Annales Geophysicae (2004) 22: 923–934 SRef-ID: 1432-0576/ag/2004-22-923 Annales © European Geosciences Union 2004 Geophysicae Investigation of subauroral ion drifts and related field-aligned currents and ionospheric Pedersen conductivity distribution S. Figueiredo, T. Karlsson, and G. T. Marklund Alfven´ Laboratory, Royal Institute of Technology, Stockholm, Sweden Received: 4 December 2002 – Revised: 10 September 2003 – Accepted: 8 October 2003 – Published: 19 March 2004 Abstract. Based on Astrid-2 satellite data, results are pre- ther as a pure voltage generator or as a pure current genera- sented from a statistical study on subauroral ion drift (SAID) tor, applied to the ionosphere. While the anti-correlation be- occurrence. SAID is a subauroral phenomenon characterized tween the width and the peak intensity of the SAID structures by a westward ionospheric ion drift with velocity greater than with substorm evolution indicates a magnetospheric source 1000 m/s, or equivalently, by a poleward-directed electric acting as a constant voltage generator, the ionospheric mod- field with intensity greater than 30 mV/m. SAID events occur ifications and, in particular the reduction in the conductivity predominantly in the premidnight sector, with a maximum for intense SAID structures, are indicative of a constant cur- probability located within the 20:00 to 23:00 MLT sector, rent system closing through the ionosphere. The ionospheric where the most rapid SAID events are also found. They are feedback mechanisms are seen to be of major importance for substorm related, and show first an increase in intensity and sustaining and regulating the SAID structures. a decrease in latitudinal width during the expansion phase, Key words. Ionosphere (mid-latitude ionosphere; electric followed by a weakening and widening of the SAID struc- fields and currents; ionosphere-magnetosphere interactions) tures during the recovery phase. The potential drop across a SAID structure is seen to remain roughly constant during the recovery phase. 1 Introduction The field-aligned current density and the height-integrated Pedersen conductivity distribution associated with the SAID Strong poleward electric fields associated with intense west- events were calculated. The results reveal that the strongest ward ionospheric plasma flow are a relatively common fea- SAID electric field peaks are associated with the lowest Ped- ture observed at subauroral latitudes in the evening to pre- ersen conductivity minimum values. Clear modifications are midnight local time sector during substorm activity. Denoted seen in the ionospheric Pedersen conductivity distribution as- as subauroral ion drifts (SAID) or equivalently, as subauroral sociated with the SAID structure as time evolves: the SAID electric fields (SAEF), this phenomenon has been the subject peak is located on the poleward side of the corresponding of many studies reported by several authors during the last region of reduced Pedersen conductivity; the shape of the re- three decades. Although different measurement techniques gions of reduced conductivity is asymmetric, with a steeper and different definitions were used, there are several features poleward edge and a more rounded equatorward edge; the of this phenomenon generally agreed upon. The SAID phe- SAID structure becomes less intense and widens with evolu- nomenon is a manifestation of magnetosphere-ionosphere tion of the substorm recovery phase. From the analysis of the coupling outside of the auroral region, characterized by west- SAID occurrence relative to the mid-latitude trough position, ward ion drifts with velocities greater than 1000 m/s by many SAID peaks are seen to occur relatively close to the corre- of the authors (Anderson et al., 1991; Galperin et al., 1997; sponding mid-latitude trough minimum. Both these features Burke et al., 2000). Some authors have focused on the corre- show a similar response to magnetospheric disturbances, but sponding intense poleward-directed electric fields, reporting on different time scales - with increasing magnetic activity, events with intensities greater than approximately 30 mV/m the SAID structure shows a faster movement towards lower (Smiddy et al., 1977; Maynard, 1978; Maynard et al., 1980; latitudes than that of the mid-latitude trough. Rich et al., 1980; Karlsson et al., 1998). SAID structures are From the combined analysis of these results, we conclude confined to latitudinally narrow regions (0.1◦–2◦ CGLat – that the SAID generation mechanism cannot be regarded ei- Corrected Geomagnetic Latitude), equatorwards of the auro- ral zone (Karlsson et al., 1998; Galperin et al., 1997; An- Correspondence to: S. Figueiredo derson et al., 2001), occur predominantly in the premid- (sonia.fi[email protected]) night MLT (Magnetic Local Time) sector (18:00–02:00 MLT, 924 S. Figueiredo et al.: Investigation of subauroral ion drifts according to Spiro et al. (1979), Karlsson et al. (1998) and a more recent study, Anderson et al. (1991) showed that in Anderson et al. (2001)) and are substorm related (Smiddy most cases a mid-latitude trough was present prior to SAID et al., 1977; Maynard, 1978; Spiro et al., 1979; Maynard formation. Very deep troughs were observed associated with et al., 1980; Rich et al., 1980; Anderson et al., 1993; Karlsson SAID events, resulting from a deepening caused by the SAID et al., 1998; Keyser et al., 1998; Keyser, 1999; Burke et al., of the poleward most extent of the preexisting trough. 