On a possible relationship between density depletions in the SAA region and storm-enhanced densities in the conjugate hemisphere

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1 ARTICLE IN PRESS Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) On a possible relationship between density depletions in the SAA region and storm-enhanced densities in the conjugate hemisphere C.S. Lin a,, H.-C. Yeh b, C.K. Chao b a Institute for Scientific Research, Boston College, Chestnut Hill, MA, USA b Institute of Space Science, National Central University, Jhongli City, Taiwan Received 16 March 2005; accepted 31 July 2006 Available online 6 December 2006 Abstract During the great magnetic storm of July 15, 2000, the ROCSAT-1 satellite with an orbital altitude of 640 km detected a large density depletion region at low latitudes (o351) in the southern hemisphere while storm-enhanced density (SED) was observed in the northern hemisphere. Convective electric fields deduced from measurements of the ROCSAT-1 drift meters and retarding potential analyzer (RPA) indicate negligible electric fields in the eastern portion of the SED region where density enhancement was evident, suggesting a lack of ion upward motion at the equator of the SED longitudes. However, a localized electric field enhancement was observed in association with South Atlantic magnetic anomaly density structures (MADS) during the event. The zonal electric field component was eastward corresponding to radial convection, and the radial electric field component on the magnetic meridional plane was outward corresponding to westward convection. In general, both components of the observed convective electric fields were enhanced inside the density depletion region and reduced inside the density enhancement. The electric field patterns are consistent with those arising from a circular region of flux tubes with enhanced conductance in the presence of a background electric field. The mapping of the MADS region and convective electric fields from the southern hemisphere to the northern hemisphere suggests that these localized electric fields could drive ionospheric plasma from the conjugate MADS region to the SED at higher latitudes. We thus propose a new idea that the conjugate MADS region could be the plasma source for producing the SED. r 2006 Elsevier Ltd. All rights reserved. Keywords: Storm-enhanced density; Magnetic anomaly density structures; Convective electric field; South Atlantic anomaly; Low-latitude ionosphere 1. Introduction Low-earth orbiting satellites have detected largescale ion density dropouts with a spatial width Corresponding author. Present address: Air Force Research Laboratory, Space Vehicles Directorate, AFRL/VSBXI, 29 Randolph Road, Hanscom AFB, MA Tel.: ; fax: address: chin.lin@hanscom.af.mil (C.S. Lin). greater than 800 km in the low-latitude ionosphere during several magnetic superstorms (Greenspan et al., 1991; Basu et al., 2001; Lin et al., 2001, Lee et al., 2002; Su et al., 2002; Lin and Yeh, 2005). Lin et al. (2001) demonstrated that these large-scale density dropouts corotated with the South Atlantic anomaly (SAA) and were adjacent to regions of density enhancement, forming a magnetic anomaly density structure (MADS). While the large-scale density dropouts were detected in the southern /$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi: /j.jastp

2 152 ARTICLE IN PRESS C.S. Lin et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) hemisphere, ground-based global positioning satellite (GPS) total electron content (TEC) measurements have indicated large-scale storm-enhanced density (SED) in the northern hemisphere during magnetic superstorms (Foster et al., 2002; Kelley et al., 2004; Yin et al., 2004). The TEC map of the SED for the July 2000 Bastille Day superstorm recently presented by Kelley et al. (2004) and Yin et al. (2004) showed a large TEC enhancement at latitudes from 51 to 401N in North America. The GPS TEC map published by Kelley et al. (2004) is reproduced in the top panel of Fig. 1. During this magnetic superstorm, ROCSAT-1 detected a large ion density depletion at low latitudes (o351s) in the nighttime ionosphere after magnetic storm activity reached its peak intensity, lasting 8 h through the end of the magnetic storm (Lin et al., 2001). The bottom panel of Fig. 1 taken from Lin et al. (2001) is a composite overview of ion density structures measured by the ROCSAT-1 ion trap sensor for this event. Large density depletions and enhancements (MADS) were detected from 3001 to 3401E geographic longitude, east of the center of the SAA. In general, ion density decreased by a factor of from the ambient to the bottom of the density dropout (Lin et al., 2001). In this paper, we investigate the possible relationship between the SED and MADS. The SED has been shown to be associated with the plasmaspheric drainage plume (Foster et al., 2002). However, the responsible electric field structures for the SED and plumes are not known. Several