Understanding the unique equatorial electrodynamics in the African Sector

Transcription

1 Understanding the unique equatorial electrodynamics in the African Sector Endawoke Yizengaw, Keith Groves, Tim Fuller-Rowell, Anthea Coster Science Background Satellite observations (see Figure 1) show unique equatorial ionospheric structures only in the African sector [Hei et al., 2005; Su, 2005; Burke et al., 2006; Yizengaw et al., 2010]. It has been observed that in this region the bubbles are much deeper and occur more frequently than bubbles observed in any other longitudinal sector [Hei et al., 2005 and Su, 2005]. It was also reported that the depletions in the African region rise to high altitudes (up to km) more frequently compared to other longitudes [Burke et al., 2004]. However, these observations have not been confirmed, validated or studied in detail by observations from the ground due to lack of suitable ground-based instrumentation in the region, and the question of what causes or drives these unique density irregularities in the region is still not yet fully understood. Figure 1. Satellite data suggest both the spatial structure and seasonal behavior of disturbances above Africa are unique. (left panels) seasonal behavior of the density irregularity occurrence probability using ROCSAT in situ density observation [after Su, 2005]. (right panels) the typical bubbles observation using AEE satellite in situ density, occurring in the fall (August, September, October (ASO)): (a) and (b) in the Pacific sector and (c) in the African sector [after Hei et al., 2005]. Despite the fact that much progress has been made in the study of ionospheric density structure and dynamics in the last decade, there are many gaps in our global understanding of the fundamental electrodynamics that governs equatorial ionospheric density irregularities. The uneven distribution of ground-based instruments has been the main barrier that hinders our ability to obtain a global understanding of the dynamics and structures of the ionosphere. Although ground-based instrumentations, such as GPS receivers and magnetometers are located in dense regional arrays, these arrays are primarily in North America and Europe, and only recently with the LISN network in South America. However, in regions like Africa, observations 1

2 of the ionosphere are currently not possible due to lack of ground-based instruments. Because of this, detailed investigation and validation of those unique dynamics and structure of the thermosphere and ionosphere in the African region cannot be done. The ionospheric density structure in the African sector has been traditionally estimated by model interpolation over vast geographic areas. This makes it very difficult to observe small-to-medium scale ionospheric irregularities, such as equatorial plasma bubbles, equatorial spread F (ESF), and bottom side spread F (BSSF) at low/mid-latitudes in the Figure 2 Available ground-based instruments in the African sector. region. In fact, our communication and navigation technologies depend on understanding, modeling, and mitigating the effects of these irregularities [Doherty et al., 2004]. For example, when scintillation (the rapid amplitude and phase fluctuations of radio signals from space due to turbulence generated by ionospheric irregularities) [Groves et al., 1997] happens the following technological systems will be affected: (1) regional SATCOM outages for extended periods (hours), (2) increased Global Positioning Satellite (GPS) navigation errors, (3) degraded High Frequency (HF) radio communication. Required instrumentation To have a complete and global understanding of the electrodynamics of the ionosphere in a cost-effective way, the deployment of small instruments, like ground-based GNSS receivers, is essential. GNSS receivers have a capacity to receive signals from GLONASS, COMPASS, EGNOS, in addition from the more popular GPS satellite constellations, providing higher accuracy and more reliable information in all environments. Therefore, for the upcoming decadal science objectives we propose the expansion of the ground-based instrumentation in the African sector, a region that has been severely lacked ground-based instrumentation for space science (see Figure 2). As can be seen in the figure, a significant number of GPS receivers have been deployed by the Tectonics community, but these receivers can also serve as important data resources for the space science community. However, the most dynamic region in terms of ionospheric irregularities, in the northern and equatorial part of Africa is still devoid of groundbased GPS receivers. Therefore, closing the largest land-based gaps in the northern and equatorial region of Africa will play a vital role in studying the fundamental governing electrodynamics of equatorial ionospheric motion. In addition, augmenting the proposed GNSS deployment in this region with magnetometer networks will strengthen our investigations. This is because, magnetometer can provide information about the electrodynamics that governs the equatorial ionospheric motion [Anderson et al., 2004]. Moreover, three ionosondes are either deployed or soon to be deployed in the equatorial region of the African continent. The beauty of these ionosondes is that they provide 2

