Initial deflection of middle‐latitude Pi2 pulsations in the premidnight sector: Remote detection of oscillatory upward field‐aligned current at substorm onset

Abstract

In this study, we examined middle‐ and low‐latitude Pi2 events to address the following two issues regarding the well‐known substorm current wedge (SCW) model for Pi2 pulsations: (1) the center of the SCW, which is estimated using the Pi2 polarization pattern, is not always collocated with that determined using the magnetic bay pattern; and (2) although ideally Pi2 hodograms would be linear, they tend to become circular. In this study, auroral breakup events were identified from Polar Ultraviolet Imager data. We assumed that the ionospheric footprint of the upward field‐aligned current (FAC) in each event was located at the position of the auroral breakup and subsequently calculated the signature of the magnetic variation at the middle‐latitude station Zyryanka (ZYK; GMLAT = 59.6°) that was generated by the upward FAC. In order to examine the magnetic effects of the upward FAC, we selected Pi2 events that were observed when ZYK was located on the duskward side of the auroral breakup location. A total of 112 events were selected and analyzed in this study. It was found that the location of the upward FAC of the SCW could be estimated more accurately by using an azimuth value predicted based on the initial deflection of the middle‐latitude Pi2. Our results suggest that the circular shapes of Pi2 polarization curves are caused by the delayed driven Alfvénic waves that are superimposed on the geomagnetic northward components of SCW oscillations.

Formation of a 3‐D Oscillatory Current System Associated With Global High‐Correlation Pi 2 Event: A Case Study

“…The middle-latitude Pi 2 pulsation in the D component detected the perturbation of the earthward portion of the upward and downward FACs in the SCW system. A major difference between T19 and our study is that T19 applied the oscillating FAC model proposed by Lester et al (1983) and Imajo et al (2015), but our study based on the oscillating FAC model developed by the successive study of Uozumi et al (2009Uozumi et al ( , 2011Uozumi et al ( , 2016 as introduced in section 1. Their observation confirmed that the presence of identical Pi 2 oscillations in the azimuthal components at topside ionosphere and ground.…”

“…Uozumi et al (2009) found that the H component of a high-latitude Pi 2 exhibits systematic group delays (|T| <~100 s) from the low-latitude Pi 2 in H. On the other hand, the delay time of the high-latitude Pi 2 in the D component to the low-latitude Pi 2 from the H component was not significant (|T|<~10 s) in the entire nighttime sector, and the high-latitude D component Pi 2s switched their polarity around 1 hr MLT dawnward of the auroral breakup center that was identified by global auroral image of the Polar satellite. Uozumi et al (2009Uozumi et al ( , 2011Uozumi et al ( , 2016 proposed a generation and propagation mechanism of global high-correlation Pi 2 event based on ground observations. Uozumi et al (2009Uozumi et al ( , 2011Uozumi et al ( , 2016 proposed a generation and propagation mechanism of global high-correlation Pi 2 event based on ground observations.…”

“…U09 applied the oscillating FAC model proposed by Lester et al (1983) to interpret the characteristics of the high-latitude Pi 2 in the D component. The present study is based on the oscillating FAC model developed by the successive studies of Uozumi et al (2009Uozumi et al ( , 2011Uozumi et al ( , 2016. What causes the mismatch between the SCW locations estimated by the McPherron and Lester methodology?…”

“…U16 pointed out that there remained open questions in Lester's model to be answered; that is, (1) the centers of the SCWs predicted based on the Lester's model were not always collocated with those predicted by the SCW model proposed by McPherron et al (1973). Uozumi et al (2009Uozumi et al ( , 2011Uozumi et al ( , 2016 proposed three possible wave elements that exist in global high-correlation Pi 2 events as follows: (1) fast-mode waves (δB FW ) that propagate from the Pi 2 source region in the nightside magnetosphere and observed mainly in low-latitude Pi 2 in the H component, (2) SCW oscillations (δB SCW ) observed mainly in the low-to high-latitude Pi 2 in the D component, and (3) directly driven Alfvénic waves (δB DA ) generated by δB FW through the mode-conversion process and observed as the main oscillation in the middle-to high-latitude Pi 2 in the H component with some group delays (see Uozumi et al, 2016, for more details). (2) Ideally, the ellipticity of a Pi 2 hodogram should be linear if the middle-latitude Pi 2 pulsation is directly associated with the substorm current system.…”

Nightside Magnetosphere‐Ionosphere Current Circuit: Implications for Auroral Streamers and Pi2 Pulsations

“…The Pi2 pulsations are a damped magnetic oscillation with a period of 40-150 s. They are observed at the onsets of substorms very often along with midlatitude positive bays. The spatial distributions of magnetic Journal of Geophysical Research: Space Physics 10.1002/2017JA024792 signatures suggest that the positive bays are the DC component of the substorm wedge current (e.g., Clauer & McPherron, 1974), whereas the Pi2's are its AC component (e.g., Lester et al, 1983;Uozumi et al, 2016). If the Pi2 pulsations are the manifestation of the damped oscillation of the substorm wedge current, it appears to be inconsistent with our conclusion that the M-I current circuit is overdamped.…”