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

Abstract

Abstract. A solar-flare-induced, high-latitude (peak at 70–75∘ geographic latitude – GGlat) ionospheric current system was studied. Right after the X9.3 flare on 6 September 2017, magnetic stations at 68–77∘ 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) BY is not the driver, with the BY 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. 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−1) 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. 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. 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. The effect is also seen in the AU index for nearly half of the > X2-class solar flares. A case study suggests that the new crochet is related to the Sq0 (tidal-driven part) current.

Lower-thermosphere–ionosphere (LTI) quantities: current status of measuring techniques and models

“…Solar flares are also known to enhance electron density and hence JH in the LTI (e.g. Pudovkin and Sergeev, 1977;Sergeev, 1977;Curto et al, 1994;Yamazaki and Maute, 2017). While the solar-flare-driven ionospheric current, or crochet current, near the subsolar region has been intensively studied (e.g.…”

“…The modification of the auroral electrojets by solar flares can be more than a mere enhancement. Recently, Yamauchi et al (2020) found that the solar flares can change the direction of the electrojet, and the resulting geomagnetic deviation sometimes exceeds 200 nT. The European Incoherent Scatter (EISCAT) radar observation suggested that even the altitude of JH can be changed for these events.…”

“…While the general principles are described, the significant day-to-day variability of their magnitudes is still the subject of investigation (e.g. Yamazaki and Maute, 2017). A special category of the LTI variability and dynamics within the low latitudes are post-sunset F-region equatorial plasma irregularities, in which the LTI and lower F region are believed to play an important role.…”

“…Due to high electrical conductivity in the direction of the magnetic field, σ is very large, up to tens of S m −1 (e.g. Yamazaki and Maute, 2017). This means that the geomagnetic field lines are nearly equipotential in the ionosphere, facilitating the approximation of the ionospheric parameters in the height-integrated form.…”

Lower thermosphere – ionosphere (LTI) quantities: Current status of measuring techniques and models

“…Solar flares are also known to enhance electron density and hence JH in the LTI (e.g. Pudovkin and Sergeev, 1977;Sergeev, 1977;Curto et al, 1994;Yamazaki and Maute, 2017). While the solar-flare-driven ionospheric current, or crochet current, near the subsolar region has been intensively studied (e.g.…”