SABRE observations of structured ionospheric flows during substorm expansion phase onset

Abstract. The irregularity velocity patterns observed by the SABRE coherent radar at substorm expansion phase onset, which is identified by magnetometer observations of Pi2 pulsations, are occasionally highly structured. In all the examples of structured velocity patterns examined, the SABRE viewing area is located at longitudes within the inferred substorm current wedge. Three types of structured velocity regime are apparent depending on the level of magnetic activity and the position of the radar viewing area relative to the substorm enhanced currents and the Pi2 pulsation generation region. Firstly, vortex-like velocity patterns are observed and these may be caused by the field-aligned currents associated with the substorm current wedge. Secondly, regions of equatorward velocity are also observed at times of substorm expansion phase onset moving longitudinally across the SABRE viewing area. The longitudinal movement is usually westward although an example of eastward motion has been observed. The phase velocity of these regions of equatorward flow is typically 1-3 km s-1. The observed equatorward velocities occur at the poleward edge or poleward of the background convection velocities observed by SABRE. These equatorward velocities may be related to the westward travelling surge and to the expansion (eastwards as well as westwards) of the brightening arc region at substorm onset. Thirdly, the flow rotates equatorward within the field of view but does not then appear to move longitudinally. These equatorward velocities may relate to the earthward surge of plasma from the magnetotail at substorm onset.

Download Full-text

SABRE observations of Pi2 pulsations: case studies

Annales Geophysicae ◽

10.1007/s00585-997-0040-3 ◽

1997 ◽

Vol 15 (1) ◽

pp. 40-53 ◽

Cited By ~ 2

Author(s):

E. G. Bradshaw ◽

M. Lester

Keyword(s):

Case Studies ◽

Magnetic Activity ◽

Auroral Zone ◽

Field Of View ◽

Ionospheric Currents ◽

Pi2 Pulsation ◽

First Case ◽

Pi2 Pulsations ◽

Generation Region

Abstract. The characteristics of substorm-associated Pi2 pulsations observed by the SABRE coherent radar system during three separate case studies are presented. The SABRE field of view is well positioned to observe the differences between the auroral zone pulsation signature and that observed at mid-latitudes. During the first case study the SABRE field of view is initially in the eastward electrojet, equatorward and to the west of the substorm-enhanced electrojet current. As the interval progresses, the western, upward field-aligned current of the substorm current wedge moves westward across the longitudes of the radar field of view. The westward motion of the wedge is apparent in the spatial and temporal signatures of the associated Pi2 pulsation spectra and polarisation sense. During the second case study, the complex field-aligned and ionospheric currents associated with the pulsation generation region move equatorward into the SABRE field of view and then poleward out of it again after the third pulsation in the series. The spectral content of the four pulsations during the interval indicate different auroral zone and mid-latitude signatures. The final case study is from a period of low magnetic activity when SABRE observes a Pi2 pulsation signature from regions equatorward of the enhanced substorm currents. There is an apparent mode change between the signature observed by SABRE in the ionosphere and that on the ground by magnetometers at latitudes slightly equatorward of the radar field of view. The observations are discussed in terms of published theories of the generation mechanisms for this type of pulsation. Different signatures are observed by SABRE depending on the level of magnetic activity and the position of the SABRE field of view relative to the pulsation generation region. A twin source model for Pi2 pulsation generation provides the clearest explanation of the signatures observed.

Download Full-text

Substorm behavior of the auroral electrojet indices

Annales Geophysicae ◽

10.5194/angeo-22-2135-2004 ◽

2004 ◽

Vol 22 (6) ◽

pp. 2135-2149 ◽

Cited By ~ 30

Author(s):

J. W. Gjerloev ◽

R. A. Hoffman ◽

M. M. Friel ◽

L. A. Frank ◽

J. B. Sigwarth

Keyword(s):

