Solar flare effects at Ebre: Regular and reversed solar flare effects, statistical analysis (1953 to 1985), A global case study and a model of elliptical ionospheric currents

Abstract

This paper presents the results of the analysis of geomagnetic effects of solar flares (sfe) recorded at Ebre observatory (40.8° latitude N, 0.5° longitude E) during 33 years (1953‐1985). At Ebre, located near the focus latitude, two types of sfe can be observed: regular and reversed sfe. Regular sfe are those which have phase differences less than 90° with the regular diurnal magnetic variation of the day, SR. Reversed sfe are those which have phase differences greater than 90° with SR. From these 33 years, 140 sfe events were selected and a statistical study was performed. We found a local time dependence of the phase differences between the sfe and SR vectors. Morning hours have slightly positive values and afternoon hours have slightly negative ones. Reversed sfe, with a phase difference exceeding 90°, concentrate between 10 and 12 hours. Reversed sfe show a dominant equinoctial character. Also, a weaker correlation was found between solar activity with reversed sfe (r=0.47) than with regular sfe (r=0.68). Using data from 67 observatories, we performed a global study of a sfe case, seen at Ebre as reversed sfe. In this case, in the northern hemisphere, the sfe system was about 1 hour of local time eastward of the SR system and formed 4° higher in latitude. Finally, we present a model of two elliptical ionospheric equivalent current systems with focus offset about 1 hour in local time to explain the phase difference between the sfe and Sq magnetic vectors observed at Ebre. The parameters of this model have been fitted from the results of a previous statistical analysis from Ebre data. Spatial and temporal distribution of the sfe and Sq vector phases are calculated with this model, and conditions for reversed sfe occurrence are predicted.

Sq and EEJ—A Review on the Daily Variation of the Geomagnetic Field Caused by Ionospheric Dynamo Currents

“…Later studies recognized the difference in the position of the current foci between equivalent current systems of Sq and geomagnetic crochet (e.g., Volland and Taubenheim 1958;Veldkamp and Van Sabben 1960;Yasuhara and Maeda 1961;van Sabben 1961van Sabben , 1968Curto et al 1994;Gaya-Piqué et al 2008). The differences were generally attributed to the differ- Cliver and Svalgaard (2004) ent heights where the two types of ionospheric currents flow.…”

Automatic detection of Sfe: a proposal

“…The diurnal magnetic variation, S q , is always superposed on Sfe's and has a similar, although not identical, circular current system shape (Curto et al, 1994a). S q and Sfe current systems are not exactly coincident in space; generally in the Northern Hemisphere, the Sfe vortex is delayed from the S q vortex (Curto et al, 1994b).…”

“…By taking advantage of this, we can perform a coordinate transformation using the vortex point as the new origin of the coordinate system (center) instead of the geographic pole. We take the values given in Table 2 from Curto et al (1994a) as the position of the current vortex where the seasonal variation in the Sfe vortex was computed using a parametric model. This table is the result of the analysis of 30 years of data.…”

“…However, the perfect circular current model is a simplification of the reality. As shown by Curto et al (1994a), the current vortex (loop) is usually elliptical with moderate eccentricities in solstices but achieving important eccentricities in equinox. The overall performance will necessarily be reduced due to the slight misalignment of the observatory signals outside the main axes of the ellipse.…”

“…To this end, we can take advantage of the different time durations of the signals. While S q variations last 1 day (T S q = 24 h), Sfe's usually last for several minutes (T Sfe = 0.2 h) (Curto et al, 1994a), so the frequency rate of S q is one hundredth that of a Sfe. With a similar approach as in electricity, we can use a high-pass filter to reject signals under the cutoff frequency.…”