High-latitude Substorm Research Articles

Sign-Singularity Analysis of Field-Aligned Currents in the Ionosphere

Field-aligned currents (FACs) flowing in the auroral ionosphere are a complex system of upward and downward currents, which play a fundamental role in the magnetosphere–ionosphere coupling and in the ionospheric heating. Here, using data from the ESA-Swarm multi-satellite mission, we studied the complex structure of FACs by investigating sign-singularity scaling features for two different conditions of a high-latitude substorm activity level as monitored by the AE index. The results clearly showed the sign-singular character of FACs supporting the complex and filamentary nature of these currents. Furthermore, we found evidence of the occurrence of a topological change of these current systems, which was accompanied by a change of the scaling features at spatial scales larger than 30 km. This change was interpreted in terms of a sort of symmetry-breaking phenomenon due to a dynamical topological transition of the FAC structure as a consequence of FACs and substorm current wedge intensification during substorms.

Supersubstorms and Conditions in the Solar Wind

This article examines the effect of various large-scale solar-wind structures and streams on the occurrence of a special type of substorms—the so-called supersubstorms (SSS), which are very intense substorms defined by the indices SML –50 nT, the events occur mostly right after the sudden onset (SC) of a storm (11%) and, very rarely, happen late in the storm recovery phase (1.2%). The space weather conditions associated with SSS events differ sharply from those associated with other types of high-latitude substorms, such as polar and expanded substorms.

Solar Wind Streams of Different Types and High-Latitude Substorms

The effects of different large-scale solar wind structures on magnetospheric substorms at high geomagnetic latitudes are studied. Two types of high-latitude substorm disturbances are considered: substorms observed in quiet conditions, when the auroral oval contracts and shifts towards high latitudes (contracted oval substorms, or polar substorms), and those observed in disturbed conditions, when the auroral oval expands (expanded oval substorms, or expanded substorms). Ground-based magnetic observations from the Scandinavian IMAGE network are compared with the OMNI solar wind database and the catalog of large-scale solar wind phenomena ( ftp://ftp.iki.rssi.ru/omni/ ). The study involves the following types of solar wind streams: (1) high-speed streams from coronal holes (FAST); (2) interplanetary manifestations of coronal mass ejections, i.e., magnetic clouds (MCs) or EJECTA; and (3) plasma compression regions ahead of these streams, i.e., corotating interaction regions (CIRs) and SHEATH. The study includes 186 polar and 202 expanded substorms in 1995, 1996, 1999, and 2000. It is shown that ~75% of expanded substorms are observed in FAST streams or plasma compression regions ahead of these streams (CIRs), and only ~18% of such substorms are observed in interplanetary manifestations of coronal mass ejections EJECTA and in SHEATH compression regions. While ~67% of polar substorms are observed in SLOW streams, ~19% of such substorms have been recorded in SHEATH and EJECTA but only against the background of a slow solar wind.

High-latitude substorm dependence on space weather conditions in solar cycle 23 and 24 (SC23 and SC24)

The occurrence of the magnetic substorms at high geomagnetic latitudes (>70° CGC) has been visually analyzed using the ground-based IMAGE magnetometer chain and OMNI solar wind and Interplanetary Magnetic Field (IMF) data. We studied the time intervals closely to the minimum and maximum of the 23-th and 24-th solar cycles. The considered high-latitude substorms have been divided into two different types according to their occurrence relatively to the auroral oval dynamics. The first type - the substorms which expand from the auroral geomagnetic latitudes to the polar ones (named the “expanded” substorms, according to an expanded oval dynamics); the second type - the substorms which are observed only at the geomagnetic latitudes higher ∼ 70° CGC under the absence of simultaneous magnetic disturbances at lower latitudes (named the “polar” substorms, according to a polar latitude contracted oval dynamics). Both substorm types are identified at almost identical latitudes, however, with different latitude of their onset locations. It was found that the “polar” substorms show behavior opposite to the “expanded” ones. The “polar” substorms are observed under a low solar wind speed (V < 500 km/s) and the “expanded” substorms - under a high speed (V > 500 km/s). We found, that the PC-index of the polar cap activity, which could be used as a proxy of changes in solar wind electric field, was lower before the “polar” substorms than before the “expanded” ones. The summer maxima in the seasonal occurrence of the “polar” substorms correspond to the minima of the “expanded” substorm. We did not reveal any significant cycle differences in the distributions of the IMF and solar wind parameters before the both types of substorms observed near the maxima and minima of the 23-th and 24-th cycles of the solar activity.

