The details of the inversion technique applied to invert the F3/C occultation soundings to ionospheric electron density profiles are given by Schreiner et al. [1999] and Lei et al. [2007]. The validation of F3/C radio occultation observations has been carried out by Schreiner et al. [2007] and Lei et al. [2007] and found that the derived electron density profiles are in agreement with Ionosonde and Incoherent Scatter Radar observations. [6] The F3/C data is unique because it provides global coverage of vertical electron density distribution from 60 to 800 km. This facilitates us the investigation of the altitudinal dependence of the ionospheric response to recurrent geomagnetic activity during the solar minimum of 2008, when the solar activity was extreme low [Heelis et al., L of5

2 Figure 1. (a) Latitudinal variation of zonal mean electron density at 350 km altitude as a function of day number in 2008 and (b) the corresponding Lomb-Scargle (LS) periodogram. Daily mean Kp-index and its LS spectral amplitudes are overlapped as black curves (right hand scale). The unit of electron density is m ]. In this study, only daytime electron density profiles are used to minimize the local time effect, given that the ionosphere response to geomagnetic activity is significantly different in daytime and at night [e.g., Pedatella et al., 2009]. All the day time profiles (0600 LT < 1800) in each day are zonally (longitudinally) averaged into 16 latitudinal bins from 80 to +80 geographic latitudes (with 10 window) and 40 altitudinal bins from 100 to 500 km (with 10 km window). 3. Results [7] Figure 1 shows the latitudinal variation of day time electron density at 350 km altitude as a function of day number in 2008 and the corresponding Lomb-Scargle (LS) periodogram. Imbedded in the large time scale seasonal and semi-annual variations, shorter time scale (with the periods of 27-days as well as a few days) variations can also be seen in the electron density (Figure 1a). From Figure 1b, predominant spectral peaks can be found at the periods of 27-, and 9-days, which are correlated with the spectral peaks in the Kp-index (black curve). This corroborates with the earlier observations by Lei et al. [2008c] that the periodic oscillations at sub-harmonics of a solar rotation period in the ionosphere are associated with recurrent geomagnetic activity. It should be mentioned that, although the 27-day and 13.5-day periodicities are present in TEC [Lei et al., 2008c] and also electron density (Figure 1b), they may not be solely attributed to recurrent geomagnetic activity because 13.5-day and 27-day periodicities also exist in solar EUV radiation which directly impacts ion production. Hence, we will focus our investigation mainly on the 9-day periodicity in the F-region electron density associated with recurrent geomagnetic activity. [8] In order to examine the relationship between the 9-day periodicities in the F-region ionosphere and the geomagnetic activity index, Kp, we applied a bandpass filter to each latitude-altitude bin of electron density as well as F-region parameters. The bandpass filter was centered at 9-days, with half-power points at 6 and 12 days in the same manner as Lei et al. [2008a]. Figures 2a 2c show the latitudinal variations of bandpass filtered 9-day perturbations in the F-region parameters such as F2 layer peak density (N m F2), F2 layer peak height (h m F2), and the thickness of the topside F-layer (H T ), respectively. Here, H T is taken as the difference between h m F2 and the height where the topside electron density is reduced by a factor of e with respect to the F2 peak density. Interestingly, the perturbations in N m F2 exhibit significant latitudinal variation. They are in-phase with Kp oscillations at low latitudes (<40 ), while out of phase at high latitudes (50 80 ). The perturbations in H T and h m F2 correspond well with the perturbations in Kp from pole to pole, although the perturbations are larger at high latitudes compared to mid-low latitudes. [9] The altitudinal variations of the 9-day perturbations in electron density from 100 to 500 km at high- (70 80 N), mid- (40 50 N) and low- (0 10 N) latitudes are shown in Figure 3. Interesting tilt structures can be noticed at high 2of5

3 and they are in phase at low-mid latitudes, whereas the 9-day perturbations in the h m F2 and H T exhibit good correspondence with the perturbations in Kp from pole to pole. Firstly, we turn our attention to the discussion of the latitudinal variations in ionospheric response to the recurrent geomagnetic activity. Crowley et al. [2008] showed a depletion in P O/N2 ratio at high latitudes and an enhancement at low latitudes in response to the recurrent geomagnetic activity in This response was attributed to an upwelling wind driven by Joule and particle heating at high latitudes which carries molecular-rich air to higher altitudes resulting in the P O/N 2 ratio reduction. Divergent horizontal wind blows equatorward, where downwelling then transports atomic-oxygen-rich air parcels to low altitudes, enhancing the P O/N 2 ratio. The N m F2 perturbations presented in Figure 2a show latitudinal features similar to those observed in P O/N 2 ratio by Crowley et al. [2008]. Given that N m F2 during the day time is more or less determined by the balance of production due to photoionization of O and loss due to N 2, this correspondence between