Solar Illumination Dependence of the Auroral Electrojet Intensity: Interplay Between the Solar Zenith Angle and Dipole Tilt

Ohtani, S.; Gjerloev, J. W.; Johnsen, Magnar Gullikstad; Yamauchi, M.; Brändström, Urban; Lewis, Andrew M.

doi:10.1029/2019ja026707

Cited by 7 publications

(14 citation statements)

References 31 publications

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“…The results shown in Fig. 7 are in agreement with the findings by Ohtani et al (2019) that "the nightside westward electrojet (WEJ) is more intense when the ionosphere is dark. "…”

Section: Discussionsupporting

confidence: 90%

“…With the removal of such fluctuations, the global-scale patterns are consistent and repeatable. For example, the electric potential patterns obtained from double-probe measurements on the Dynamics Explorer 2 satellite (Weimer, 1995(Weimer, , 2005a are very similar to results obtained from ground radar Greenwald, 1996, 2005;Cousins and Shepherd, 2010), an electron drift instrument on the Cluster spacecraft (Haaland et al, 2007), and ion drift meters on the DMSP (Papitashvili and Rich, 2002). The field-aligned current maps obtained from different data sets are also similar and repeatable.…”

Section: Discussionsupporting

confidence: 69%

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Testing the electrodynamic method to derive height-integrated ionospheric conductances

Weimer

Edwards

2021

Ann. Geophys.

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Abstract. We have used empirical models for electric potentials and the magnetic fields both in space and on the ground to obtain maps of the height-integrated Pedersen and Hall ionospheric conductivities at high latitudes. This calculation required use of both “curl-free” and “divergence-free” components of the ionospheric currents, with the former obtained from magnetic fields that are used in a model of the field-aligned currents. The second component is from the equivalent current, usually associated with Hall currents, derived from the ground-level magnetic field. Conductances were calculated for varying combinations of the interplanetary magnetic field (IMF) magnitude and orientation angle, as well as the dipole tilt angle. The results show that reversing the sign of the Y component of the IMF produces substantially different conductivity patterns. The Hall conductivities are largest on the dawn side in the upward, Region 2 field-aligned currents. Low electric field strengths in the Harang discontinuity lead to inconclusive results near midnight. Calculations of the Joule heating, obtained from the electric field and both components of the ionospheric current, are compared with the Poynting flux in space. The maps show some differences, while their integrated totals match to within 1 %. Some of the Poynting flux that enters the polar cap is dissipated as Joule heating within the auroral ovals, where the conductivity is greater.

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“…The results shown in Fig. 7 are in agreement with the findings by Ohtani et al (2019) that "the nightside westward electrojet (WEJ) is more intense when the ionosphere is dark. "…”

Section: Discussionsupporting

confidence: 90%

Section: Discussionsupporting

confidence: 69%

Testing the electrodynamic method to derive height-integrated ionospheric conductances

Weimer

Edwards

2021

Ann. Geophys.

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“…Their seasonal variations are consistent with our results, whereas the local time differences are due to the narrower MLT sectors concentrated in their study. Ohtani et al (2019) found that in the daytime, both eastward and westward electrojets were more intense when the SZA was smaller, and in the midnight-to-dawn sector, the eastward electrojet was more intense for smaller SZA. These findings are consistent with our results.…”

Section: Ionospheric Conductance Effectmentioning

confidence: 90%

“…Ohtani et al. (2019) provided us with an in‐depth understanding of the formation of the auroral electrojet. However, Guo et al.…”

Section: Introductionmentioning

confidence: 99%

Effects of Solar Illumination and Substorms on Auroral Electrojets Based on CHAMP Observations

Wang

Lühr

2021

JGR Space Physics

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We investigate the effects of solar illumination and the substorm process on the auroral electrojets using CHAMP high‐resolution magnetic field data obtained over a period of 10 years. The eastward electrojet was found to be proportional to the ionospheric conductance induced by solar irradiation over the whole day. More notably, a stronger westward current was detected in times of lower fluxtube‐integrated conductance in both hemispheres. The relative spatial distribution and evolution of the westward electrojet and field‐aligned current (FAC) during the substorm periods were also investigated. The position of the westward current coincided with that of the upward FAC in 2000–2200 magnetic local time (MLT), in keeping with the structure of a westward traveling surge. Furthermore, the upward FAC in the local time sector of the poleward of the westward electrojet was supplied mostly by the remote closure current. The downward FACs in the poleward and upward FACs in the equatorward of the westward electrojet existed in the MLT sectors after 2200 h. Finally, the ratio of the peak intensity of the westward electrojet to that of the meridional Pedersen current, in the 2000–2200 MLT sector increased by about 250%, and the value in the 2200–0200 MLT increased by about 10%–20%. These results provide observational evidence of the increased efficiency of the Cowling channel in the night‐time during substorm periods and contribute to our understanding of the formation mechanism of the auroral electrojet.

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“…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, hence, 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.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2020

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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.

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Solar Illumination Dependence of the Auroral Electrojet Intensity: Interplay Between the Solar Zenith Angle and Dipole Tilt

Cited by 7 publications

References 31 publications

Testing the electrodynamic method to derive height-integrated ionospheric conductances

Testing the electrodynamic method to derive height-integrated ionospheric conductances

Effects of Solar Illumination and Substorms on Auroral Electrojets Based on CHAMP Observations

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

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