theoretical ideas have been discussed (Su et al., 2001; Foster et al., 2002). Recently, Kelley et al. (2004) proposed a plausible explanation for producing the SED. They suggested that plasma, which originated in the fully sunlit equatorial ionosphere first spills over into the anomaly and then is driven poleward by a penetrating zonal electric field. Because the poleward electric field cannot extend into the dayside due to high conductivity, the flow stagnates, causing the plasma to build up. The SED is thus produced as large convective flow is impeded when passing through a region of enhanced conductivity. When flow velocity is reduced, ion density increases to maintain the constancy of density flux since plasma production is believed to be negligible. In this paper, we present features of the electric fields in association with the SED and MADS structures observed during the great July 2000 magnetic storm. These electric fields could, in principle, drive ionospheric plasma to produce large-scale MADS in the southern ionosphere and a SED in the northern ionosphere. Based on the observed electric field signatures in the SED and MADS regions, we propose an alternative explanation for producing the SED. 2. Observations and analysis The ROCSAT-1 satellite launched in 1999 had a circular orbit with an altitude of nearly 640 km and an inclination angle of 351. The ionospheric plasma and electrodynamics instrument (IPEI) payload contained an ion trap, two drift meters and a retarding potential analyzer (RPA) (Yeh et al., 1999). The ion trap sensor measured the total ion density, and the two ion drift meters measured the two perpendicular cross-track components of ion drift velocities. The RPA data are used to infer ion velocity in the satellite ram direction. The three perpendicular components of ion velocity in the satellite frame are then transformed to obtain two perpendicular components of ion convective velocity and the field-aligned flow velocity in the geomagnetic field-aligned coordinate system according to the international geomagnetic reference field (IGRF) magnetic field model. We calculate the two perpendicular components of convective electric fields E M and E Z at the satellite altitude by using E ¼ V B, where V is the measured convective velocity and B is the IGRF magnetic field vector. Because corotation velocity has been subtracted in calculating V, E M and E Z are defined in the Earth s corotating frame of reference. The electric field E M component is chosen to be perpendicular to the magnetic field on the meridional plane and to be positive when it is in the outward direction. Since E M is close to the radial direction, it is referred to as the radial component in this paper. The other electric field component E Z, perpendicular to both E M and the magnetic field direction, is the zonal electric field component, which is defined to be positive in the eastward direction. We have reprocessed the data by removing bias in the convective velocity data and reduced noise in fitting the RPA data by removing spikes and smoothly averaging the ram velocity in the analysis (Lin and Yeh, 2005). During the July 2000 Bastille Day event ROC- SAT-1 detected large-scale density structures from 18 to 02 LT between 3101 and 3401 in longitude and from 20 to 401 in latitude. DMSP detected large-scale density depletions near the equator

3 ARTICLE IN PRESS C.S. Lin et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) GPS TEC Map from 15-Jul :10:00 to 15-Jul :25:00 TEC Geodetica Latitude, Deg (a) Geodetic Longitude, Deg GLAT GLON (b) Log (Density) (#/c.c) Fig. 1. (a) GPS TEC map at 2216 UT on July 15, 2000 from Kelley et al. (2004). The TEC unit is electrons per square centimeters. (b) Ion density map from ROCSAT-1 measurements from 22 UT on July 15, 2000 to 06 UT on July 16, 2000 from Lin and Yeh (2005). Ion density is in units of #/cm 3. The GPS TEC map was based on ground-based data in the northern hemisphere; ROCSAT-1 data were taken in the southern hemisphere. (Basu et al., 2001). The D st index signaled the beginning of the storm s main phase at 2040 UT on July 15, 2000, and reached its lowest value ( 300 nt) around 2200 LT. Fig. 1(a) shows the GPS TEC map at 2216 UT near the peak of storm activity. Fig. 2 plots the radial and zonal electric