Polar and high latitude substorms and solar wind conditions

All substorm disturbances observed in polar latitudes can be divided into two types: polar, which are observable at geomagnetic latitudes higher than 70° in the absence of substorms below 70°, and high latitude substorms, which travel from auroral ( 70°) geomagnetic latitudes. The aim of this study is to compare conditions in the IMF and solar wind, under which these two types of substorms are observable on the basis of data from meridional chain of magnetometers IMAGE and OMNI database for 1995, 2000, and 2006–2011. In total, 105 polar and 55 high latitude substorms were studied. It is shown that polar substorms are observable at a low velocity of solar wind after propagation of a high-speed recurrent stream during the late recovery phase of a magnetic storm. High latitude substorms, in contrast, are observable with a high velocity of solar wind, increased values of the Bz component of the IMF, the Ey component of the electric field, and solar wind temperature and pressure, when a high-speed recurrent stream passes by the Earth.

A broad climatology of very high latitude substorms

Magnetic data from a newly commissioned Indian Antarctic station Bharati (corrected geomagnetic (CGM) coordinates 74.7°S, 97.2°E) and closely-spaced IMAGE chain observatories (∼100° magnetic meridian in Northern hemisphere) has been analyzed to study the climatology of substorms which were localized poleward of the standard auroral oval. We considered four austral summers (year 2007–2010) when data from Bharati was available. Several very high latitude substorms were observed in this duration when the solar activity remained unexpectedly low for a long time. Various features of very high latitude substorms, e.g., local time dependence, interplanetary state, hemispherical asymmetry and their nightside low latitude signatures are examined. Events studied here, suggested the following properties of substorms occurring at very high latitudes: (1) maximum occurrence was observed near magnetic midnight (21:00–02:00 MLT). (2) In contradiction to earlier reports, many substorms were observed even during negative IMF Bz condition. In addition, majority of substorms occurred during low or moderate solar wind streams. (3) Magnetic signatures were often pronounced in the winter hemisphere. (4) Even if widely used standard AE indices fail to monitor very high latitude substorms, their low latitude signatures are often evident.

Non-typical ground-based quasi-periodic VLF emissions observed at L∼5.3 under quiet geomagnetic conditions at night

Non-typical long lasting quasi-periodic (QP) VLF emissions have been recorded in Northern Finland at L∼5.3 during the recent Finnish VLF campaign held in December 2011. Contrary to the typical daytime QP emissions, the night-time and early morning (00–05UT) event reported here for the first time is a sequence of 1.5–3.5kHz noise bursts lasting for several tens of seconds with an unusually long repetition period which gradually decreases from ∼700s to ∼50s. These QP emissions were observed under conditions of very quiet geomagnetic activity (Kp=0). In spite of that, the interplanetary magnetic field generally had a small southward component, and a high-latitude substorm occurred on the night-side. After this substorm, the repetition period of the VLF bursts suddenly dropped from ∼200s to∼60s and the spectral structure of QP wave changed. We attribute these QP emissions to auto-oscillations of the cyclotron instability of the Earth's radiation belts. According to the theory, the repetition period of the QP should be inversely proportional to the flux of the gyroresonant energetic electrons. Thus the increased flux of energetic electrons injected by the substorm probably led to the decreasing QP repetition periods.

Flux transport, dipolarization, and current sheet evolution during a double-onset substorm

[1] We study a substorm with two onsets (at 0220 and 0243 UT) that occurred during a gradual northward interplanetary magnetic field (IMF) turning on 16 February 2008. At these times, Time History of Events and Macroscale Interactions during Substorms (THEMIS) and GOES spacecraft were distributed between 6.6 and 18 RE downtail. Prior to the weak auroral electrojet onset at 0220 UT, a thin current sheet was extended near 10–11 RE. After the onset, Earthward fast flows with dipolarization fronts followed by signatures of magnetic flux pileup were detected in this region. The 0243 UT onset disturbances were more intense and centered at higher latitudes. The reconnection region tailward of 18 RE became activated and reached the lobe flux. We suggest that activations of reconnection Earthward of 18 RE associated with the 0220 UT event led to pileup of flux and redistribution of BZ to form a thin current sheet with small BZ in the midtail region. This made conditions favorable for reconnection tailward of 18 RE involving lobe flux for the 0243 UT onset. The reconfiguration process in the current sheet between the two onsets possibly enabled a relatively strong, high-latitude substorm despite the rather weak IMF driver. The near-Earth dipolarization observed after the 0243 UT onset was accompanied by more localized Earthward flows and flow reversals. Differences in dipolarization signatures could be caused by ambient plasma condition and field configuration between these two events. Our observations of the double-onset substorm suggest that the plasma sheet can be preconditioned by both the IMF driver and internal magnetotail processes.

