EISCAT observations of unusual flows in the morning sector associated with weak substorm activity

Abstract. A discussion is given of plasma flows in the dawn and nightside high-latitude ionospheric regions during substorms occurring on a contracted auroral oval, as observed using the EISCAT CP-4-A experiment. Supporting data from the PACE radar, Greenland magnetometer chain, SAMNET magnetometers and geostationary satellites are compared to the EISCAT observations. On 4 October 1989 a weak substorm with initial expansion phase onset signatures at 0030 UT, resulted in the convection reversal boundary observed by EISCAT (at ~0415 MLT) contracting rapidly poleward, causing a band of elevated ionospheric ion temperatures and a localised plasma density depletion. This polar cap contraction event is shown to be associated with various substorm signatures; Pi2 pulsations at mid-latitudes, magnetic bays in the midnight sector and particle injections at geosynchronous orbit. A similar event was observed on the following day around 0230 UT (~0515 MLT) with the unusual and significant difference that two convection reversals were observed, both contracting poleward. We show that this feature is not an ionospheric signature of two active reconnection neutral lines as predicted by the near-Earth neutral model before the plasmoid is "pinched off", and present two alternative explanations in terms of (1) viscous and lobe circulation cells and (2) polar cap contraction during northward IMF. The voltage associated with the anti-sunward flow between the reversals reaches a maximum of 13 kV during the substorm expansion phase. This suggests it to be associated with the polar cap contraction and caused by the reconnection of open flux in the geomagnetic tail which has mimicked "viscous-like" momentum transfer across the magnetopause.

Download Full-text

Aurora and open magnetic flux during isolated substorms, sawteeth, and SMC events

Annales Geophysicae ◽

10.5194/angeo-25-1865-2007 ◽

2007 ◽

Vol 25 (8) ◽

pp. 1865-1876 ◽

Cited By ~ 30

Author(s):

A. D. DeJong ◽

X. Cai ◽

R. C. Clauer ◽

J. F. Spann

Keyword(s):

Magnetic Flux ◽

Temporal Variations ◽

Auroral Oval ◽

Polar Cap ◽

Expansion Phase ◽

Magnetospheric Convection ◽

Open Magnetic Flux ◽

Open Flux ◽

The Individual

Abstract. Using Polar UVI LBHl and IMAGE FUV WIC data, we have compared the auroral signatures and polar cap open flux for isolated substorms, sawteeth oscillations, and steady magnetospheric convection (SMC) events. First, a case study of each event type is performed, comparing auroral signatures and open magnetic fluxes to one another. The latitude location of the auroral oval is similar during isolated substorms and SMC events. The auroral intensity during SMC events is similar to that observed during the expansion phase of an isolated substorm. Examination of an individual sawtooth shows that the auroral intensity is much greater than the SMC or isolated substorm events and the auroral oval is displaced equatorward making a larger polar cap. The temporal variations observed during the individual sawtooth are similar to that observed during the isolated substorm, and while the change in polar cap flux measured during the sawtooth is larger, the percent change in flux is similar to that measured during the isolated substorm. These results are confirmed by a statistical analysis of events within these three classes. The results show that the auroral oval measured during individual sawteeth contains a polar cap with, on average, 150% more magnetic flux than the oval measured during isolated substorms or during SMC events. However, both isolated substorms and sawteeth show a 30% decrease in polar cap magnetic flux during the dipolarization (expansion) phase.

Download Full-text

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

Annales Geophysicae ◽

10.5194/angeo-32-333-2014 ◽

2014 ◽

Vol 32 (4) ◽

pp. 333-351 ◽

Cited By ~ 11

Author(s):

P. E. Sandholt ◽

C. J. Farrugia ◽

W. F. Denig

Keyword(s):