4 154 ARTICLE IN PRESS C.S. Lin et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) Fig. 2. ROCSAT-1 IPEI observations of ion density (top), the E Z electric field component (second), the E M component of convective electric field (third), and the satellite geographic latitude (bottom) during the magnetic storm main phase. E Z is the zonal component and positive in the eastward direction. E M is defined to be perpendicular to the magnetic field on the meridional plane and positive in the outward direction. The top panel plots the logarithm of ion density in units of #/cm 3. Electric fields are in units of mv/m. Geographic latitude and longitudes are in degrees. field components versus geographic longitude for UT, which covers the time interval of the GPS TEC map in Fig. 1(a). When the SED was observed in North America, ROCSAT-1 detected a small density enhancement from 2401 to 2901 longitude in the northern hemisphere (between lines A and B in the top panel of Fig. 2). The electric fields E Z and E M were negligible in the eastern portion of the SED ( longitude) when the density enhancement was most obvious. This implies no upward drift of plasma from the equator at the SED longitude. It is interesting to note that both E Z and E M had large negative values near the western boundary of the SED (line A), indicating strong ion eastward and downward drift motion at the SED western boundary. The ROCSAT-1 observations indicate large electric fields in association with the MADS, which is shown between vertical lines C and E in the top panel of Fig. 2. The density depletion was detected from 3101 to 3201 geographic longitude (between lines C and D), whereas the density enhancement was observed from 3201 to 3401 (between lines D and E). ROCSAT-1 measured a steady increase in E M and E Z, corresponding to an ion density gradual decrease in the density depletion region. The radial electric field component E M shown in the third panel had a peak magnitude 7.5 mv/m at the boundary between the depletion and the enhancement, which was at 3201 geographic longitude (line D). The peak magnitude of E Z was 4.5 mv/m (second panel, Fig. 2). These electric field magnitudes are unusually large since ROCSAT-1 typically measured electric fields of o1 mv/m in the nightside. The bottom panel indicates that the electric field enhancement was detected from 201 to 351 geographic latitude. Both

5 ARTICLE IN PRESS C.S. Lin et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) E M and E Z had quasi-periodic fluctuations in the density enhancement region (between lines D and E). The zonal component E Z fluctuation was more pronounced around a mean value of 2 mv/m with a wavelength of 128 km. The electric field fluctuations might be produced by velocity shear due to the Kelvin Helmholtz instability. The magnitude of the observed electric fields was largest in the beginning of the magnetic storm main phase and decreased with time until the end of the storm. We examine the time history of the two electric field components E * M and E * Z, respectively, at the boundary between density depletion and enhancement. The radial E M component decreased * from 9 mv/m at the main phase onset to less than 2 mv/m during the recovery phase in about 8 h. In the beginning of the storm, E * Z was 6 mv/m and decreased more gradually than E * M. During the recovery phase between 02 and 06 h UT on July 16, E * Z was less than 2 mv/m. Based on ROCSAT-1 observations during the July 2000 magnetic storm, we sketch a schematic depicting regions of density depletion and enhancement in the southern hemisphere and their conjugate locations in the northern hemisphere (Fig. 3). We also plot four contours of constant magnetic field around the SAA (dashed lines) to show that these density structures were located close to the SAA center. The density structures are mapped from the ROCSAT-1 altitude (640 km) to 200 km altitude in both hemispheres. The conjugate region of density depletion and electric field enhancement is estimated to be localized at latitude and longitude. In Fig. 3, we also illustrate the direction of convective plasma flow expected from the electric fields. Plasma flow was observed to be south-westward in the southern hemisphere and became north-westward in the northern hemisphere. Flow velocity in the low-density region is higher than that in the high-density region. We estimate the flow velocity to be 120 m/s in the northern hemisphere density depletion region. From the ground-based GPS TEC map shown in Fig. 1(a), we sketch the SED region in Fig. 3, which indicates that the conjugate MADS was located at latitudes lower than that of SED and plasma flowed from the conjugate MADS to the SED. This raises the possibility that the conjugate MADS could be the origin of the plasma producing the SED. Note that ionospheric plasma equatorward of the SED flowed eastward, opposite to the flow direction inside the SED, as indicated by the arrows in Fig. 3. Large Fig. 3. Schematic summarizing the ROCSAT-1 observations of density structures in relation with the SED during the July 2000 magnetic storm. The SED region is sketched using the GPS TEC map of Kelley et al. (2004). Density depletion regions are mapped from the depletion observed by ROCSAT-1 to an altitude of 200 km in both hemispheres. Similarly, the density enhancement regions marked as DE are mapped from the enhancement observed by ROCSAT-1 satellite to 200 km altitude. Four dashed lines represent contours around the SAA center with magnetic field magnitudes of 0.23, 0.24, 0.25 and 0.26 G at 200 km, respectively, from the inside outward. The black dashed line traces the magnetic equator at 200 km altitude. The arrows inside DD and DE illustrate flow directions based on ROCSAT-1 electric field observations. Eastward arrows indicate the direction of ionospheric plasma equatorward of the SED. eastward flow equatorward of the SED was caused by large negative E M observed at the SED western boundary (Fig. 2), which might manifest the overshielding of magnetospheric electric fields. 