2000; Anderson et al., 2001; Galperin, 2002). Different models for the SAID production mechanism Events were reported to occur more than 30 min after sub- have been presented. Anderson et al. (1993) suggested a storm onset during the substorm early recovery phase (May- model where the downward flowing region 2 FAC close via nard et al., 1980; Anderson et al., 1993). Karlsson et al. ionospheric Pedersen currents with the outward flowing re- (1998) found that events close to 22:00 MLT occurred ear- gion 1 currents. As the Pedersen currents flow in the region lier during the recovery phase. Galperin (2002) also refers of low conductivity equatorward of the electron precipitation, to a similar result. Close to the midnight MLT sector, SAID a region of relatively large poleward-directed electric fields is events (or equivalently denoted as polarization jets when it produced. These electric fields, in turn, produce large west- was first described by Galperin et al. (1973)) can appear ∼10 ward ion drifts that will, by frictional heating of the ions, min after a large AE-index variation (>500 nT). However, cause an increase in the recombination of NO+ with elec- the average SAID delay from substorm onset was inferred to trons, consequently reducing the already low subauroral con- be about 30 min in the near midnight sector, reaching values ductivity. As the substorm evolves, very large electric fields up to 1–2 h towards the evening MLT sector. are eventually produced between the converging equatorial Rich et al. (1980) refer to the absence of any particular cor- boundaries of the electron and ion precipitation, giving rise relation between observations of SAID events and the magni- to the latitudinally narrow regions of rapid SAID. Karlsson tude of the activity index Kp, while the statistical results re- et al. (1998) reported results from a statistical study based on ported by Karlsson et al. (1998) showed a tendency of SAID Freja satellite data. These results are in agreement with the occurrence moving towards lower latitudes with increasing model proposed by Anderson et al. (1993) and clearly indi- Kp value, as well as an increase of the associated poleward cate that the SAID phenomenon has a close relationship to electric field intensity with increasing Kp, up to Kp≈5. the mid-latitude trough and the currents flowing through this Some authors have associated the SAID occurrence with region. phenomena related to the ionospheric projection of the Keyser et al. (1998) and Keyser (1999) presented a differ- plasmapause, such as the electron trough. Heelis et al. ent mechanism for the formation and evolution of SAID in (1976) reported large, localized westward plasma flow ve- the course of a substorm. In this model, SAID is considered locities in the evening electron trough. Smiddy et al. (1977) as the ionospheric signature of a voltage difference gener- also reported some events where large electric field signals ated across a magnetospheric current sheet, interfacing the were detected near the electron trough. Maynard (1978) re- cold plasma trough and hot injected plasma moving inward. ported intense poleward-directed electric fields in the pre- This model accounts for the westward direction and intensity midnight F-region, close to the Harang discontinuity. The of the SAID, for its latitudinal width, its lifetime (generally author concluded that these intense fields could result from <3 h), and also predicts its predominant occurrence in the the substorm-related expansion of the convection patterns to premidnight sector, as well as its movement towards lower lower latitudes being impeded by some means such as the latitudes with increasing magnetic activity. lower conductivity in the trough. At the same time, South- In a recent study by Galperin (2002) a semi-quantitative wood and Wolf (1978) presented a model that predicts the model was presented, which describes the main features of occurrence of intense electric fields in the region between a SAID event, and also the recent observational results by the proton inner edge and the low-latitude edge of elec- Khalipov et al. (2001). The last author reported on SAID for- tron precipitation (i.e. the trough), if these boundaries are mation near midnight shortly after substorm onset (∼10 min close to each other but not coincident, as a consequence or less), accompanied by a fast westward displacement of of a substorm-enhancement of the cross-tail magnetospheric the Harang discontinuity. The model presented by Galperin electric field. (2002) is based on a main hypothesis: the convection pat- The effects of large electric fields imposed on the iono- tern equipotentials in the evening to midnight MLT sector are sphere have also been investigated.