High‐latitude reconnection effect observed at the dayside dip equator as a precursor of a sudden impulse

D.-S Han,J Liang,T Iyemori,T Araki,H.-G Yang,S W H Cowley

doi:10.1029/2009ja014787

Abstract

Geomagnetic sudden impulses (SI) observed at the dayside dip equator normally show a decrease and then an increase in the H magnetic field component, i.e., a preliminary reverse impulse (PRI) followed by a main impulse. Using global geomagnetic field measurements, we examine an unusual SI event observed at the dayside dip equator, which shows a clear precursor ∼1 min before the PRI onset. The precursor was observed simultaneously at both the dayside dip equator and in the southern polar region but was not observed at all in the northern polar region. The global ground variations after the PRI onset were, however, consistent with a conventional SI model of the magnetospheric response to a sudden enhancement of the solar wind dynamic pressure. Considering that the interplanetary magnetic field Bx, By, and Bz components were positive with the By component dominant for this event, we suggest that the SI precursor was caused by high‐latitude magnetic reconnection that occurred only (or first) in the dawn quadrant of the southern hemisphere, the effect of which has for the first time been clearly identified at the dayside dip equator. This implies that electric field effects occurring in the polar ionosphere due to magnetopause reconnection may be rapidly monitored at the dayside dip equator. In addition, we argue that the quasi‐simultaneous (time difference is less than 10 s) appearance of the disturbance fields both in the polar region and at the dip equator confirmed in this study is extremely important for theoretically explaining the transmission of the polar electric field to the dayside dip equator, because different theories give different travel times for a disturbance field propagating from polar region to the dip equator. Our observations lend strong evidence for validity of the waveguide model.

Highlights

Introduction of the Unusual sudden impulses (SI)Event [8] Figure 2a shows the SI event studied in this paper as observed at Jicamarca (JIC, dip latitude = 0.0°) on the dayside on 20 March 2002 in the H component
[29] In this study, we examined a unique but significant SI event using extensive measurements of the geomagnetic field, together with spacecraft measurements and simulation results
What is unique is that the SI observed in the H component at the dayside dip equator shows a clear precursor prior to the preliminary reverse impulse (PRI) onset

Summary

Introduction

[2] It is well known that a sudden enhancement of the solar wind dynamic pressure (SEPdyn) can cause steplike increases in the geomagnetic H (northward) field component, both in the magnetosphere [e.g., Wang et al, 2007] and on the ground [e.g., Araki, 1994 and references therein]. By examining an unusual SI event using global magnetic field observations, we will show that a SEPdyn may trigger high‐latitude reconnection, and that the electric field in the polar ionosphere caused by the reconnection can be essentially instantaneously transmitted to the dayside dip equator.

Observations

Discussion

Summary and Conclusions