A. T. Aikio

Characteristics of pseudobreakups and substorms observed in the ionosphere, at the geosynchronous orbit, and in the midtail

We present a comprehensive study of a sequence of two substorms and multiple pseudobreakups using optical, magnetic and incoherent scatter radar measurements, energetic particles from two geosynchronous satellites and particle and field data from the Geotail spacecraft located at Xgsm ∼ −86 RE. Following conventional nomenclature, we classified as pseudobreakups those auroral breakups which did not exhibit significant poleward expansion (< 2° magnetic latitude). Auroral intensifications following substorm breakups were also observed, and were classified separately. Pseudobreakups were found not to differ from substorm breakups in longitudinal extent (from 1.3 to 6.1 hours of magnetic local time), or in duration (from 5 to 16 minutes). In general, the ionospheric currents producing ground magnetic disturbances were more intense during substorms than pseudobreakups. We found that pseudobreakups are associated with the same magnetospheric processes as substorm breakups which involve current wedge formation, midlatitude magnetic Pi2 pulsations and energetic particle injections at the geosynchronous altitude. Moreover, pseudobreakups are associated with magnetic reconnection in the near-Earth region, evidenced by the typical subsequent detection of a plasmoid at Geotail. This implies that the magnetotail volume influenced by a pseudobreakup is quite large in radial distance. We conclude that there is no definitive qualitative distinction between pseudobreakups and substorms but there is a continuum of states between the small pseudobreakups and large substorms.

Ground-based measurements of an arc-associated electric field

Abstract The electrodynamics of a pre-midnight auroral arc, situated close to the equatorward border of the convection reversal, are studied by ground-based measurements. The optical intensity and the location of the arc is determined by a scanning photometer and an all-sky camera. Ionospheric electric fields around the arc are measured by the STARE radar. The EISCAT radar provides measurements of electric fields and conductivities equatorward of the arc. The EISCAT magnetometer chain is used to observe the development of ionospheric currents. An enhanced northward component of the electric field is observed equatorward of the arc. The width of this arc-associated electric field is estimated to be less than, or equal to, 45 km. A quite new result is the simultaneous intensification of the arc-associated electric field and the optical brightening of the auroral arc. The brightening of the arc is related to an intensification of the upward current from the arc, carried by energetic electrons. Previous satellite and rocket measurements have shown that auroral arcs are associated with matched pairs of currents with the upward current flowing from the arc and the downward current, carried by cold ionospheric electrons, flowing on the equatorward (poleward) side of the arc in the evening (morning) sector. The field-aligned currents are connected by a Pedersen current flowing in the direction of the electric field in the ionosphere. If the magnetospheric generator region acts as a current generator, the ionospheric electric field has to modify itself in a way that the current continuity in the ionosphere is preserved. When the ionospheric conductivity is low adjacent to the arc, as the EISCAT measurement indicates, the meridional electric field has to increase in order to produce an enhanced Pedersen current and an enhanced downward current (by the gradient of the Pedersen current) in response to the increased upward current. Thus we suggest that the enhanced electric field adjacent to the arc is a result of the ionosphere-magnetosphere coupling, influenced by the low ionospheric conductivity.

Multistation correlation of ULF pulsation spectra associated with sudden impulses

Abstract We have made a survey of ULF emissions associated with sudden compressions of the magnetosphere due to sudden impulses (SI or ssc) on the basis of induction coil magnetometer recordings collected in Finland and Alaska during 1976–1979. We show several examples to illustrate the utility in classifying the emissions according to their spectral characteristics. The most common emissions are Pc1 pulsations. These show a maximum occurrence in the noon-afternoon sector and may undergo drastic variations in form as a function of latitude along one meridian. Pulsations which occur both in Finland and Alaska, at the same latitude separated by 175°, show very little similarity. A SI amplitude threshold of approximately 10 nT is necessary for the stimulation of Pel pulsations, however, if there is previous Pc1 activity this threshold is lowered. As SI amplitudes rise above 25 nT the probability of observing ULF emissions rises dramatically. We note that SI often trigger band-limited bursts of ULF emissions near 1 Hz. These bursts are short-lived with durations of a few minutes and occur a few minutes after the SI. They are most common during years of high solar activity. We conclude that the stimulation of different types of ULF emissions by SIs reflects the state of the magnetosphere at the time of the SI.

A substorm observed by EISCAT and other ground-based instruments — evidence for near-Earth substorm initiation

Abstract We present observations of the development of a substorm in the ionosphere made by the EISCAT radar, optical and magnetic instruments. A typical growth phase started 30 min before the substorm onset with equatorward drifting arcs. The arc which crossed the EISCAT beam drifted equatorward with approximately the same velocity as the ambient plasma and it approached the most equatorward discrete arc. The most equatorward discrete arc (the breakup arc) stopped its drift at L = 5.3. Arcs about 1° of latitude poleward of the breakup arc continued their equatorward motion with no significant changes in intensity after the onset of the substorm, which was timed on the basis of the explosive intensification in the breakup arc. Similarly, no dramatic changes in electric fields or other plasma parameters measured by EISCAT were observed poleward of the breakup arc. The observations indicate that the instability that triggered the substorm onset was localized in the near-Earth magnetotail. A westward travelling surge (WTS) developed in the breakup arc and moved westward with a very high velocity, 13 ± 3 km s−1. Extremely high conductances were measured by EISCAT from the poleward boundary of the poleward expanding bulge with a maximum value of ΣH = 214 S obtained with a time resolution of 0.2 s. The westward electrojet (WEJ) was observed to be latitudinally very inhomogenous and concentrated near conductivity enhancements, especially close to the head of the WTS. The localization of the WEJ close to discrete arcs and consequent motions with the arcs gave the charasteristic spiky appearance of the magnetic X-component in the pre-midnight sector.