3 important geophysical parameters, such as winds, electric fields, conductivity, and density profiles, from the E- and F- ionosphere for the first time in the African sector [Valladares et al., 2004; Abdu et al., 2006]. Adding a single incoherent scatter radar (ISR) to this mix of space science instrumentation in the equatorial African sector would be of significant scientific benefit. ISR s deduce height- and time-resolved plasma drift velocities, electron and ion temperatures, electron densities, ion composition, and ion-neutral collision frequencies. These parameters provide further information about the neutral gas, neutral temperatures and winds, and electric fields present in the medium. No other instrument provides all of this ionospheric information. An ISR in Africa would greatly enhance the physics output of these other instruments and would provide information that would greatly enhance our understanding of equatorial electrodynamics. To date, the vast majority of equatorial ISR measurements have been collected at Jicamarca, so that much of what we know about equatorial physics is based on Jicamarca ISR observations. However, Jicamarca is in the American sector where the geomagnetic equator dips, and there is a fairly large excursion between the geomagnetic and geodetic equator. On the other hand, in the African sector the geomagnetic equator is fairly well aligned with the geodetic equator. New data from satellites (e.g. C/NOFS) have also indicated that the equatorial ionosphere in the African sector responds differently than other sectors. Therefore, having ISR in the African sector will significantly enhance our understanding of the physics behind the global equatorial electrodynamics. Finally, there is a fast growing number of LEO satellites that are equipped with GPS receivers that will also augment the proposed ground-based GNSS network by providing additional information about the structure and dynamics of topside ionospheric and plasmaspheric density [Mannucci et al., 2005; Yizengaw et al., 2006]. In general, within the next decade the combined ground- and space-based observations will allow us, for the first time in the region, to answer the following longstanding specific questions. Relevant Critical problems to be addressed With the deployment of the proposed GPS receivers along with other available ground- and space-based multi-instruments, the space science community will be able to address the following main and major critical scientific problems. 1. What is special in Africa that produces unique ionospheric irregularities, such as the largescale bubble properties (zonal width, depletion level, and spacing)? Is it because of special seeding conditions? Or because of special electrodynamics in the region? 2. Why does the depleted plasma (or bubbles) penetrate to higher altitudes in the African sector than in any other sectors? Does it relate to the ambient ionospheric plasma uplift? How often and under what conditions does vertical bubble penetration happen? What controls the lifetime of the plasma bubbles? What is the role of gravity waves in influencing the upward extension of the bubbles? 3. What are the possible governing mechanisms that create unique equatorial structures in Africa? 4. How intense and widely separated is the equatorial ionospheric anomaly (EIA) in the African sector? How often do such widely separated intense EIA structures occur in the African sector? Is it a year-round phenomenon as the occurrence of bubbles appears to be? 5. What is the role of the neutral atmosphere, including winds, tides and waves, in creating the unique equatorial electrodynamics of the African ionosphere? What are the characteristics of the neutral winds over Africa in the context of the global four-cell non- 3

