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

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.

Relation of auroral substorm onset to local AL index and dispersionless particle injections

“…When the proton flux injections are binned according to MLT the earliest injection appears to be in the 22-01 MLT bin. This sector is similar to the location of dipolarization observed by the GOES geosynchronous spacecraft (Cai et al, 2006) and the magnetic field line mapped location of the auroral onset observed with Viking auroral images (Elphinestone et al, 1991). Our location of the proton injection, however, is duskward of the mean location determined from the Polar ENA images of seven isolated substorm injections (Reeves and Henderson, 2001).…”

“…Liou et al (1999Liou et al ( , 2000Liou et al ( , 2002 found that 65% of the Pi 2 pulsations begin about 0-3 min after the start of the auroral breakup identified with the Polar ultraviolet images, while the GOES satellites observed dipolarizations 1.772.7 min after this time. The most probable location of the geosynchronous location of this dipolarization was found to be about 23 MLT (Cai et al, 2006). When the Pi 2 pulsations were compared with the standard AL index, Hsu and McPherron (2003) and Hsu and McPherron (2007) indicate that there is no clear relationship between the start of the Pi 2 pulsations and the AL index.…”

M–I coupling across the auroral oval at dusk and midnight: repetitive substorm activity driven by interplanetary coronal mass ejections (CMEs)

“…However, during the expansion phase of certain substorm activities, including the category we discuss in this paper (see e.g. Cai et al, 2006;Henderson et al, 2006;and Andalsvik et al, 2011), the WEJ and the Harang reversal boundary expand westward across the 18:00 MLT meridian (Andalsvik et al, 2011), as we shall also demonstrate below. According to Nielsen and Greenwald (1979) the Harang discontinuity (our Harang reversal boundary) tended to be observed by the Stare radar in the 21:00-24:00 MLT sector, but it was observed as early as 18:30 MLT under very disturbed conditions.…”

Spatial distributions of electromagnetic field variations and injection regions during the 20 November 2007 sawtooth event

“…Based on a statistical study with GOES data, Cai et al (2006) argued that the dipolarisation is seen from 16 h to 4 h MLT, and the initiation is in the sector between 22 h and 0 h MLT. Kitamura et al (2005) deduced the substorm current wedge (SCW) of a full width (at full maximum) of ∼12 h in MLT from ground-based magnetometer data.…”