Diagnostics of auroral oval boundaries on the basis of the magnetogram inversion technique

It is shown that the convection reversal boundary is a fundamental parameter of the magnetosphere-ionosphere coupling, which determines a strong analogy between the electrostatic potential of the ionosphere and the equivalent current function in the dipole geomagnetic field approximation and the uniform ionospheric conductance. We have developed a new ground-based method for automatically diagnosing boundaries of the auroral oval using output data obtained with the magnetogram inversion technique (MIT). Using maps of the current function and field-aligned currents, calculated at the first stage of MIT with uniform ionospheric conductance, we determine the convection reversal boundary, polar cap boundary, equatorial boundary of the auroral oval, and line of maximum density of auroral electrojets. These parameters have previously been determined by a visual-manual method: analyzing maps of field-aligned and equivalent currents on the monitor screen and carrying out predetermined boundaries with the mouse — this took a very long time (weeks and months). The comparison between manually and automatically obtained boundaries has shown that the correlation coefficient between the two boundaries is, on average, 0.85, and the root-mean-square deviation does not exceed 2° in latitude. By providing an adequate accuracy for the boundary determination, the automatic method reduces the time for map processing by a factor of 2–3 (to minutes and hours), releasing a researcher from laborious visual work. The new method is implemented as one of the important modules in the updated MIT software.

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Studying the dynamics of electric currents and polar caps in ionospheres of two hemispheres during the August 17, 2001 geomagnetic storm

Solar-Terrestrial Physics ◽

10.12737/stp-52201903 ◽

2019 ◽

Vol 5 (2) ◽

pp. 15-27

Author(s):

Сергей Лунюшкин ◽

Sergey Lunyushkin ◽

Владимир Мишин ◽

Vladimir Mishin ◽

Юрий Караваев ◽

...

Keyword(s):

Magnetic Storm ◽

Geomagnetic Storm ◽

Electric Fields ◽

Large Scale ◽

Hall Current ◽

Auroral Oval ◽

Polar Cap ◽

Advanced Technique ◽

Current Function ◽

Polar Caps

The magnetogram inversion technique (MIT), developed at ISTP SB RAS more than forty years ago, has been used until recently only in the Northern Hemisphere. In recent years, MIT has been improved and extended to make instantaneous calculations of 2D distributions of electric fields, horizontal and field-aligned currents in two polar ionospheres. The calculations were carried out based on one-minute ground-based geomagnetic measurements from the worldwide network of stations in both hemispheres (SuperMAG). In this paper, this extended technique is used in the approximation of uniform ionospheric conductance and is applied for the first time to calculations of equivalent and field-aligned currents in two hemispheres through the example of the August 17, 2001 geomagnetic storm. We have obtained the main and essential result: the advanced MIT-ISTP can calculate large-scale distributions of ionospheric convection and FACs in the Northern (N) and Southern (S) polar ionospheres with a high degree of expected hemispheric similarity between these distributions. Using the said event as an example, we have established that the equivalent and field-aligned currents obtained with the advanced technique exhibit the expected dynamics of auroral electrojets and polar caps in two hemispheres. Hall current intensities in polar caps and auroral electrojets, calculated from the equivalent current function, change fairly synchronously in the N and S hemispheres throughout the magnetic storm. Both (westward and eastward) electrojets of the N hemisphere are markedly more intense than respective electrojets of the S hemisphere, and the Hall current in the north polar cap is almost twice as intense as that in the south one. This interhemispheric asymmetry is likely to be due to seasonal conductance variations, which is implicitly contained in the current function. From FAC distributions we determine auroral oval boundaries and calculate magnetic fluxes through the polar caps in the N and S hemispheres. These magnetic fluxes coincide with an accuracy of about 5 % and change almost synchronously during the magnetic storm. In the N hemisphere, the magnetic flux in the dawn polar cap is more intense that that in the dusk one, and vice versa in the S hemisphere. These asymmetries (dawn–dusk and interhemispheric) in the polar caps are consistent with the theory of reconnection for IMF By>0 and with satellite images of auroral ovals; both of these asymmetries decrease during the substorm expansion phase.