3. Discussion The ROCSAT-1 observations indicate penetration of convective electric fields to low latitudes (o351) with distinct localized features in association with density depletion and enhancement structures (MADS) in the SAA region. Although we report the observations during the July 15 16, 2000 storm, ROCSAT-1 had observed similar features during other magnetic superstorms. The mean magnitude of the electric fields increased steadily as density decreased in the depletion region, but it is reduced inside the enhancement. Since electric fields in the direction of the external electric field are generally reduced in the high Pedersen conductivity region, the density enhancement probably corresponds to a high field-line integrated Pedersen conductivity

6 156 ARTICLE IN PRESS C.S. Lin et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) region. This interpretation is consistent with the fact that Pedersen conductivity in the F region is proportional to density. To understand the mechanisms underlying the observed electric field enhancement and reduction, Lin and Yeh (2005) studied electric field solutions when Pedersen and Hall conductances are enhanced in a circular region of flux tubes in the ionosphere in presence of an ambient electric field. Although electric field variation in a one-dimensional model of conductance is well known, electric field signatures around a circular high conductive region are not well understood. Electric field structures have previously been modeled using flux-tube-integrated conductivities (Haerendel et al., 1992; Keskinen et al., 1998; Abdu et al., 1998) by requiring that the divergence of the perpendicular current J? is zero. In terms of Pedersen and Hall conductances, the governing equation is rj? ¼rðS P ~E þ S H b ~EÞ ¼0, (1) where ~E is the convective electric field vector, b is the unit vector in the direction of magnetic field assumed to be along the cylindrical axis, S P and S H are the field-line integrated Pedersen and Hall conductivity, respectively. The first term on the right side of Eq. (1) represents Pedersen current and the second Hall current. Effects of gravitational and field-aligned current are neglected. When the perpendicular electric field ~E is expressed as rf in the cylindrical coordinate (r,y), Eq. (1) can be written as q qr rs qf P qr þ S q 2 f P qy 2 d P H qf ¼ 0. (2) dr qy This equation has been solved for the classical problem with a uniform isotropic dielectric cylinder under a constant external electric field (for example, see Smythe (1989)). It is straightforward to solve Eq. (2) for electric potential f when S P and S H are constant inside the dielectric cylinder. The electric field solutions of Eq. (2) depend on the magnitude of conductances and the orientation of the external electric field E 0. Here we consider the simple case of height-integrated Pedersen and Hall conductivities are enhanced uniformly in a cylinder of flux tubes with a radius R. Conductances inside the flux tube cylinder are assumed to be much larger than those outside the cylinder, S i P cs o i P, S H cs o i H. Here S P and S o P are Pedersen conductance inside and outside the i cylinder, respectively. Similarly S H and S o H are the Hall conductance inside and outside the cylinder, respectively. For constant background electric field ~E o in the +x direction ~E o ¼ E 0 ~x, the solution of Eq. (2) can be expressed as! f ¼ E 0 Cr cos y Si H S i sin y P ¼ E 0 x þ E 0 ð1 CÞ R2 r cos y þ CE 0 S i H S i P rpr R 2 sin y for rxr; ð3þ r = S i 2 P þ with the constant C ¼ 2 S i P So P þ Si H So H S i2 HÞ. Inside the cylinder (ror) the electric field is constant with the x-component of the electric field E x reduced from E o to CE o and the y-component E y equal to CE 0 S i H =Si P. Outside the cylinder (r4r) the electric fields vary as 1/r 2, E x ¼ 1 þ R2 R2 S i H ð1 CÞcos 2y þ C E 0 r2 r 2 sin 2y, E y ¼ R2 R2 S i H ð1 CÞsin 2y C E 0 r2 r 2 S i cos 2y. ð4þ P Fig. 4 illustrates the electric field solutions along the x-axis for three values of S i P /S o P. The solutions inside the conductance enhancement region have well-known features including a reduction in E x and a small enhancement in E y. Eq. (4) also indicates that E x peaks at the conductivity boundary and Fig. 4. Normalized electric fields E x and E y as a function of radial distance from the origin along the +x-axis for various ratios of Pedersen conductances between inside and outside S i P / S o P. The electric fields are normalized by the external electric field E o, which is in the +x direction. The radius of conductance enhancement is assumed to be 300 km. The ratios of Pedersen and Hall conductances are S o P /S o i H ¼ 6 and S H /S o H ¼ 3. S i P

