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.
Highlights
Solar flares are known to enhance the ionospheric electron density and, influence the electric currents in the D- and E-region
The subsolar crochet is most likely caused by a redistribution of the electron density at a < 120 km altitude that is enhanced by the flare X-ray (e.g. Curto et al, 1994; Yamasaki and Maute, 2017), resulting in a twin vortex ionospheric current that is similar to the tidal-driven part of solar quiet (Sq) ionospheric current, Sq0, which dominates the low-latitude diurnal convection
Using magnetometer data from northern Europe, Russia, and Greenland, as well as EISCAT data, we found a new type of solar flare effect (SFE, or crochet) on the geomagnetic disturbance in response to the X9.3 flare on 6 September 2017 at high latitudes (65–75◦ GGlat)
Summary
Solar flares are known to enhance the ionospheric electron density and, influence the electric currents in the D- and E-region. The auroral crochet is most likely caused by the modification of a pre-existing ionospheric current (or electrojet) by the enhanced electron density (Pudovkin, 1974) This effect increases as background plasma convection (ionospheric electric field) increases and, is most visible during the polar disturbances of DP1 and DP2 (Akasofu, 1964; Nishida, 1968), as long as the ionosphere is sunlit, for example, near summer solstice when X-ray flux reaches high latitudes. As shown in this paper, we found that the crochet at high latitudes is not a simple extension or sub-effect of, but is independent from, the subsolar crochet with a larger amplitude and longer duration We show this from a case study of the X9.3 flare on 6 September 2017, using highlatitude magnetometer data in the dayside and EISCAT radar data. The amplitude is larger at BJN than DIK for the second and third peaks, BJN is located far poleward of the AE stations and did not contribute to AU
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