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Studying the dynamics of electric currents and polar caps in ionospheres of two hemispheres during the August 17, 2001 geomagnetic storm

Solnechno-Zemnaya Fizika ◽

10.12737/szf-52201903 ◽

2019 ◽

Vol 5 (2) ◽

pp. 17-29

Author(s):

Сергей Лунюшкин ◽

Sergey Lunyushkin ◽

Владимир Мишин ◽

Vladimir Mishin ◽

Юрий Караваев ◽

...

Keyword(s):

Magnetic Storm ◽

Geomagnetic Storm ◽

Electric Fields ◽

Large Scale ◽

Hall Current ◽

Auroral Oval ◽

Polar Cap ◽

Advanced Technique ◽

Current Function ◽

Polar Caps

The magnetogram inversion technique (MIT), developed at ISTP SB RAS more than forty years ago, has been used until recently only in the Northern Hemisphere. In recent years, MIT has been improved and extended to make instantaneous calculations of 2D distributions of electric fields, horizontal and field-aligned currents in two polar ionospheres. The calculations were carried out based on one-minute ground-based geomagnetic measurements from the worldwide network of stations in both hemispheres (SuperMAG). In this paper, this extended technique is used in the approximation of uniform ionospheric conductance and is applied for the first time to calculations of equivalent and field-aligned currents in two hemispheres through the example of the August 17, 2001 geomagnetic storm. We have obtained the main and essential result: the advanced MIT-ISTP can calculate large-scale distributions of ionospheric convection and FACs in the Northern (N) and Southern (S) polar ionospheres with a high degree of expected hemispheric similarity between these distributions. Using the said event as an example, we have established that the equivalent and field-aligned currents obtained with the advanced technique exhibit the expected dynamics of auroral electrojets and polar caps in two hemispheres. Hall current intensities in polar caps and auroral electrojets, calculated from the equivalent current function, change fairly synchronously in the N and S hemispheres throughout the magnetic storm. Both (westward and eastward) electrojets of the N hemisphere are markedly more intense than respective electrojets of the S hemisphere, and the Hall current in the north polar cap is almost twice as intense as that in the south one. This interhemispheric asymmetry is likely to be due to seasonal conductance variations, which is implicitly contained in the current function. From FAC distributions we determine auroral oval boundaries and calculate magnetic fluxes through the polar caps in the N and S hemispheres. These magnetic fluxes coincide with an accuracy of about 5 % and change almost synchronously during the magnetic storm. In the N hemisphere, the magnetic flux in the dawn polar cap is more intense that that in the dusk one, and vice versa in the S hemisphere. These asymmetries (dawn–dusk and interhemispheric) in the polar caps are consistent with the theory of reconnection for IMF By>0 and with satellite images of auroral ovals; both of these asymmetries decrease during the substorm expansion phase.

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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.

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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

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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.

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

Annales Geophysicae ◽

10.1007/s00585-994-0541-2 ◽

1994 ◽

Vol 12 (6) ◽

pp. 541-553 ◽

Cited By ~ 4

Author(s):

N. J. Fox ◽

M. Lockwood ◽

S. W. H. Cowley ◽

M. P. Freeman ◽

E. Friis-Christensen ◽

...

Keyword(s):

Auroral Oval ◽

Geosynchronous Orbit ◽

Polar Cap ◽

Expansion Phase ◽

Geomagnetic Tail ◽

Density Depletion ◽

Significant Difference ◽

Pi2 Pulsations ◽

Substorm Activity ◽

Open Flux

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.

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The relation between the polar cap auroral arc and the auroral oval arc

Journal of Geophysical Research Atmospheres ◽

10.1029/ja081i022p04004 ◽

1976 ◽

Vol 81 (22) ◽

pp. 4004-4006 ◽

Cited By ~ 39

Author(s):

C. -I. Meng ◽

S. -I. Akasofu

Keyword(s):

Auroral Oval ◽

Polar Cap

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EISCAT and Cluster observations in the vicinity of the dynamical polar cap boundary

Annales Geophysicae ◽

10.5194/angeo-26-87-2008 ◽

2008 ◽

Vol 26 (1) ◽

pp. 87-105 ◽

Cited By ~ 9

Author(s):

A. T. Aikio ◽

T. Pitkänen ◽

D. Fontaine ◽

I. Dandouras ◽

O. Amm ◽

...