latitudinal structures in N m F2 and P O/N 2 is as expected. [11] The oscillations in h m F2 from pole to pole (Figure 2b) are well in-phase with the oscillations in Kp, suggesting that the ionospheric response is global and undergoes periodic Figure 2. Bandpass filtered 9-day perturbations in (a) F2- layer peak density (N m F2), (b) F2-layer peak height (h m F2), and (c) the thickness of the topside F-layer (H T ) as a function of latitude and day number. We only show the data from DOY 1 to 100 in 2008 for better visualization. The 9-day perturbations in Kp are also overlapped (right hand scale). In order to better visualize the 9-day periodicity, the N m F2 perturbations are represented in percent perturbations relative to 11-day running mean, whereas the H T and h m F2 perturbations are shown in absolute values. latitudes (Figure 3a), where the electron density perturbations are out of phase with the perturbations in Kp near the F2-layer peak altitudes. Further, their phase relationship above and below the F2-layer peak varies with height and becomes nearly in-phase at both higher and lower altitudes. At mid-latitudes (Figure 3b), the electron density perturbations are generally in-phase with the perturbations in Kp above the F2-layer peak, while they are out of phase below the F2-layer peak. At low latitudes (Figure 3c), the electron density perturbations are in general greater above the F2-peak and can exceed 40% with respect to background levels. Further, these perturbations are in-phase with Kp above 250 km and their phase relationship changes with height below 250 km. 4. Discussion [10] Our results in Figure 2 show that the 9-day oscillations in N m F2 are out of phase with Kp at high latitudes, Figure 3. Bandpass filtered 9-day perturbations in electron density at (a) high (70 80 N), (b) middle (40 50 N), and (c) low (0 10 N) latitudes as a function of altitude and day number. The overlapped white dotted curves represent the zonal mean h m F2. 3of5

4 Figure 4. (a) Latitudinal and altitudinal variation of correlation coefficient (r) obtained from the zero-lag cross correlation of bandpass filtered 9-day perturbations in electron density with the perturbations in Kp during the entire year of The overlapped black dashed line indicates the latitudinal variation of annual mean h m F2. (b) Schematic diagram to demonstrate possible dominant mechanisms at different altitude and latitude regions. The black dashed lines represent the reference electron density profiles and the red lines are for the disturbed profiles during the enhanced geomagnetic activity. expansion/contraction coherently with the variations in geomagnetic activity. These results are in good agreement with the thermospheric expansion/contraction as indicated by the oscillations in the neutral mass density measured by the CHAMP accelerometer at 400 km, reported by Lei et al. [2008a, 2008b], who found that the thermospheric density response is global and in phase with Kp at all latitudes. [12] The enhanced Joule and particle heating that occurs at high latitudes during the enhanced geomagnetic activity results in the expansion of the thermosphere and a resultant increase in the neutral gas scale height [Lei et al., 2008c]. Recently, Sojka et al. [2009] reported that the ion temperature in the F-region shows strong enhancements due to high speed solar wind streams. Thus, an enhancement in the electron density scale height would be expected. The inphase correlation between the thickness parameter H T and Kp (Figure 2c) further supports this argument. We speculate that the larger perturbations in H T at high latitudes are due to larger heat deposition and hence greater plasma temperature variation in this region. Note that the observed changes in ionospheric electron density are not only related to temperature and neutral composition changes, but also to winds and electric fields which can be significantly disturbed during the recurrent geomagnetic activity periods, as suggested by Lei et al. [2008c]. [13] The remaining question is what causes the phaserelation between the perturbations in electron density and those in Kp to vary with altitude (Figure 3). This may be due to various physical processes that are dominant at different latitude-altitude regions. Figure 4a shows the correlation coefficient (r) at zero lag between the bandpass filtered 9-day perturbations in electron density and the perturbations in Kp at each latitude-altitude bin during the entire year The electron density response above 300 km shows good positive correlation (r > 0.7) at all latitudes. However, the correlation is negative between 180 and 270 km at high latitudes, with the maximum negative correlation (r < 0.6) near the F2-layer peak height (200 to 250 km) from the polar region to 50 latitude in both hemispheres. Although we interpreted the variation of N m F2 in terms of the changes in neutral composition shown by Crowley et al. [2008], the perturbations in electron densities in neither the topside nor the bottomside of the F2 region can be simply interpreted in the same way. [14] The altitudinal dependence of the relationship between the perturbations in electron density and those in Kp can be explained by using the schematic diagram in Figure 4b. At high latitudes, electron densities above the F2 peak height increase as the geomagnetic activity enhances, even though N m F2 decreases due