A statistical study of magnetic dipolarization for sawtooth events and isolated substorms at geosynchronous orbit with GOES data

X Cai,C R Clauer,M G Henderson

doi:10.5194/angeo-24-3481-2006

X Cai, C R Clauer + Show 1 more

Open Access

https://doi.org/10.5194/angeo-24-3481-2006

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Abstract

Abstract. We investigate whether the dipolarization process during sawtooth events is global or not through an examination of the local time distribution of the magnetic tilt angle (i.e. the angle between the B vector and the equatorial plane in dipole VDH coordinates) at geostationary orbit. From the statistical analysis of 207 individual teeth and 212 isolated substorms, we find that individual teeth follow a dipolarization pattern similar to that observed for isolated substorms. The dipolarization for individual teeth initiates in the sector between 22:00 magnetic local time (MLT) and 00:00 MLT. Then the dipolarization expands both eastward and westward. However, relative to isolated substorms, we find that the nightside magnetosphere is much more stretched prior to the onset, and the change of the tilt angle during the dipolarization process is much larger during individual teeth. The magnetic dipolarization is seen over a wider local time sector during individual teeth than during isolated substorms. However, the magnetic dipolarization is rarely observed near local noon during individual teeth. This suggests that the magnetic dipolarization process during individual teeth is still confined primarily to the nightside.

Highlights

Sawtooth events are so named due to their global and periodic variations, i.e. slow decreases followed by rapid increases, in the (>50 keV) energetic proton fluxes measured at geosynchronous orbit by the Synchronous Orbit Particle Analyzer (SOPA) instruments on the Los Alamos National Laboratory (LANL) geosynchronous satellites
It is generally accepted that the gradual decrease and rapid increase in energetic particle fluxes near local midnight at geosynchronous orbit is a typical feature of magnetospheric substorms (e.g. Sauvaud and Winckler, 1980)
For isolated substorms, the dipolarization is clearly seen in sectors 10, 11, and 0 (20:00 magnetic local time (MLT)– 02:00 MLT)

Summary

Introduction

It is generally accepted that the gradual decrease and rapid increase (i.e. particle injection) in energetic particle fluxes near local midnight at geosynchronous orbit is a typical feature of magnetospheric substorms (e.g. Sauvaud and Winckler, 1980). Cai et al.: Magnetic tilt angle for sawtooth and substorm dipolarization is typically observed when the satellite is approximately in the pre-midnight sector They argued that the dipolarization process during individual teeth is still approximately confined to the nightside. Borovsky et al (2006) statistically analyzed the magnetic-field rotation angle – which they defined as the final magnetic tilt angle minus the initial magnetic tilt angle – for 181 teeth from 1986 to 2002 using GOES data They concluded that the rotation angle for individual teeth is smaller at local noon than it is on the nightside but they did find signatures on the dayside. We undertake a statistical investigation of the averaged local time – universal time distribution of the magnetic tilt angle during individual teeth and during isolated magnetospheric substorms, using GOES magnetometer data at geostationary orbit. The local time extent of dipolarization process and the changes in tilt angle are investigated in detail using a superposed epoch technique

Data and methodology

Isolated substorms and individual teeth

The first tooth and all subsequent teeth

Conclusion and summary