Keyword(s):

Neutral Line ◽

Broad Band ◽

Low Frequency ◽

Recovery Phase ◽

Auroral Oval ◽

Polar Cap ◽

Early Recovery ◽

F Region ◽

Cluster Data ◽

Cluster Satellite

Abstract. The dynamics of the polar cap boundary and auroral oval in the nightside ionosphere are studied during late expansion and recovery of a substorm from the region between Tromsø (66.6° cgmLat) and Longyearbyen (75.2° cgmLat) on 27 February 2004 by using the coordinated EISCAT incoherent scatter radar, MIRACLE magnetometer and Cluster satellite measurements. During the late substorm expansion/early recovery phase, the polar cap boundary (PCB) made zig-zag-type motion with amplitude of 2.5° cgmLat and period of about 30 min near magnetic midnight. We suggest that the poleward motions of the PCB were produced by bursts of enhanced reconnection at the near-Earth neutral line (NENL). The subsequent equatorward motions of the PCB would then represent the recovery of the merging line towards the equilibrium state (Cowley and Lockwood, 1992). The observed bursts of enhanced westward electrojet just equatorward of the polar cap boundary during poleward expansions were produced plausibly by particles accelerated in the vicinity of the neutral line and thus lend evidence to the Cowley-Lockwood paradigm. During the substorm recovery phase, the footpoints of the Cluster satellites at a geocentric distance of 4.4 RE mapped in the vicinity of EISCAT measurements. Cluster data indicate that outflow of H+ and O+ ions took place within the plasma sheet boundary layer (PSBL) as noted in some earlier studies as well. We show that in this case the PSBL corresponded to a region of enhanced electron temperature in the ionospheric F region. It is suggested that the ion outflow originates from the F region as a result of increased ambipolar diffusion. At higher altitudes, the ions could be further energized by waves, which at Cluster altitudes were observed as BBELF (broad band extra low frequency) fluctuations. The four-satellite configuration of Cluster revealed a sudden poleward expansion of the PSBL by 2° during ~5 min. The beginning of the poleward motion of the PCB was associated with an intensification of the downward FAC at the boundary. We suggest that the downward FAC sheet at the PCB is the high-altitude counterpart of the Earthward flowing FAC produced in the vicinity of the magnetotail neutral line by the Hall effect (Sonnerup, 1979) during a short-lived reconnection pulse.

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Evidence for thermospheric gravity waves in the southern polar cap from ground-based vertical velocity and photometric observations

Annales Geophysicae ◽

10.5194/angeo-19-533-2001 ◽

2001 ◽

Vol 19 (5) ◽

pp. 533-543 ◽

Cited By ~ 6

Author(s):

J. L. Innis ◽

P. A. Greet ◽

P. L. Dyson

Keyword(s):

Gravity Waves ◽

Vertical Velocity ◽

Auroral Oval ◽

Local Times ◽

Polar Cap ◽

Photometric Observations ◽

Fabry Perot ◽

Waves And Tides ◽

Thermospheric Dynamics ◽

Vertical Winds

Abstract. Zenith-directed Fabry-Perot Spectrometer (FPS) and 3-Field Photometer (3FP) observations of the λ630 nm emission (~240 km altitude) were obtained at Davis station, Antarctica, during the austral winter of 1999. Eleven nights of suitable data were searched for significant periodicities common to vertical winds from the FPS and photo-metric variations from the 3FP. Three wave-like events were found, each of around one or more hours in duration, with periods around 15 minutes, vertical velocity amplitudes near 60 ms–1 , horizontal phase velocities around 300 ms–1 , and horizontal wavelengths from 240 to 400 km. These characteristics appear consistent with polar cap gravity waves seen by other workers, and we conclude this is a likely interpretation of our data. Assuming a source height near 125 km altitude, we determine the approximate source location by calculating back along the wave trajectory using the gravity wave property relating angle of ascent and frequency. The wave sources appear to be in the vicinity of the poleward border of the auroral oval, at magnetic local times up to 5 hours before local magnetic midnight.Key words. Meteorology and atmospheric dynamics (thermospheric dynamics; waves and tides)

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