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Preprints
https://doi.org/10.5194/angeo-2020-48
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/angeo-2020-48
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  09 Jul 2020

09 Jul 2020

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This preprint is currently under review for the journal ANGEO.

High-latitude crochet: solar flare-induced magnetic disturbance independent from low-latitude

Masatoshi Yamauchi1, Magnar G. Johnsen2, Carl-Fredrik Enell3, Anders Tjulin3, Anna Willer4, and Dmitry A. Sormakov5 Masatoshi Yamauchi et al.
  • 1Swedish Institute of Space Physics (IRF), Kiruna, Sweden
  • 2Tromsø Geophysical Observatory (TGO), UiT the Arctic University of Norway, Tromsø, Norway
  • 3EISCAT Scientific Association, Kiruna, Sweden
  • 4National Space Institute, Technical University of Denmark (DTU Space), Kgs. Lyngby, Denmark
  • 5Arctic and Antarctic Research Institute (AARI), St. Petersburg, Russia

Abstract. Solar flare-induced High latitude (peak at 70–75° geographic latitude) ionospheric current system was studied. Right after the X9.3 flare on 6 September 2017, magnetic stations at 68–77° geographic latitudes (GGlat) near local noon detected northward geomagnetic deviations (ΔB) for more than 3 hours, with peak amplitudes > 200 nT, without any accompanying substorm activities. From its location, this solar flare effect, or crochet, is different from previously studied ones, namely, subsolar crochet (seen at lower latitude), auroral crochet (pre-requires auroral electrojet in sunlight), or cusp crochet (seen only in the cusp). The new crochet is much more intense and longer in duration than the subsolar crochet. The long duration matches with the period of high solar X-ray flux (more than M3-class flare level). Unlike the cusp crochet, interplanetary magnetic field (IMF) BY is not the driver with BY only 0–1 nT out of 3 nT total field. The equivalent ionospheric current flows eastward in a limited latitude range but extended at least 8 hours in local time (LT), forming a zonal current region equatorward of the polar cap on the geomagnetic closed region.

EISCAT radar measurements over the same region as the most intense ΔB near local noon show enhancements of electron density (and hence ion-neutral ratio) at these altitudes (~ 100 km) where the background Sq ion convection (> 100 m/s) pre-existed. Therefore, this new zonal current can be related to the Sq convection and the electron density enhancement, e.g., by descending E-region height. However, we have not found why the new crochet is found in a limited latitudinal range, and therefore the mechanism is still unclear compared to the subsolar crochet that is maintained by transient re-distribution of electron density.

The signature is sometimes seen in the Auroral Electrojet (AE) index. A quick eye-survey for X-class flares during solar cycle 23 and 24 shows clear AU increases for about half the > X2 flares during non-substorm time, although the latitudinal coverage of the AE stations is not favorable to detect this new crochet. Although some of them could be due to auroral crochet, this new crochet can be rather common feature for X flares.

Masatoshi Yamauchi et al.

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Masatoshi Yamauchi et al.

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Latest update: 11 Aug 2020
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Short summary
The paper found strong Space Weather effect on the ionospheric current driven by the solar flare at geographic location around 70–75 degree. Because of that narrow range at high-latitude, this effect has been overlooked during past half century although data was available, but the effect is as large as a substorm that is know to cause strong aurora.
The paper found strong Space Weather effect on the ionospheric current driven by the solar flare...
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