4 migrating tide structure? Do neutral winds provide seeding that may account for the unique characteristics of African bubbles? These questions are relevant broadly to the global ionospheric community, but given the intensity of local terrestrial weather forcing in Africa and the aforementioned characteristics of the African ionosphere, we believe it may be possible to establish an unambiguous connection between the terrestrial and space weather domains in Africa. How to address the Critical problems? These fundamental problems can be addressed by filling the critical ground-based data gaps. While the GPS and ionosondes observe the dynamics and structure of ionospheric density (such as bubbles and scintillation activities), magnetometers and ionosondes will estimate the driving force ( E B drift) simultaneously. More importantly, in addition to ground-based GPS receivers, in situ density observations track the bubbles at different altitudes; such data can be supplied from in situ density observations by LEO satellites, such as C/NOFS (400 x 800 km altitude) and DMSP (~850 km altitude). This will provide information about the altitude penetration of the bubbles which occurs quite often in the African sector [Burke et al., 2004]. Moreover, with the proposed ISR the plasma drift velocities, electron and ion temperatures, electron densities, ion composition, and ion-neutral collision frequencies as function of altitude and time will be measured or estimated. The ISR would also provide the altitude structural images of the equatorial bubbles and ionospheric irregularities. We will also use the opportunity of the combined ground- and space-based instruments in the African region to compare equatorial electrodynamics with similar events at other longitudes and, specifically, the global response to major magnetic storms. Impact on Research Infrastructure and Education This proposal has significant broader impacts in creating international research collaborations. In addition to its scientific importance, this proposal will have other imperative impacts for both the U.S. and Africa. It has direct impact in advancing space science research into Africa by establishing and furthering sustainable research/training infrastructure within Africa so that more young scientists will be educated in their own country. This will not only enhance the research/training infrastructure but it will also play a vital role in the future socioeconomic development of Africa. For the United State, this project will serve as a vehicle to create opportunities for graduate and undergraduate students to participate in different phases of the development and deployment process. This will provide US students with high quality international research experiences, which will be quite important for the US scientific community to continue its leading international scientific stature in the future. The experience may provide opportunities for students to gain expertise in instrument calibration, deployment, data analysis, remote-sensing, computer modeling, data assimilation, and grid-based computing and data retrieval. 4

5 References Abdu, M. A., T. K. Ramkumar, I. S. Batista, C. G. M. Brum, H. Takahashi, B. W. Reinisch, and J. H. A. Sobral (2006), Planetary wave signatures in the equatorial atmosphere ionosphere system, and mesosphere- E- and F-region coupling, J. Atmos. Solar-Terr. Phys., 68(3-5), Anderson, D., A. Anghel, J. Chau, and O. Veliz (2004), Daytime vertical E B drift velocities inferred from ground-based magnetometer observations at low latitudes, Space Weather, 2, S11001, doi: /2004sw Burke, W. J., L. C. Gentile, C. Y. Huang, C. E. Valladares, and S. Y. Su (2004), Longitudinal variability of equatorial plasma bubbles observed by DMSP and ROCSAT-1, J. Geophys. Res., 109, A12301, doi: /2004ja Doherty, P., A. J. Coster, and W. Murtagh (2004), Space weather effects of October November 2003, GPS Sol., 8(3), doi: /s Groves, K. M., S. Basu, E. J. Weber, M. Smitham, H. Kuenzler, C. E. Valladares, R. Sheehan, E. MacKenzie, J. A. Secan, P. Ning, W. J. McNeill, D. W. Moonan, and M. J. Kendra (1997), Equatorial scintillation and systems support, Radio Sci., 32(5), , doi: /97rs Hei, M. A., R. A. Heelis, and J. P. McClure (2005), Seasonal and longitudinal variation of largescale topside equatorial plasma depletions, J. Geophys. Res., 110, A12315, doi: /2005ja Mannucci, A. J., B. T. Tsurutani, B. A. Iijima, A. Komjathy, A. Saito, W. D. Gonzalez, F. L. Guarnieri, J. U. Kozyra, and R. Skoug (2005), Dayside global ionospheric response to the major interplanetary events of October 29 30, 2003 Halloween Storms, Geophys. Res. Lett., 32, L12S02, doi: /2004gl Valladares, C. E., R. Sheehan, and J. Villalobos (2004), A latitudinal network of GPS receivers dedicated to studies of equatorial spread F, Radio Sci., 39, RS1S23, doi: /2002rs Yizengaw, E., M. B. Moldwin, A. Mebrahtu, B. Damtie, E. Zesta, C. E. Valladares, and P. H. Doherty (2010), Comparison of storm time equatorial ionospheric electrodynamics in the African and American sectors, J. Atmos. Solar-Terr. Phys., in press. Yizengaw, E., M. B. Moldwin, P. L. Dyson, and T. J. Immel (2005), Southern Hemisphere ionosphere and plasmasphere response to the interplanetary shock event of October 2003, J. Geophys. Res., 110, A09S30, doi: /2004ja Yizengaw, E., M. B. Moldwin, P. L. Dyson, B. J. Fraser, and S. K. Morley (2006), First Tomographic Images of Ionospheric Outflows, Geophys. Res. Lett., 33, L20102, doi: /2006gl