Method to locate the polar cap boundary in the nightside ionosphere and application to a substorm event

Abstract

Abstract. In this paper we describe a new method to be used for the polar cap boundary (PCB) determination in the nightside ionosphere by using the EISCAT Svalbard radar (ESR) field-aligned measurements by the 42-m antenna and southward directed low-elevation measurements by the ESR 32m antenna or northward directed low-elevation measurements by the EISCAT VHF radar at Tromsø. The method is based on increased electron temperature (T e ) caused by precipitating particles on closed field lines. Since the Svalbard field-aligned measurement provides the reference polar cap T e height profile, the method can be utilised only when the PCB is located between Svalbard and the mainland. Comparison with the Polar UVI images shows that the radar-based method is generally in agreement with the PAE (poleward auroral emission) boundary from Polar UVI.The new technique to map the polar cap boundary was applied to a substorm event on 6 November 2002. Simultaneous measurements by the MIRACLE magnetometers enabled us to put the PCB location in the framework of ionospheric electrojets. During the substorm growth phase, the polar cap expands and the region of the westward electrojet shifts gradually more apart from the PCB. The substorm onset takes place deep within the region of closed magnetic field region, separated by about 6-7 • in latitude from the PCB in the ionosphere. We interpret the observations in the framework of the near-Earth neutral line (NENL) model of substorms. After the substorm onset, the reconnection at the NENL reaches within 3 min the open-closed field line boundary and then the PCB moves poleward together with the poleward boundary of the substorm current wedge. The poleward expansion occurs in the form of individual bursts, which are separated by 2-10 min, indicating that the reconnection in the magnetotail neutral line is impulsive. The poleward expansions of the PCB are followed by latitude dispersed intensifications inCorrespondence to: A. T. Aikio (anita.aikio@oulu.fi) the westward electrojet with high latitudes affected first and lower latitudes later. We suggest that reconnection bursts energize plasma and produce enhanced flows toward the Earth. While drifting earthward, part of the plasma population precipitates to the ionosphere producing latitude-dispersed enhancements in the WEJ.

Comparison of the open-closed field line boundary location inferred using IMAGE-FUV SI12 images and EISCAT radar observations

“…By itself, this is not a determination of the PCB location, nor of the OCB location. However, this boundary depends on the ionospheric current system and on the ionization produced by the auroral precipitation, so that the motion of this boundary can be expected to mimic that of the OCB, and in some circumstances, the boundaries may well match each other (Amm et al, 2003;Aikio et al, 2006Aikio et al, , 2008.…”

“…The second method used in this paper to estimate the PCB location utilizes EISCAT radar facility observations of the ionospheric electron temperature (Aikio et al, 2006). The energy flux of electron precipitation inside the oval is typically larger than that of the polar rain precipitating in the polar cap, resulting in an elevated electron temperature T e within the oval.…”

“…Several techniques have been considered, based on observations of the auroral emissions (Blanchard et al, 1995;Wild et al, 2004;Hubert et al, 2006a;Boakes et al, 2008), in situ particle detection (Newell et al, 1991;Blanchard et al, 1997), analysis of radar backscatter from the moving ionospheric plasma (Milan et al 2003, and references therein; Chisham et al, 2005), or a combination of in situ particle measurements and radar backscatter analysis (Chisham et al, 2004). Estimates of the electron temperature were also used by Østgaard et al (2005) and Aikio et al (2006) to determine the location of the polar cap boundary. Newell et al (1991) characterized the properties of the precipitating auroral particles as measured using the DMSP satellites to estimate the location of the PCB.…”

“…The electron temperature enhancement in the nightside ionosphere has also been used as a proxy for the OCB by Østgaard et al (2005), who combined FUV images of the electron aurora with electron temperature measurements by the EISCAT radar. This method was extended by Aikio et al (2006) as discussed in more detail in Sect. 2.…”

Determining the boundaries of the auroral oval from CHAMP field-aligned current signatures – Part 1

“…The boundaries of the auroral oval can be determined locally by ground-based observations such as radars (Pinnock and Rodger, 2000;Moen et al, 2004;Aikio et al, 2006) or low-altitude satellite observations (Newell et al, 1996;Wang et al, 2005), while optical imagers from high-altitude satellites can provide an instantaneous global view of the auroral oval (Newell et al, 2001;Østgaard et al, 2007). There are also some models for predicting the location of the auroral oval (e.g.…”