On the origin of the high-altitude electric field fluctuations in the auroral zone

Intense fluctuations in the electric field at high altitudes in the auroral zone are frequently measured by the Viking satellite. We have made an analysis of the origin of electric and magnetic flu ...

Near‐Earth substorm onset: A coordinated study

We present simultaneous satellite and ground-based measurements of a substorm. Throughout the initial substorm expansion, southward drifting arcs are observed poleward of the expanding substorm aurora, indicating two independent systems of particle precipitation. Freja passes the brightening onset arc in the topside ionosphere near the moment of the substorm onset, observing an Alfven wave, field aligned current and oxygen ion outflow. The substorm onset occurs at low magnetospheric L-shells, near the poleward edge of the region of trapped particles. The location and time for the substorm injection are confirmed by geostationary spacecraft together with magnetometers, all-sky cameras and radar on the ground. We believe that the substorm onset may be triggered by modification of the oxygen content of the inner magnetosphere during the growth-phase caused by ionospheric ion outflow.

Ionospheric Conductances and Currents of a Morning-Sector Auroral Arc From Swarm-A Electric and Magnetic Field Measurements†

We show the first ionospheric Hall and Pedersen conductances derived from Swarm magnetic and electric field measurements during a crossing of a morning sector auroral arc. Only Swarm-A was used, with assumptions of negligible azimuthal gradients and vanishing eastward electric field. We find upward field-aligned current, enhanced Hall and Pedersen conductances, and relatively weak electric field coincident with the arc. Poleward of the arc the field-aligned current was downward, conductances lower, and the electric field enhanced. The arc was embedded in a westward electrojet, immediately equatorward of the peak current density. The equatorward portion of the electrojet could thus be considered conductance dominant and the poleward portion electric field dominant. Although the electric field measured by Swarm was intense, resulting in conductances lower than those typically reported, comparable electric fields have been observed earlier. These results demonstrate how Swarm data can significantly contribute to our understanding of the ionospheric electrodynamics.

Response of the quiet auroral arc motion to ionospheric convection variations

Equatorward motion of quiet auroral arcs was studied together with the F region ionospheric plasma drift observed by the European Incoherent Scatter radar in Tromso. Variations in the arc velocity lagged behind corresponding variations in the ionospheric convection velocity by 5–9 min. The same result was obtained from comparison of the arc velocity with variations in the eastward component of ground magnetic field. The variations in the arc and plasma drift velocities followed interplanetary magnetic field Bz variations. The observations indicate that the dawn-dusk electric potential difference arises at the open polar cap magnetic field lines due to the solar wind - magnetosphere interaction and then propagates to the closed magnetotail field lines through the ionosphere.

Radar observations of simultaneous traveling ionospheric disturbances and atmospheric gravity waves

Simultaneous observations of atmospheric gravity waves (AGWs) and traveling ionospheric disturbances (TIDs) measured by an incoherent scatter radar at high latitudes are shown. The measurements were made using a beam swing experiment of the EISCAT UHF radar. The F region TID is seen as wavefronts in electron density, whereas the E region AGW is seen in the oscillations of the neutral wind. The wave vector of the TID has a downward component indicating that energy propagates upward. The periods of AGWs and TIDs are approximately the same (52–57min), so it is concluded that the observed gravity wave in the E region propagates to the F region causing the TID there. Two interesting properties of the waves are observed. First, the neutral wind oscillations have an amplitude minimum at about 115km. It is suggested that this could be related to the minimum of the vertical refractive index around 120km. Second, in the course of time, the wave vector of the TID turns more in the downward direction, which leads to an increase in the horizontal wave length from 400 to 1450km. A possible explanation is that the background wind increases with altitude and turns the wavefronts more horizontal when distance from a stationary source increases. We suggest that the source is the sunrise terminator, since the horizontal direction of propagation of the TID in the morning hours is from the west, where both the auroral and thunderstorm activity are low.

Solar wind effect on Joule heating in the high‐latitude ionosphere

The effect of solar wind on several electrodynamic parameters, measured simultaneously by the European Incoherent Scatter (EISCAT) radars in Tromso (TRO, 66.6° cgmLat) and on Svalbard (ESR, 75.4° cgmLat), has been evaluated statistically. The main emphasis is on Joule heating rate QJ, which has been estimated by taking into account the neutral wind. In addition, a generally used proxy QE, which is the Pedersen conductance times the electric field squared, has been calculated. The most important findings are as follows. (i) The decrease in Joule heating in the afternoon-evening sector due to winds reported by Aikio et al. (2012) requires southward interplanetary magnetic field (IMF) conditions and a sufficiently high solar wind electric field. The increase in the morning sector takes place for all IMF directions within a region where the upper E neutral wind has a large equatorward component and the F region plasma flow is directed eastward. (ii) At ESR, an afternoon hot spot of Joule heating centered typically at 14–15 magnetic local time (MLT) is observed during all IMF conditions. Enhanced Pedersen conductances within the hot spot region are observed only for the IMF Bz + /By− conditions, and the corresponding convection electric field values within the hot spot are smaller than during the other IMF conditions. Hence, the hot spot represents a region of persistent magnetospheric electromagnetic energy input, and the median value is about 3 mW/m2. (iii) For the southward IMF conditions, the MLT-integrated QE for By− is twice the value for By+ at TRO. This can plausibly be explained by the higher average solar wind electric field values for By−.