Transition region of TEC enhancement phenomena during geomagnetically disturbed periods at mid-latitudes

K Unnikrishnan,M Yamamoto,Y Otsuka,A Saito,S Fukao

doi:10.5194/angeo-23-3439-2005

K Unnikrishnan, M Yamamoto + Show 3 more

Open Access

https://doi.org/10.5194/angeo-23-3439-2005

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Abstract

Abstract. Large-scale TEC perturbations/enhancements observed during the day sectors of major storm periods, 12-13 February 2000, 23 September 1999, 29 October 2003, and 21 November 2003, were studied using a high resolution GPS network over Japan. TEC enhancements described in the present study have large magnitudes (≥25×1016 electrons/m2) compared to the quiet-time values and long periods (≥120 min). The sequential manner of development and the propagation of these perturbations show that they are initiated at the northern region and propagate towards the southern region of Japan, with velocities >350 m/s. On 12 February 2000, remarkably high values of TEC and background content are observed at the southern region, compared to the north, because of the poleward expansion of the equatorial anomaly crest, which is characterized by strong latitudinal gradients near 35° N (26° N geomagnetically). When the TEC enhancements, initiating at the north, propagate through the region 39-34° N (30-25° N geomagnetically), they undergo transitions characterized by a severe decrease in amplitude of TEC enhancements. This may be due to their interaction with the higher background content of the expanded anomaly crest. However, at the low-latitude region, below 34° N, an increase in TEC is manifested as an enhanced ionization pattern (EIP). This could be due to the prompt penetration of the eastward electric field, which is evident from high values of the southward Interplanetary Magnetic Field component (IMF Bz) and AE index. The TEC perturbations observed on the other storm days also exhibit similar transitions, characterized by a decreasing magnitude of the perturbation component, at the region around 39-34° N. In addition to this, on the other storm days, at the low-latitude region, below 34° N, an increase in TEC (EIP feature) also indicates the repeatability of the above scenario. It is found that, the latitude and time at which the decrease in magnitude of the perturbation component/amplitude of the TEC enhancement are matching with the latitude and time of the appearance of the high background content. In the present study, on 12 February 2000, the F-layer height increases at Wakkanai and Kokubunji, by exhibiting a typical dispersion feature of LSTID, or passage of an equatorward surge, which is matching with the time of occurrence of the propagating TEC perturbation component. Similarly, on 29 October 2003, the increase in F-layer heights by more than 150km at Wakkanai and 90 km at Kokubunji around 18:00 JST, indicates the role of the equatorward neutral wind. On that day, TEC perturbation observed at the northern region, after 18:30 JST, which propagates towards south, could be caused mainly by the equatorward neutral wind, leading to an F-layer height increase. These observations imply the role of the equatorward neutral wind, which increases the F-layer height, by lifting the ionization to the regions of lower loss during daytime, when production is still taking place, which, in turn, increases the TEC values. Large-scale traveling ionospheric disturbances (LSTIDs) are considered as ionospheric manifestations of the passage of Atmospheric Gravity Waves (AGWs) that are generated at the high latitude by energy input from the magnetosphere to the low-latitude ionosphere. This study shows that large-scale TEC perturbations observed here are produced at the northern region due to the combined effects of the equatorward neutral wind, the subsequent F-layer height increase, and LSTIDs. When these perturbation components propagate through the region, 39-34° N, they undergo transitions characterised by a decrease in magnitude. Also, at the low-latitude region, below 34° N, an increase in the TEC exhibits EIP feature, due to the prompt penetration of the eastward electric field.

Highlights

In the last decade, several new features, like turbulent upwelling, electrodynamics of spread F, airglow observations of mesoscale low-velocity traveling ionospheric disturbances (TIDs), damping of Large-scale traveling ionospheric disturbances (LSTIDs), etc., of the mid-latitude ionosphere have been discovered by observations of radars, satellites, all-sky imagers and Global Positioning System (GPS) receivers (Fukao et al, 1991; Miller et al, 1997; Garcia et al, 2000)
On 12 February 2000, remarkably high values of TEC and background content are observed at the southern region, compared to the north, because of the poleward expansion of the equatorial anomaly crest, which is characterized by strong latitudinal gradients near 35◦ N (26◦ N geomagnetically)
This could be due to the prompt penetration of the eastward electric field, which is evident from high values of the southward Interplanetary Magnetic Field component (IMF Bz) and AE index

Summary

Introduction

Several new features, like turbulent upwelling, electrodynamics of spread F, airglow observations of mesoscale low-velocity traveling ionospheric disturbances (TIDs), damping of LSTIDs, etc., of the mid-latitude ionosphere have been discovered by observations of radars, satellites, all-sky imagers and GPS receivers (Fukao et al, 1991; Miller et al, 1997; Garcia et al, 2000). Ho et al (1996) studied the global distribution of TEC variations during a magnetically disturbed period with more than 60 worldwide GPS receivers. TEC variations in a local area have been studied with GPS receivers in equatorial (Kelley et al, 1996; Musman et al, 1997), mid-latitude (Beach et al, 1997), and high-latitude (Aarons, 1997) regions. It is known that TIDs are the ionospheric manifestations of the passage of Atmospheric Gravity Waves (AGWs) that are generated at the high latitude by energy input from magnetosphere to the low-latitude ionosphere. Saito et al (1998; 2001; 2002) have conducted studies on nighttime TIDs propagating through the Japanese mid-latitude sector with velocities up to 150 m/s, which produce a magnitude of perturbation structures (with respect to the background content) up to 1 TECU (1 TECU=1016 electrons/m2)

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Conclusion