Excitation of twin-vortex flow in the nightside high-latitude ionosphere during an isolated substorm

Abstract. We present SuperDARN radar observations of the ionospheric flow during a well-observed high-latitude substorm which occurred during steady northward IMF conditions on 2 December 1999. These data clearly demonstrate the excitation of large-scale flow associated with the substorm expansion phase, with enhanced equatorward flows being observed in the pre-midnight local time sector of the expansion phase auroral bulge and westward electrojet, and enhanced return sunward flows being present at local times on either side, extending into the dayside sector. The flow pattern excited was thus of twin-vortex form, with foci located at either end of the substorm auroral bulge, as imaged by the Polar VIS UV imager. Estimated total transpolar voltages were ~40 kV prior to expansion phase onset, grew to ~80 kV over a ~15 min interval during the expansion phase, and then decayed to ~35 kV over ~10 min during recovery. The excitation of the large-scale flow pattern resulted in the development of magnetic disturbances which extended well outside of the region directly disturbed by the substorm, depending upon the change in the flow and the local ionospheric conductivity. It is estimated that the nightside reconnection rate averaged over the 24-min interval of the substorm was ~65– 75 kV, compared with continuing dayside reconnection rates of ~30–45 kV. The net closure of open flux during the sub-storm was thus ~0.4–0.6 × 108 Wb, representing ~15–20% of the open flux present at onset, and corresponding to an overall contraction of the open-closed field line boundary by ~1° latitude.Key words. Ionosphere (auroral ionosphere; ionosphere-magnetosphere interactions; plasma convection)

Possible enhancement of previously unobserved (NII) emissions in the mid‐day aurora associated with nightside substorm activity

We report here the presence of previously unobserved NII emissions in several spectra of aurora from the dayside cusp region taken near Longyearbyen, Spitzbergen (74.3°N geomagnetic). The presence of the 3d³P° ‐ 4p³S transition giving rise to the triplet (6809, 6834, 6847A) is particularly obvious, but the spectra also show strong evidence of other NII emissions such as those from the 3d³D° ‐ 4p³D level. The forbidden [NII] (6548, 6584A) doublet seems also to be present, but less obvious than the NII (6809, 6834, 6847A) triplet. The presence or the enhanced level of these permitted emissions seems, on two occasions, to coincide with a sudden decrease in the characteristic energy of the local particle precipitation and with a high latitude substorm on the nightside. Integration time is so large (30 min) as to preclude a more exact correlation with the substorm. However, two otherwise identical situations occurred with no nightside substorm and neither increased NII emissions nor decreased particle energies were observed on the dayside.

Some characteristics of high-latitude substorm

The occurrence characteristics of substorms appearing at invariant latitudes greater than 79° and the movement of associated ionospheric electrojets are investigated. These substorms appear mainly in the winter months during the descending phase of the sun-spot cycle. The substorms generally are localized and occur on days of moderate magnetic activity. The diurnal variation of their occurrence reflects the proximity of the station to the poleward edge of the auroral bulge in the midnight-dawn sector. A study of several selected events suggests that electrojets may occur simultaneously at the equatorward and poleward edges of the auroral oval.

High-latitude geomagnetic variations and substorms

The present review concerns high-latitude geomagnetic variations, and in particular those caused by fluctuations of the interplanetary magnetic field. It is shown that IMFB z,B x,B y and |B y| produce four kinds of geomagnetic effects, which qualitatively agree with the expected consequences of the reconnection hypothesis. At quiet time the cleft into the Earth's magnetosphere has a quasi-circular shape at Φ ≈ 78° that also fits qualitatively the reconnection model by Stern (1973) but does not agree with the conception ofdayside cusps. From the analysis of the geomagnetic effects of IMFB z it was found also that the contribution of magnetospheric dynamo to the electric field of the dayside plasmasphere (middle and low latitudes) does not exceed 15–20%. Characteristics of this contribution are given (i.e.DP-2 fields). The magnetic substorm models are reviewed as well. Geomagnetic data confirm the existence of the substorm growth phase both atB z < 0 and atB z > 0. The expansion phase of most substorms evidently involves the processes of reconnection and neutral line formation near the inner edge of the plasma sheet (LT ≈ 23h), resulting from instability of field-aligned currents of the westward electrojet ath ~ 1000 km. Such a mechanism accounts for a number of signatures for local development of substorms, including coastal effect, jumplike development of the electrojet, etc. The second kind of substorms, not involving the magnetic disturbances, is probably caused by the development of ion tearing instability in the plasma sheet. The original results are presented against a general review background, which includes a method for mathematical description of global fields of magnetic variations and substorms.