“Polar” Substorms and the Harang Discontinuity

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Abstract

We analyzed 214 cases of “polar” substorms on the Scandinavian meridian IMAGE, i.e. substorms recorded by magnetometers located at geomagnetic latitudes above ~70° MLAT at 19−02 MLT under magnetically quiet time in the absence of negative magnetic bays at lower latitudes. The Harang Discontinuity, which separates the westward and eastward electrojets by latitude, is a typical structure for the indicated MLT sector of the high-latitude ionosphere. The global distribution of ionospheric electrojets and the location of the Harang discontinuity during the development of “polar” substorms were studied by the maps constructed from the results of spherical harmonic analysis of the magnetic measurements on 66 simultaneous ionospheric communications satellites of the AMPERE project. Based on these maps analysis, it is shown that the instantaneous location of the equatorial boundary of the ionospheric current of a “polar” substorm determines the instantaneous location of the polar boundary of the Harang Discontinuity, and the polar boundary of the eastward electrojet determines its equatorial boundary. It has been established that the appearance of 90% of the “polar” substorms is observed simultaneously with increasing of the planetary substorm activity according to the AL-index and the development of a magnetospheric substorm in the post-midnight sector. At the same time, the development of the evening “polar” substorms is associated with the formation of near-midnight magnetic vortices at geomagnetic latitudes of ~70° MLAT (near the “nose” of the Harang discontinuity), indicating a sharp local enhancement of the field-aligned currents. This leads to the formation of a new substorm in the evening sector of near-polar latitudes, called a “polar” substorm with typical features of the onset of a substorm (Pi2 geomagnetic pulsation bursts, an abrupt onset of the substorm close to the equatorial boundary of the constructed oval (the development of a “substorm current wedge” – etc.)

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About the authors

N. G. Kleimenova

Schmidt Institute Physics of the Earth RAS

Author for correspondence.
Email: ngk1935@yandex.ru
Russian Federation, Moscow

L. I. Gromova

Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation

Email: ngk1935@yandex.ru
Russian Federation, Moscow, Troitsk

S. V. Gromov

Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation

Email: ngk1935@yandex.ru
Russian Federation, Moscow, Troitsk

L. M. Malysheva

Schmidt Institute Physics of the Earth RAS

Email: ngk1935@yandex.ru
Russian Federation, Moscow

I. V. Despirak

Polar Geophysical Institute

Email: ngk1935@yandex.ru
Russian Federation, Apatity

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2. Fig. 1. (a)- Schematic of the convective bay and magnetospheric substorm from [Baumjohann, 1983]; (b)- Schematic of currents in the Harang gap from [Koskinen and Pulkkinen, 1995]

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3. Fig. 2. Typical examples of the distribution of global ionospheric currents from measurements on the AMPERE satellites and variations of the planetary substorm AL-index during 4 “polar” substorms. Details in the text.

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4. Fig. 3. Example of one of the typical “polar” substorms on December 5, 2020: magnetograms of some IMAGE stations and midlatitude obs. Borok (BOX) and Irkutsk (IRT), as well as a map of the distribution of ionospheric currents from AMPERE data and variations of the AL-index.

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