Current System ◽

Three Dimensional ◽

Recovery Phase ◽

Auroral Electrojet ◽

Spatial Behavior ◽

Early Recovery ◽

Substorm Current Wedge ◽

Expansion Phase ◽

Systematic Behavior ◽

Current Wedge

Abstract. The behavior of the auroral electrojet indices AU and AL during classical substorms is investigated by the use of global auroral images. A superposition of the 12 AE stations onto global auroral images and identification of the AL and AU contributing stations enable an understanding of the temporal as well as spatial behavior of the indices with respect to the substorm coordinate system and timeframe. Based on this simple technique it was found that at substorm onset the AL contributing station makes a characteristic jump from a location near the dawn terminator to the onset region, typically bypassing one or more AE stations. During the expansion phase this station typically lies at the poleward edge of the surge region. This is the location of the intense substorm current wedge electrojet in the semiempirical self-consistent substorm model of the three-dimensional current system by Gjerloev and Hoffman (2002). This current wedge is fed primarily pre-midnight by an imbalance of the Region 0 and Region 1 field-aligned currents, not from the dawnside westward electrojet. Then during the early recovery phase the AL contributing station jumps back to the dawn sector. The defining AU station does not show any similar systematic behavior. We also find that the dawn side westward electrojet seems to be unaffected by the introduction of the substorm current wedge. According to our model, much of this current is closed to the magnetosphere as it approaches midnight from dawn. Based on the characteristics of the AL station jumps, the behavior of the dawn-side electrojet, and the understanding of the three-dimensional substorm current system from our model, we provide additional experimental evidence for, and an understanding of, the concept of the two component westward electrojet, as suggested by Kamide and Kokubun (1996).

Download Full-text

A numerical simulation of the Pi2 pulsations associated with the substorm current wedge

Journal of Geophysical Research Atmospheres ◽

10.1029/2001ja900137 ◽

2002 ◽

Vol 107 (A3) ◽

Cited By ~ 37

Author(s):

S. Fujita

Keyword(s):

Numerical Simulation ◽

Substorm Current Wedge ◽

Pi2 Pulsations ◽

Current Wedge

Download Full-text

The response of ionospheric convection in the polar cap to substorm activity

Annales Geophysicae ◽

10.1007/s00585-995-0147-3 ◽

1995 ◽

Vol 13 (2) ◽

pp. 147-158 ◽

Cited By ~ 5

Author(s):

M. Lester ◽

M. Lockwood ◽

T. K. Yeoman ◽

S. W. H. Cowley ◽

H. Lühr ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Flux ◽

Neutral Line ◽

Flow Reversal ◽

Polar Cap ◽

Substorm Current Wedge ◽

Expansion Phase ◽

Ionospheric Convection ◽

Eiscat Radar ◽

Current Wedge

Abstract. We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°λ-78°λ. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71°λ by the time of the expansion phase onset. A westward electrojet, centred at 65.4°λ, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1°λ at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the "distant" neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.

Download Full-text

The substorm current wedge and midnight sector partial ring current near substorm onset: A synthesis based on a magnetotail magnetic field geometry model

ADVANCES IN POLAR SCIENCE ◽

10.3724/sp.j.1085.2013.00032 ◽

2014 ◽

Vol 24 (1) ◽

pp. 32-41 ◽

Cited By ~ 1

Author(s):

J Sofko George ◽

A McWilliams Kathryn ◽

R Bryant Chad

Keyword(s):

Magnetic Field ◽

Ring Current ◽

Substorm Onset ◽

Substorm Current Wedge ◽

Geometry Model ◽

Field Geometry ◽

Current Wedge

Download Full-text

The electric field response to the growth phase and expansion phase onset of a small isolated substorm

Annales Geophysicae ◽

10.1007/s00585-997-0289-6 ◽

1997 ◽

Vol 15 (3) ◽

pp. 289-299 ◽

Cited By ~ 17

Author(s):

R. V. Lewis ◽

M. P. Freeman ◽

A. S. Rodger ◽

G. D. Reeves ◽

D. K. Milling

Keyword(s):