to the depletion of O/N2, as discussed previously. The increased electron densities in the topside ionosphere are possibly due to an increase of scale height resulting from the enhanced plasma temperatures [Sojka et al., 2009]. Thus the tilt structures seen in the bandpass filtered data at high latitudes (Figure 3a) occur as a result of the transition from the photo-chemical dominated region near the F2 peak height to the dynamic and diffusion governed region of the topside ionosphere. [15] We suggest the positive correlation (r > 0.8) between electron density oscillations and Kp perturbations around km at high latitudes is related to particle precipitation during the enhanced geomagnetic activity. Mlynczak et al. [2008] demonstrated that integrated NO and CO2 IR emissions were enhanced during the periods of high speed solar wind streams. Particle precipitation strongly drives the NO emission around km and an increase of ionization due to particle precipitation may be responsible for the observed enhancements in electron density during the periods of high speed solar wind streams. [16] At low and middle latitudes, as mentioned before, the increase in O/N2 ratio, which is associated with downwelling during enhanced geomagnetic activity, leads to the enhancement of electron densities in the F2 region (N m F2). Higher peak height and scale height at these 4of5

5 regions (Figures 2b and 2c) indicate the upward movement of the F2 region and then the resultant increase of electron density. A weak negative correlation observed below 200 km at low latitudes (30 S to 30 N) may be related to increased chemical loss rates due to enhanced temperatures during the periods of geomagnetic activity. Note that the Abel inversion technique may also introduce error in this region due to the assumption of spherical symmetry. Additionally, the effect of neutral composition changes in this region is currently unknown. Further investigation is required in this aspect. 5. Conclusions [17] This study addresses the altitudinal and latitudinal dependence of daytime ionospheric response to 9-day quasi periodic geomagnetic activity associated with high speed solar wind forcing using the unique dataset of global electron density profiles from COSMIC. The 9-day oscillations in N m F2 are in-phase with Kp at low latitudes and out of phase at high latitudes; whereas the oscillations in the h m F2 and H T are in-phase with Kp from pole to pole suggesting that the ionospheric response is global and undergoes periodic expansion/contraction coherently with periodic forcing. The ionospheric response above 300 km is similar (in-phase with Kp) at all latitudes primarily due to the changes in the scale height resulting from the changes in the neutral temperature. However, the electron density perturbations around F2-peak region are dominated by the changes in neutral composition (O/N 2 density ratio) due to upwelling (downwelling) winds at high (low) latitudes. Further, the particle precipitation during the enhanced geomagnetic activity probably drives the corresponding periodic enhancements in the electron density around km at high latitudes. [18] Our results are the first to reveal the interesting altitudinal dependence of the ionospheric response to recurrent geomagnetic activity at different latitude regions, which provides important insights for future modeling works on the solar wind-magnetosphere-ionosphere-thermosphere coupling. Further, understanding this periodic modulation in the ionosphere associated with solar wind forcing will have crucial applications in predicting the ionospheric behavior to aid the operational needs of space-based communication and navigational systems. [19] Acknowledgments. S. Tulasi Ram is a Post-doctoral Fellow at ASIAA, Academia Sinica. This work is partly supported by NSPO project 98-NSPO(B)-IC-FA07-01(S) and NSC project NSC M J. Lei s effort was supported by AFOSR MURI Award FA The authors wish to thank N. Pedatella for his useful comments and UCAR/CDAAC, NSPO, for providing FORMOSAT-3/COSMIC data. References Crowley, G., A. Reynolds, J. P. Thayer, J. Lei, L. J. Paxton, A. B. 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Rocken (1999), Analysis and validation of GPS/MET radio occultation data in the ionosphere, Radio Sci., 34, , doi: /1999rs Schreiner, W., C. Rocken, S. Sokolovskiy, S. Syndergaard, and D. Hunt (2007), Estimates of the precision of GPS radio occultations from the COSMIC/FORMOSAT-3 mission, Geophys. Res. Lett., 34, L04808, doi: /2006gl Sojka, J. J., R. L. McPherron, A. P. van Eyken, M. J. Nicolls, C. J. Heinselman, and J. D. Kelly (2009), Observations of ionospheric heating during the passage of solar coronal hole fast streams, Geophys. Res. Lett., 36, L19105, doi: /2009gl Thayer, J. P., J. Lei, J. M. Forbes, E. K. Sutton, and R. S. Nerem (2008), Thermospheric density oscillations due to periodic solar wind high-speed streams, J. Geophys. Res., 113, A06307, doi: /2008ja W. S. Chen and S.-Y. Su, Institute of Space Science, National Central University, Chung-Li 32001, Taiwan. J. Lei, Aerospace Engineering Sciences, University of Colorado, Boulder, CO 80309, USA. C. H. Lin, Plasma and Space Science Center, National Cheng Kung University, Tainan 701, Taiwan. C. H. Liu, Academia Sinica, Taipei 115, Taiwan. S. Tulasi Ram, Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan. (tulasiram@asiaa.sinica.edu.tw) 5of5