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dc.contributor.authorYamauchi, Masatoshi
dc.contributor.authorJohnsen, Magnar Gullikstad
dc.contributor.authorEnell, Carl-Fredrik
dc.contributor.authorTjulin, Anders
dc.contributor.authorWiller, Anna
dc.contributor.authorSormakov, Dmitry A.
dc.date.accessioned2021-03-01T12:51:51Z
dc.date.available2021-03-01T12:51:51Z
dc.date.issued2020-11-03
dc.description.abstractA solar-flare-induced, high-latitude (peak at 70–75<sup>∘</sup> geographic latitude – GGlat) ionospheric current system was studied. Right after the X9.3 flare on 6 September 2017, magnetic stations at 68–77<sup>∘</sup> GGlat near local noon detected northward geomagnetic deviations (ΔB) for more than 3 h, with peak amplitudes of >200 nT without any accompanying substorm activities. From its location, this solar flare effect, or crochet, is different from previously studied ones, namely, the subsolar crochet (seen at lower latitudes), 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, the interplanetary magnetic field (IMF) <i>B<sub>Y</sub></i> is not the driver, with the <i>B<sub>Y</sub></i> values of only 0–1 nT out of a 3 nT total field. The equivalent ionospheric current flows eastward in a limited latitude range but extended at least 8 h in local time (LT), forming a zonal current region equatorward of the polar cap on the geomagnetic closed region.<p> <p>EISCAT radar measurements, which were conducted over the same region as the most intense ΔB, show enhancements of electron density (and hence of ion-neutral density ratio) at these altitudes (∼100 km) at which strong background ion convection (>100 m s<sup>−1</sup>) pre-existed in the direction of tidal-driven diurnal solar quiet (Sq0) flow. Therefore, this new zonal current can be related to this Sq0-like convection and the electron density enhancement, for example, by descending the 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 a transient redistribution of the electron density.<p> <p>The signature is sometimes seen in the auroral electrojet (AE = AU − AL) index. A quick survey for X-class flares during solar cycle 23 and 24 shows clear increases in AU for about half the > X2 flares during non-substorm time, despite the unfavourable latitudinal coverage of the AE stations for detecting this new crochet. Although some of these AU increases could be the auroral crochet signature, the high-latitude crochet can be a rather common feature for X flares.<p> <ol> <p><li>We found a new type of the solar flare effect on the dayside ionospheric current at high latitudes but equatorward of the cusp during quiet periods.</li> <li>The effect is also seen in the AU index for nearly half of the > X2-class solar flares.</li> <li>A case study suggests that the new crochet is related to the Sq0 (tidal-driven part) current.</li> </ol>en_US
dc.identifier.citationYamauchi, Johnsen, Enell, Tjulin A, Willer, Sormakov. High-latitude crochet: Solar-flare-induced magnetic disturbance independent from low-latitude crochet. Annales Geophysicae. 2020;38(6):1159-1170en_US
dc.identifier.cristinIDFRIDAID 1862021
dc.identifier.doi10.5194/angeo-38-1159-2020
dc.identifier.issn0992-7689
dc.identifier.issn1432-0576
dc.identifier.urihttps://hdl.handle.net/10037/20621
dc.language.isoengen_US
dc.publisherCopernicus Publications, European Geosciences Unionen_US
dc.relation.journalAnnales Geophysicae
dc.rights.accessRightsopenAccessen_US
dc.rights.holderCopyright 2020 The Author(s)en_US
dc.subjectVDP::Mathematics and natural science: 400::Physics: 430en_US
dc.subjectVDP::Matematikk og Naturvitenskap: 400::Fysikk: 430en_US
dc.titleHigh-latitude crochet: Solar-flare-induced magnetic disturbance independent from low-latitude crocheten_US
dc.type.versionpublishedVersionen_US
dc.typeJournal articleen_US
dc.typeTidsskriftartikkelen_US
dc.typePeer revieweden_US


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