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

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.

Realtime geomagnetic indices for mid-latitudes. MID-R, MID-E, MID-U and MID-L

<p>Mid latitudes around 40 degree are influenced by effects typically found at both high and low latitudes. Moreover, the focus of the Solar Quiet ionospheric current system, drifts around these mid-latitudes. Consequently they have been considered as a complicated place to infer the geospace state from the ground and also complicated for practical procedures to generate geomagnetic indices. <br>The procedure designed at the University of Alcala specially focused on removing solar regular variations at mid-latitudes is delivering a geomagnetic Local Disturbance index (LDi) in realtime. The same procedure can be used to produce global geomagnetic indices when applied to several geomagnetic stations at these latitudes. <br>We present in this work the high-resolution (one minute) realtime production of ring current and auroral indices (MID-R, MID-E, MID-U and MID-L) similar to the well known Dst and AE indices for mid-latitudes which will help in the understanding of the complex physical processes that emerge at these latitudes. At the same time they fill a gap in the current operational space weather products available for these latitudes.</p>

Remote Sensing of Magnetic Fields Induced by Electrojets From Space: Measurement Techniques and Sensor Design Requirements

<p>The Zeeman effect of the O<sub>2</sub> 118 GHz spectral radiance measurements can be utilized to remotely measure the magnetic field perturbations at altitudes close to the auroral electrojets. The technique has been demonstrated using the measurements provided by the Microwave Limb Souncer onboard the Aura spacecraft.  The derived current-induced magnetic field perturbations were found to be highly correlated with those coincidently obtained by ground magnetometers and to be consistent with the well-known auroral electrojet current distribution thereby providing a strong argument for the validity of the technique. With today's technology, a 118 GHz instrument, can be miniaturized allowing it to fly on small satellites such as CubeSats.  A constellation of small satellites with each one carrying a number of these identical mini-radiometers would have the ability to provide simultaneous multipoint measurement of the magnetic field perturbations at altitudes close to the electrojet, thereby greatly advancing our understanding of the ionospheric current system.  In this paper, we present the Zeeman magnetic field sensing technique, the requirements and specifications of the instrument, and an example of a cost effectively cubesat mission that provides unprecedented measurements of the evolution and structure of the auroral electrojet system.</p>