Abstract
Abstract. We compare the location of the polar cap boundary (PCB) determined using two different techniques, and use them as proxies for the open-closed field line boundary (OCB). Electron temperatures from observations of the EISCAT radar facility are used to estimate the latitude of the PCB along the meridian of the EISCAT VHF beam. The second method utilizes global images of proton aurora obtained by the IMAGE satellite FUV SI12 instrument. These methods are applied to three different intervals. In two events, the agreement between the methods is good and the mean of the difference is within the resolution of the observations. In a third event, the PCB estimated from EISCAT data is located several degrees poleward of that obtained from the IMAGE FUV SI12 instrument. Comparison of the reconnection electric field estimated from the two methods shows that high-resolution measurements both in time and space are needed to capture the variations in reconnection electric field during substorm expansion. In addition to the two techniques introduced above to determine the PCB location, we also use a search for the location of the reversal of the east-west component of the equivalent current known as the magnetic convection reversal boundary (MCRB). The MCRB from the MIRACLE magnetometer chain mainly follows the motion of the polar cap boundary during different substorm phases, but differences arise near the Harang discontinuity.
Highlights
Techniques which allow the identification of the the polar cap boundary (PCB), the boundary that separates the ionospheric polar cap from the auroral oval, are used as a proxy for the open-closed field line boundary (OCB) to analyze the cycle of accumulation and release of open magnetic flux and energy by the magnetosphere in its interaction with the solar wind
We have presented a comparison between the PCB location determined using Imager for Magnetopause to Aurora Global Exploration (IMAGE)-Far Ultraviolet (FUV) Spectrographic Imager at 121.8 nm (SI12) and EISCAT radar data, and have compared these with the magnetic convection reversal boundary (MCRB) deduced from the MIRACLE magnetometers
On 22 September 2001 and 17 February 2002 the average difference between the OCB proxies obtained from EISCAT and SI12 data are within the resolutions of observations, but individual proxies may differ even by several degrees
Summary
Techniques which allow the identification of the the polar cap boundary (PCB), the boundary that separates the ionospheric polar cap from the auroral oval, are used as a proxy for the open-closed field line boundary (OCB) to analyze the cycle of accumulation and release of open magnetic flux and energy by the magnetosphere in its interaction with the solar wind. The main difficulty comes from the fact that the boundary itself is not directly observable, involving a change in the topology of the magnetic field lines linking differing regions of space This can only be inferred from the resulting differences in the plasma populations, as observed through auroral emissions, heating, direct detection of particles etc., so that one rather identifies the polar cap boundary (PCB), used as a proxy for the OCB. Low-elevation VHF radar observations from Tromso (corrected geomagnetic coordinates: 66.6◦ MLAT, 103◦ MLON) in the poleward direction are used to identify the most poleward location that has higher Te than at the corresponding altitude within the polar cap, measured by the field-aligned EISCAT Svalbard radar (ESR) at Longyearbyen This estimate of the PCB location is taken as a proxy for the OCB location. We analyze three intervals for which these data are available, namely 17:30 to 23:15 UT on 22 September 2001, 20:30 UT on February to 00:30 UT on February 2002, and 20:20 to 22:00 UT on 7 December 2000
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