Browsing by Author "Reeves, G. D."
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Item Open Access Contribution of storm time substorms to the prompt electric field disturbances in the equatorial ionosphere(American Geophysical Union, 2017-04-18) Hui, Debrup; Chakrabarty, D.; Sekar, R.; Reeves, G. D.; Yoshikawa, A.; Shiokawa, K.This study tries to bring out the fact that storm time substorms can compete and at times significantly contribute to the geomagnetically disturbed time prompt penetration electric field effects on low and equatorial latitudes. Observations of unusual equatorial plasma drift data from Jicamarca Unattended Long‐term Investigations of the Ionosphere and Atmosphere during two space weather events show that substorms can induce both eastward and westward penetration electric fields under steady southward interplanetary magnetic field (IMF Bz) conditions. During the first event on 2 January 2005, the enhancement of the daytime eastward electric field over Jicamarca due to substorm is found to be comparable with the Sq and interplanetary electric field (IEFy) generated electric fields combined. During the second event on 19 August 2006, the substorm is seen to weaken the daytime eastward field thereby inducing a westward field in spite of the absence of northward turning of IMF Bz (overshielding). The westward electric field perturbation in the absence of any overshielding events is observationally sparse and contrary to the earlier results. Further, the substorm‐induced field is found to be strong enough to compete or almost nullify the effects of storm time IEFy fields. This study also shows quantitatively that at times substorm contribution to the disturbed time prompt electric fields can be significant and thus should be taken into consideration in evaluating penetration events over low latitudes.Item Restricted Coordinated observations of magnetospheric reconfiguration during an overshielding event(American Geophysical Union, 2008-08-14) Wei, Y.; Hong, M.; Wan, W.; Du, A.; Pu, Z.; Thomsen, M. F.; Ren, Z.; Reeves, G. D.During 1400–1800 UT on September 15, 2005, the solar wind exhibited large variations and caused a moderate magnetic storm. The Jicamarca incoherent scatter radar, located near the local noon, observed an overshielding event after the northward turning of interplanetary magnetic field. The overshielding event featured distinct increase and decrease phases. We have examined the magnetospheric configuration variation with the geosynchronous observations provided by GOES12 (dayside) and LANL 97A (nightside), as well as the tail magnetic field monitored by Double Star TC1. The results suggested that the time‐dependent magnetospheric reconfiguration process is closely related to the development of equatorial electric field. We speculate that the magnetospheric reconfiguration may cause equatorial ionosphere electric field disturbances through affecting the Region 2 field aligned current.Item Restricted Role of IMF By in the prompt electric field disturbances over equatorial ionosphere during a space weather event(American Geophysical Union, 2017-02-04) Chakrabarty, D.; Hui, Debrup; Rout, Diptiranjan; Sekar, R.; Bhattacharyya, Archana; Reeves, G. D.; Ruohoniemi, J. M.On 7 January 2005 (Ap=40) prompt penetration electric field perturbations of opposite polarities were observed over Thumba and Jicamarca on a few occasions during 13:45–16:30 UT. However, the electric field was found to be eastward during 14:45–15:30 UT over both Thumba and Jicamarca contrary to the general expectation wherein opposite polarities are expected at nearly antipodal points. On closer scrutiny, three important observational features are noticed during 14:10–15:15 UT. First, during 14:10–14:45 UT, despite increasing southward interplanetary magnetic field (IMF) Bz condition, the already westward electric field over Thumba weakened (less westward) while the eastward electric field over Jicamarca intensified (more eastward). Second, the electric field not only became anomalously eastward over Thumba but also got intensified further during 14:45–15:00 UT similar to Jicamarca. Third, during 15:00–15:15 UT, despite IMF Bz remaining steadily southward, the eastward electric field continued to intensify over Thumba but weakened over Jicamarca. It is suggested that the changes in IMF By component under southward IMF Bz condition are responsible for skewing the ionospheric equipotential patterns over the dip equator in such a way that Thumba came into the same DP2 cell as that of Jicamarca leading to anomalous electric field variations. Magnetic field measurements along the Indian and Jicamarca longitude sectors and changes in high‐latitude ionospheric convection patterns provide credence to this proposition. Thus, the present investigation shows that the variations in IMF By are fundamentally important to understand the prompt penetration effects over low latitudes.Item Open Access Variations of low-latitude geomagnetic fields and Dst index caused by magnetospheric substorms(American Geophysical Union, 2004-05-25) Huang, Chao-Song; Foster, J. C.; Goncharenko, L. P.; Reeves, G. D.; Chau Chong Shing, Jorge Luis; Yumoto, K.; Kitamura, K.We present observations of periodic magnetospheric substorms and corresponding ionospheric disturbances. Since the periodic substorms occur during a stable interplanetary magnetic field, we are able to identify which ionospheric signatures are caused solely by substorms. We find that the low-latitude ionospheric electric field perturbation after substorm onsets is eastward on the dayside and westward on the nightside and that the ground magnetometer northward (H) deviations at middle and low latitudes show an increase (a positive bay) after each substorm onset, no matter whether the magnetometers are located on the dayside or on the nightside. The nightside magnetometer H deviations are closely correlated with the inner magnetospheric magnetic field Bz component during the dipolarization process. The Dst index shows a significant increase of 20–40 nT after each substorm onset. We propose that the increase in the magnetometer H field and Dst index in response to substorm onsets is related to the field dipolarization. In this scenario the nightside magnetosphere earthward of the near-Earth neutral line is highly compressed during the dipolarization, and the magnetic flux density within the inner magnetosphere is greatly enhanced, resulting in an increase in the ground magnetometer H component and in Dst.