Spatial Scales ◽

Current System ◽

Auroral Oval ◽

Local Times ◽

Type I ◽

Polar Cap ◽

Interplanetary Coronal Mass Ejections ◽

Expansion Phase ◽

Earth Magnetic Field ◽

Substorm Activity

Abstract. We study substorms from two perspectives, i.e., magnetosphere–ionosphere coupling across the auroral oval at dusk and at midnight magnetic local times. By this approach we monitor the activations/expansions of basic elements of the substorm current system (Bostrøm type I centered at midnight and Bostrøm type II maximizing at dawn and dusk) during the evolution of the substorm activity. Emphasis is placed on the R1 and R2 types of field-aligned current (FAC) coupling across the Harang reversal at dusk. We distinguish between two distinct activity levels in the substorm expansion phase, i.e., an initial transient phase and a persistent phase. These activities/phases are discussed in relation to polar cap convection which is continuously monitored by the polar cap north (PCN) index. The substorm activity we selected occurred during a long interval of continuously strong solar wind forcing at the interplanetary coronal mass ejection passage on 18 August 2003. The advantage of our scientific approach lies in the combination of (i) continuous ground observations of the ionospheric signatures within wide latitude ranges across the auroral oval at dusk and midnight by meridian chain magnetometer data, (ii) "snapshot" satellite (DMSP F13) observations of FAC/precipitation/ion drift profiles, and (iii) observations of current disruption/near-Earth magnetic field dipolarizations at geostationary altitude. Under the prevailing fortunate circumstances we are able to discriminate between the roles of the dayside and nightside sources of polar cap convection. For the nightside source we distinguish between the roles of inductive and potential electric fields in the two substages of the substorm expansion phase. According to our estimates the observed dipolarization rate (δ Bz/δt) and the inferred large spatial scales (in radial and azimuthal dimensions) of the dipolarization process in these strong substorm expansions may lead to 50–100 kV enhancements of the cross-polar-cap potential due to inductive electric field coupling.

Download Full-text

Multi-instrumentation observations of a transpolar arc in the northern hemisphere

Annales Geophysicae ◽

10.5194/angeo-26-201-2008 ◽

2008 ◽

Vol 26 (1) ◽

pp. 201-210 ◽

Cited By ~ 20

Author(s):

A. Goudarzi ◽

M. Lester ◽

S. E. Milan ◽

H. U. Frey

Keyword(s):

Flux Distribution ◽

Auroral Oval ◽

Recent Model ◽

Convection Flow ◽

Polar Cap ◽

Geomagnetic Tail ◽

Subsequent Motion ◽

Open Flux ◽

Dawn Sector ◽

The Imf

Abstract. A transpolar arc was imaged by the FUV instrument on the IMAGE spacecraft during a 3-h interval on 5 February 2002. Observations indicate that a burst of reconnection in the geomagnetic tail, which was not associated with a substorm, was responsible for the formation of the arc. The arc initially formed across the central polar cap, extending from near midnight to noon such that the polar cap was approximately divided in half. The subsequent motion of the arc was controlled by the amount of open flux being added to the dawn sector cap from a magnetopause reconnection site on the post-noon side of the magnetosphere. The dayside reconnection happened during a period when the IMF By component was dominant, although the Bz component initially remained positive, and resulted in strong westward azimuthal flows in the noon sector. The arc continued to move towards the duskside auroral oval after the IMF Bz turned southward. A keogram of the FUV/WIC auroral observations along the dawn-dusk meridian provides further evidence of the expansion and contraction of the polar cap during the period in which different IMF orientations occurred. Furthermore, comparing images from IMAGE and ionospheric convection flow from SuperDARN measurements, vortical convection flows occurred exactly at the time and location of the formation of the transpolar arc and subsequently followed the head of the transpolar arc as it moved across the polar cap. The observations are consistent with the prediction of a recent model for the formation of the transpolar cap by the closure of open flux in the geomagnetic tail, and its subsequent motion through changes in the open flux distribution within the polar cap.