7 ARTICLE IN PRESS C.S. Lin et al. / Journal of Atmospheric and Solar-Terrestrial Physics 69 (2007) decays with distance from the boundary. These features are roughly similar to the ROCSAT-1 observations. The observed radial electric field E M was reduced in the density enhancement region, most likely a region of higher conductivity. The observed electric fields were enhanced in the density depletion region and decayed away from the boundary of the density enhancement, consistent with the analytical solution shown in Fig. 4. The enhanced electric fields could be responsible for the SED observed in the conjugate northern hemisphere. We speculate that this may imply a causal relationship between SED and MADS, although this remains to be established. Some correlations seem to exist between large SED and MADS based on the reports on SED and our survey of ROCSAT-1 observations of MADS. The large SED is observed at North American longitudes as the MADS is detected close to the SAA longitude. Furthermore, both are detected during severe magnetic storms. As shown in Fig. 2, ROCSAT-1 measured little upward drift at the equator of the SED longitudes, thus ruling out the possibility that plasma uplift from the equator at the SED longitudes could account for the SED. Fig. 3 indicates instead that ions stream in the direction from the MADS to the SED. This raises the possibility that the MADS conjugate region might be the source region for the density enhancement seen in the SED. The current challenging problem in low-altitude ionospheric modeling is to characterize the formation of the SED and MADS during severe magnetic storms. Several theoretical ideas have been suggested in light of recent observations. Here we have presented ROCSAT-1 electric field measurements, which indicate the virtual absence of electric fields at SED longitudes and the presence of large westward and outward convective electric fields in connection with the MADS. The electric fields at the MADS longitudes could result from either prompt penetrating electric fields or ionospheric disturbance dynamo electric fields. Importantly, the observed electric fields are localized in the SAA region. They may be produced as a result of a localized increase in Pedersen and Hall conductivities, which in turn are due to energetic particle precipitation as suggested by Lin and Yeh (2005). We have also presented analytical calculations to show that electric fields are increased (reduced) outside (inside) a circular localized region of enhanced conductance. These localized large electric fields could drive ionospheric plasma from the MADS region to the SED region at higher latitudes. The locations of the conjugate MADS density depletion and the SED in the northern hemisphere are consistent with this interpretation. We thus propose a new idea that the SAA magnetic anomaly density structures could be the plasma source for producing the SED. Clearly, more extensive analysis is needed to determine whether the flow pattern reported here is the persistent feature for SED and MADS and how the flow pattern varies with time. It would be worthwhile to establish the connection between the SED and MADS, and to investigate further the mechanisms for simultaneously producing both density structures. Acknowledgments The work at National Central University was supported by the National Science Council of Taiwan through grant NSC M The work at Boston College was supported in part by NASA SEC-GI grant NNG04GI70G, NSF Grant ATM and Air Force Research Laboratory contract F19628-C References Abdu, M.A., Jayachandran, P.T., MacDougall, J., Cecile, J.F., Sobral, J.H.A., Equatorial F region zonal plasma irregularity drifts under magnetospheric disturbances. Journal of Geophysical Research 25, Basu, S., Groves, K.M., Yeh, H.-C., Su, S.-Y., Rich, F.J., Sultan, P.J., Keskinen, M.J., Response of the equatorial ionosphere in the South Atlantic region to the great magnetic storm of July 15, Geophysical Research Letters 28, Foster, J.C., Erickson, P.J., Coster, A.J., Goldstein, J., Rich, F.J., Ionospheric signatures of plasmaspheric tails. Geophysical Research Letters 29 (13), Greenspan, M.E., Rasmussen, C.E., Burke, W.J., Abdu, M.A., Equatorial density depletions observed at 840 km during the great magnetic storm of March Journal of Geophysical Research 96, Haerendel, G., Eccles, J.V., Cakir, S., Theory for modeling the equatorial evening ionosphere and the origin of the shear in the horizontal plasma flow. Journal of Geophysical Research 97, Kelley, M.C., Vlasov, M.N., Foster, J.C., Coster, A.J., A quantitative explanation for the phenomenon known as stormed-enhanced density. Geophysical Research Letters 31, L Keskinen, M.J., Ossakow, S.L., Basu, A., Sultan, P.J., Magnetic-flux-tube-integrated evolution of equatorial ionospheric plasma bubbles. Journal of Geophysical Research 103,

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