J. Space Weather Space Clim. 2017, 7, A31 © S.W.Y. Tam et al., Published by EDP Sciences 2017 DOI: 10.1051/swsc/2017030 Available online at: www.swsc-journal.org
RESEARCH ARTICLE
Variations of topside ionospheric electron density near the dawn terminator in relation to geomagnetic activity
Sunny W.Y. Tam1,*, Chien-Han Chen1,2 and Kaiti Wang3 1 Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan, Taiwan 2 Now at National Property Administration, Ministry of Finance, Taiwan 3 Department of Aerospace Engineering, Tamkang University, New Taipei City, Taiwan
Received 30 April 2017 / Accepted 20 October 2017
Abstract – A statistical study to determine the influence of geomagnetic disturbances on the ionosphere across the dawn terminator at subauroral and middle latitudes is performed, based on the vertical electron density profiles measured by the GPS Occultation Experiment aboard the FORMOSAT-3/COSMIC satellites from August 2006 to July 2009. Three ranges of solar zenith angles are adopted to characterize transitions between the pre- and post-dawn ionosphere. Results indicate opposing plasma density effects at the darkened and sunlit locations between 50° and 65° magnetic latitude (lm). The darkened topside ionosphere features density increases associated with geomagnetic activity, while density reductions mark its sunlit counterpart. The average electron peak density in the F2 region can increase by up to 44% in the darkened ionosphere and decrease by up to 20% in the sunlit ionosphere as Kp changes from 0–1to4–5. In the lm =55°–65° range, the dominant contributors to the density perturbation are auroral electron precipitation for the darkened region and enhanced penetration electric fields for the sunlit region, with the transition occurring across the terminator local times. Dominance shifts first to electric fields at 50°–55°, then to aurora-induced neutral wind at 45°–50°, suggesting that during disturbed times electric fields seldom penetrate below lm =50°. Findings presented in this statistical study should contribute to the study of space weather and the understanding of non-local influences of geomagnetic disturbances on topside dynamics.
Keywords: solar zenith angle / penetration electric fields / auroral electron precipitation / neutral wind / dawnside ionosphere
1 Introduction of auroral particle precipitation (Mikkelsen, 1975; Hardy et al., 2008), and/or enhanced plasma convection (Foster, 1993; As human activities increasingly rely on space-based Heelis et al., 2009). While the last of the aforementioned technology, accurate prediction of the influence of geomag- mechanisms may sometimes cause ionospheric densities to netic disturbances on the topside ionosphere has become ever decrease (Astafyeva et al., 2016), negative perturbations on the more important. Unfortunately, that is difficult to achieve for density can also be associated with variations in thermospheric subauroral and middle magnetic latitudes. Previously reported compositions (Taeusch et al., 1971; Goncharenko et al., 2006), observations showed that at these latitudes the ionosphere equatorward migrations of mid-latitude troughs (Brace et al., manifests positive and negative plasma density perturbations 1974; Shagimuratov et al., 2002), and/or increased ion and during periods of geomagnetic activity (e.g. Buonsanto, 1999; neutral temperatures that enhance recombination rates and references therein). The observed positive and negative (Thomas & Norton, 1966; Pavlov & Buonsanto, 1997). perturbations are responses to multiple driving mechanisms Physical mechanisms responsible for density perturbations associated with geomagnetic disturbances. For example, during periods of enhanced geomagnetic activity were mostly fi positive density effects may be due to traveling atmospher- identi ed through case studies of individual storm or substorm ic/ionospheric disturbances (Prölss & Jung, 1978; Cai et al., events (e.g. Basu et al., 2001; Goncharenko et al., 2006). By 2012), variations in the global wind circulation (Jones & tracking plasma properties at given ionospheric locations Rishbeth, 1971; Ngwira et al., 2012), expansion of the region throughout disturbed periods, the case-study approach can reveal critical details about the local changes that allow investigators to identify causative physical mechanisms. Case *Corresponding author: [email protected] studies are essential for identifying and quantifying the diverse
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. S.W.Y. Tam et al.: J. Space Weather Space Clim. 2017, 7, A31
Fig. 1. Color plots indicating the number of EDPs in each data group sorted by solar zenith angle (SZA), Kp index, magnetic latitude, and MLT. White indicates groups with 30 profiles. and competing mechanisms that can operate in the ionosphere develop in the topside ionosphere before and after daybreak. during periods of high geomagnetic activity. However, if one Section 3 extends our analyses to investigate the possible roles were to predict how future geomagnetic disturbances would that plasma transport mechanisms play in the formation of perturb the topside ionosphere at subauroral and middle density perturbations. A discussion of our findings and latitudes where plasma densities may increase/decrease in conclusions is provided in Section 4. response to the different driving mechanisms (Klimenko et al., 2012), it is essential to have knowledge about which of the competing mechanisms is most likely to be dominant under the 2 Statistical analysis of local density specific ionospheric conditions and to anticipate the positive or perturbations negative local density responses accordingly. The required practical knowledge can only be obtained through observa- The FORMOSAT-3/COSMIC mission consists of a tionally based statistical studies. constellation of six identical micro-satellites (Cheng et al., This study examines the statistical properties of perturba- 2006). Vertical EDPs obtained from GOX aboard the F3C tions in vertical electron density profiles (EDPs) observed near satellites cover a broad range of locations in the global the dawn meridian at subauroral latitudes due to geomagnetic ionosphere. Similar to the approach by Wang & Tam (2010),in activity. The studied EDPs, corresponding to the solar- which EDPs were sorted using combinations of three minimum period between August 2006 and July 2009, were independent parameters: magnetic latitude, magnetic local measured by the GPS occultation experiment (GOX) aboard time, the Dst index in their study of the effects of the FORMOSAT-3/COSMIC (F3C) satellites based on radio geomagnetic disturbances on the low-latitude ionosphere, this occultation (RO) techniques for probing the ionosphere study combines four sorting criteria to divide EDPs into 144 (Schreiner et al., 1999). The objectives of this study are to groups. The first two parameters are magnetic latitude lm identify typical trends of density perturbations in the local (henceforth simply referred to as “latitude”) and magnetic ionosphere and establish their statistical properties as local time (MLT). This study examines the locations in the indicators of locally dominant, disturbance mechanisms. northern hemisphere across the ranges lm =45°–65° and Section 2 presents statistical results of EDP analyses that MLT = 3:00–7:00. Data at these magnetic locations are divided demonstrate qualitatively different density perturbations into two-dimensional ranges of 5° in latitude and one hour in
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