Download Full-text

Extended period of polar cap auroral display: auroral dynamics and relation to the IMF and the ionospheric convection

Annales Geophysicae ◽

10.1007/s00585-995-0854-9 ◽

1995 ◽

Vol 13 (8) ◽

pp. 854-862 ◽

Cited By ~ 5

Author(s):

V. G. Vorobjev ◽

S. V. Leontyev ◽

Ya. I. Feldstein

Keyword(s):

Large Scale ◽

Interplanetary Space ◽

Extended Period ◽

Auroral Oval ◽

Polar Cap ◽

Substorm Activity ◽

Convection Patterns ◽

Northern And Southern Hemispheres ◽

Birkeland Currents ◽

The Imf

Abstract. An unusually extended period (5 h) of polar cap auroral display on 3 August 1986 is examined. Auroras have been investigated using ground-based data as well as measurements from the IMP-8 spacecraft in interplanetary space and simultaneous observations from the polar-orbiting satellites Viking and DE-1 in the northern and southern hemispheres, respectively. It is found that visible Sun-aligned arcs are located inside the transpolar band of the θ-aurora observed from the satellite in ultraviolet wavelengths. The transpolar band can contain several Sun-aligned arcs that move inside the band toward the morning or evening side of the auroral oval independent of the direction of the band movement. Intensifications of polar cap auroras with durations of up to about 30 min are observed. No change has been found in either IMF parameters or substorm activity that can be related to these intensifications. The θ-aurora occurred during a 2-h period when the B z-component of the IMF was negative. A tendency is noted for dawnward (duskward) displacement of the transpolar band when By>0 (By<0) in the southern hemisphere. Simultaneous observations of auroral ovals during interplanetary Bz<0, By<0 and Bx>0 in both hemispheres and convection patterns for Bz<0 and By<0 have been displayed using satellite and ground-based measurements. It was found that the transpolar band of the <theta>-aurora in the sunlit hemisphere was situated in the region of large-scale downward Birkeland currents.

Download Full-text

IMF effect on the polar cap contraction and expansion during a period of substorms

Annales Geophysicae ◽

10.5194/angeo-31-1021-2013 ◽

2013 ◽

Vol 31 (6) ◽

pp. 1021-1034 ◽

Cited By ~ 4

Author(s):

A. T. Aikio ◽

T. Pitkänen ◽

I. Honkonen ◽

M. Palmroth ◽

O. Amm

Keyword(s):

Recovery Phase ◽

Bow Shock ◽

Polar Cap ◽

Reconnection Rate ◽

Early Recovery ◽

Mhd Simulation ◽

Expansion Phase ◽

Contraction And Expansion ◽

Good Agreement ◽

The Imf

Abstract. The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase. In addition, we suggest that the IMF Bz component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.

Download Full-text

Fabry-Perot spectrometer observations of the auroral oval/polar cap boundary above Mawson, Antarctica

Journal of Atmospheric and Terrestrial Physics ◽

10.1016/0021-9169(96)00007-4 ◽

1996 ◽

Vol 58 (16) ◽

pp. 1973-1988 ◽

Cited By ~ 25

Author(s):

J.L. Innis ◽

P.A. Greet ◽

P.L. Dyson

Keyword(s):

Auroral Oval ◽

Polar Cap ◽

Fabry Perot

Download Full-text

Magnetospheric reconfiguration during the substorm cycle as inferred from the data-based modeling.

10.5194/egusphere-egu21-809 ◽

2021 ◽

Author(s):

Nikolai Tsyganenko ◽

Varvara Andreeva ◽

Mikhail Sitnov ◽

Jesper Gjerloev ◽

Xiangning Chu ◽

...

Keyword(s):

Ring Current ◽

Full Range ◽

Temporal Trends ◽

Basis Functions ◽

Polar Cap ◽

Low Latitude ◽

Quarter Century ◽

Expansion Phase ◽

First Results ◽

The Magnetic Field

<p>First results are presented of reconstructing the evolution of magnetospheric configurations through the full cycle of isolated substorms. The modeling covers the low- and mid-latitude magnetosphere in the range of radial distances from 2 to 20 Re and is based on a synthesis of (1) a high-resolution representation of the magnetic field by cylindrical basis functions, (2) the ever largest pool of magnetospheric and interplanetary data spanning the last quarter century (1995-2019), (3) an archive of concurrent ground-based indices and their temporal trends, quantifying the geomagnetic activity over the full range of latitudes, including the low-latitude ring current SMR-index, the midlatitude positive bay MPB-index, the auroral SML-index, and the polar cap PC-index, (4) the data-mining nearest-neighbour (NN) technique of the data selection and weighting in the geometric and parametric spaces. The obtained successive diagrams of magnetic depression/compression, electric current, and field line maps demonstrate all the typical features of the substorm cycle: the initial relatively slow stretching of the nightside tail during the growth phase, followed by its sudden collapse associated with a dramatic disruption of the tail current at R~11-16 Re, and finally a gradual recovery of the configuration after the expansion phase is over.</p>

