High-latitude crochet: solar flare-induced magnetic disturbance independent from low-latitude
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...
| Main Authors: | , , , , , |
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| Format: | Text |
| Language: | English |
| Published: |
2020
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| Subjects: | |
| Online Access: | https://doi.org/10.5194/angeo-2020-48 https://angeo.copernicus.org/preprints/angeo-2020-48/ |
| Summary: | 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) B Y is not the driver with B Y 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. |
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