Electric Field ◽

Growth Phase ◽

Auroral Oval ◽

Substorm Onset ◽

Field Of View ◽

Radar Backscatter ◽

Expansion Phase ◽

Ionospheric Convection ◽

Flow Component ◽

Field Response

Abstract. We capitalise on the very large field of view of the Halley HF radar to provide a comprehensive description of the electric field response to the substorm growth phase and expansion phase onset of a relatively simple isolated substorm ( |AL| < 250 nT) which occurred on 13 June 1988. The substorm phases are identified by their standard ground magnetic and spacecraft energetic particle signatures, which provide a framework for the radar measurements. The substorm is preceded by a prolonged period (>12 h) of magnetic quiescence, such that prior to the start of the growth phase, the apparent latitudinal motion of the radar backscatter returns is consistent with the variation in latitude of the quiet-time auroral oval with magnetic local time. The growth phase is characterised by an increasing, superimposed equatorward motion of the equatorward edge of the radar backscatter as the auroral oval expands. Within this backscatter region, there is a poleward gradient in the Doppler spectral width, which we believe to correspond to latitudinal structure in auroral emissions and magnetospheric precipitation. During the growth phase the ionospheric convection is dominated by a relatively smooth large-scale flow pattern consistent with the expanding DP2 (convection) auroral electrojets. Immediately prior to substorm onset the ionospheric convection observed by the radar in the midnight sector has a predominantly equatorward flow component. At substorm onset a dramatic change occurs and a poleward flow component prevails. The timing and location are quite remarkable. The timing of the flow change is within one minute of the dispersionless injection observed at geostationary orbit and the Pi2 magnetic signature on the ground. The location shows that this sudden change in flow is due to the effect of the upward field aligned current of the substorm current wedge imposed directly within the Halley radar field of view.

Download Full-text

Global manifestations of a substorm onset observed by a multi-satellite and ground station network

Annales Geophysicae ◽

10.5194/angeo-24-3491-2006 ◽

2006 ◽

Vol 24 (12) ◽

pp. 3491-3496 ◽

Cited By ~ 6

Author(s):

H. Wang ◽

S. Y. Ma ◽

H. Lühr ◽

Z. X. Liu ◽

Z. Y. Pu ◽

...

Keyword(s):

Magnetic Field ◽

Substorm Onset ◽

Double Star ◽

Auroral Ionosphere ◽

Substorm Current Wedge ◽

Station Network ◽

Ground Station ◽

Auroral Breakup ◽

Current Wedge

Abstract. With a favorable constellation of spacecraft and ground stations, a study is made on the global manifestations of a substorm onset. The onset occurred simultaneously and conjugately in both hemispheres, confirmed by observations of the auroral breakup from IMAGE FUV-WIC and a sudden intensification of a westward electrojet from ground-based magnetometers. Concurrently with the onset, field-aligned and Hall currents in the auroral ionosphere are observed by CHAMP, which are consistent with the signature of a Harang discontinuity. Immediately after the onset a magnetic field dipolarization is clearly observed by Double Star TC-1, located near the central magnetotail and subsequently, by the Cluster quartet. The observations can be explained by a dawnward propagation of the substorm current wedge at a speed of about 300 km/s.

Download Full-text

Network community structure of substorms using SuperMAG magnetometers

Nature Communications ◽

10.1038/s41467-021-22112-4 ◽

2021 ◽

Vol 12 (1) ◽

Author(s):

L. Orr ◽

S. C. Chapman ◽

J. W. Gjerloev ◽

W. Guo

Keyword(s):

Large Scale ◽

Three Dimensional ◽

Easy Access ◽

Directed Network ◽

Coherent System ◽

Dimensional System ◽

Ionospheric Currents ◽

Substorm Current Wedge ◽

Spatially Extended ◽

Current Wedge

AbstractGeomagnetic substorms are a global magnetospheric reconfiguration, during which energy is abruptly transported to the ionosphere. Central to this are the auroral electrojets, large-scale ionospheric currents that are part of a larger three-dimensional system, the substorm current wedge. Many, often conflicting, magnetospheric reconfiguration scenarios have been proposed to describe the substorm current wedge evolution and structure. SuperMAG is a worldwide collaboration providing easy access to ground based magnetometer data. Here we show application of techniques from network science to analyze data from 137 SuperMAG ground-based magnetometers. We calculate a time-varying directed network and perform community detection on the network, identifying locally dense groups of connections. Analysis of 41 substorms exhibit robust structural change from many small, uncorrelated current systems before substorm onset, to a large spatially-extended coherent system, approximately 10 minutes after onset. We interpret this as strong indication that the auroral electrojet system during substorm expansions is inherently a large-scale phenomenon and is not solely due to many meso-scale wedgelets.

Download Full-text