Download Full-text

Modulation of nightside polar patches by substorm activity

Annales Geophysicae ◽

10.5194/angeo-27-3923-2009 ◽

2009 ◽

Vol 27 (10) ◽

pp. 3923-3932 ◽

Cited By ~ 7

Author(s):

A. G. Wood ◽

S. E. Pryse ◽

J. Moen

Keyword(s):

Spatial Distribution ◽

High Latitude ◽

Ionospheric Plasma ◽

Plasma Convection ◽

Convection Pattern ◽

Polar Cap ◽

Incoherent Scatter ◽

Repetition Time ◽

Substorm Activity ◽

European Sector

Abstract. Results are presented from a multi-instrument study showing the influence of geomagnetic substorm activity on the spatial distribution of the high-latitude ionospheric plasma. Incoherent scatter radar and radio tomography measurements on 12 December 2001 were used to directly observe the remnants of polar patches in the nightside ionosphere and to investigate their characteristics. The patches occurred under conditions of IMF Bz negative and IMF By negative. They were attributed to dayside photoionisation transported by the high-latitude convection pattern across the polar cap and into the nighttime European sector. The patches on the nightside were separated by some 5° latitude during substorm expansion, but this was reduced to some 2° when the activity had subsided. The different patch separations resulted from the expansion and contraction of the high-latitude plasma convection pattern on the nightside in response to the substorm activity. The patches of larger separation occurred in the antisunward cross-polar flow as it entered the nightside sector. Those of smaller separation were also in antisunward flow, but close to the equatorward edge of the convection pattern, in the slower, diverging flow at the Harang discontinuity. A patch repetition time of some 10 to 30 min was estimated depending on the phase of the substorm.

Download Full-text

GPS phase scintillation at high latitudes during geomagnetic storms of 7–17 March 2012 – Part 1: The North American sector

Annales Geophysicae ◽

10.5194/angeo-33-637-2015 ◽

2015 ◽

Vol 33 (6) ◽

pp. 637-656 ◽

Cited By ~ 12

Author(s):

P. Prikryl ◽

R. Ghoddousi-Fard ◽

E. G. Thomas ◽

J. M. Ruohoniemi ◽

S. G. Shepherd ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Geomagnetic Storms ◽

Dynamic Pressure ◽

Auroral Oval ◽

Solar Wind Dynamic Pressure ◽

Polar Cap ◽

The North ◽

Auroral Emission ◽

Ground Magnetic

Abstract. The interval of geomagnetic storms of 7–17 March 2012 was selected at the Climate and Weather of the Sun-Earth System (CAWSES) II Workshop for group study of space weather effects during the ascending phase of solar cycle 24 (Tsurutani et al., 2014). The high-latitude ionospheric response to a series of storms is studied using arrays of GPS receivers, HF radars, ionosondes, riometers, magnetometers, and auroral imagers focusing on GPS phase scintillation. Four geomagnetic storms showed varied responses to solar wind conditions characterized by the interplanetary magnetic field (IMF) and solar wind dynamic pressure. As a function of magnetic latitude and magnetic local time, regions of enhanced scintillation are identified in the context of coupling processes between the solar wind and the magnetosphere–ionosphere system. Large southward IMF and high solar wind dynamic pressure resulted in the strongest scintillation in the nightside auroral oval. Scintillation occurrence was correlated with ground magnetic field perturbations and riometer absorption enhancements, and collocated with mapped auroral emission. During periods of southward IMF, scintillation was also collocated with ionospheric convection in the expanded dawn and dusk cells, with the antisunward convection in the polar cap and with a tongue of ionization fractured into patches. In contrast, large northward IMF combined with a strong solar wind dynamic pressure pulse was followed by scintillation caused by transpolar arcs in the polar cap.

Download Full-text