EISCAT Magnetometer Cross Research Articles

Anomalous sudden commencement on March 24, 1991

An anomalous geomagnetic sudden commencement (SC) occurred on March 24, 1991. It is characterized by an exceptionally large and sharp impulse observed in its initial part along the noon meridian in middle and low latitudes. The analysis of the SC was made by using high time resolution digital data from the 210° Meridian Magnetometer Chain in the west Pacific, Sub‐Auroral Magnetometer Network (SAMNET) in the United Kingdom and southern Scandinavia, the EISCAT Magnetometer Cross in northern Scandinavia and Svalbard, and Canopus in Canada together with other ground and satellite (GOES 6, GOES 7, CRRES, and GMS) data. The results of the analysis suggest that the pulse observed at lower‐latitude ground stations was caused by the propagation of a strong magnetospheric compression of short duration (less than l min) which has never been observed before this event. The HF Doppler observation in Kyoto near local noon seems to be consistent with existence of the bipolar electric field associated with the propagating compressional magnetic pulse. The SAMNET stations and CRRES in the early morning also detected positive pulses which delays 30–50 s from the pulses in noon sector. Although the delay in the peak time of the pulse observed on the ground is consistent with ionospheric hydromagnetic wave propagation from the dayside to the nightside with finite speed, the initial onset time of the pulse on the ground was almost simultaneous everywhere suggesting the existence of an “almost instantaneous” propagation mode below the ionosphere.

What can we tell about global auroral-electrojet activity from a single meridional magnetometer chain?

Abstract. The AE indices are generally used for monitoring the level of magnetic activity in the auroral oval region. In some cases, however, the oval is either so expanded or contracted that the latitudinal coverage of the AE magnetometer chain is not adequate. Then, a longitudinal chain in the key region would give more information of the real situation, but, of course, only during some limited UT-period. In order to find out the UT coverage of a single meridional chain, we have compared the global AL and AU indices with corresponding local indices determined using data from the meridional part of the EISCAT Magnetometer Cross during the years 1985–1987. A statistical study shows that the local indices are close (within relative error of 0.2) to the global AU and AL during periods 1500–2000 UT (~1730–2230 MLT) and 2130–0130 UT (~0000–0400 MLT), respectively. In the middle of these optimal MLT-sectors the EISCAT Cross sees more than 70% of the cases when the global AE chain records activity. Then, also the correlation between the local and global indices is generally good (>0.7). Thus we conclude that five to six evenly located meridional chains are needed for covering all the UT-periods. On the other hand, already the combination of IMAGE, CANOPUS, and the Greenland chains catches ~50% of the substorms. Case-studies show that usually during 2130–1100 UT the AL achieved from these chains reproduces the real AL with good timing, although it does not follow all transient variations.

An EISCAT study of a pulsating auroral arc: simultaneous ionospheric electron density, auroral luminosity and magnetic field pulsations

We present a case study of a pulsating auroral are using EISCAT incoherent scatter radar measurements of energetic electron precipitation ∼ 5–30 keV) combined with ground-based observations of auroral luminosity and magnetic pulsations. The event under consideration occurred during a magnetically quiet period on 1 February 1987 between 0:00 and 0:30 UT. Pronounced pulsations with a period of about 1 min were present in all measured quantities. The magnetic pulsations of this period exhibited in phase oscillations over the spatial scale of the EISCAT Magnetometer Cross (some 250 km in longitude and 1000 km in latitude). Spectral analysis revealed also variations of a shorter time scale of about 10 s in all measured quantities except for the flux of precipitating electrons having energies below 10 keV (above 10 keV the electron flux exhibited 10 s variations). In the framework of the cyclotron resonant interaction of electrons with whistler waves, the long period pulsations are attributed to temporal modulation of the energetic electron source. The simultaneously observed pulsations with a period of about 10 s are explained within the self-oscillating regime of the whistler cyclotron instability in the magnetosphere. We present computational results from a self-consistent instability model taking all the conjectured effects into account.

The response of ionospheric convection in the polar cap to substorm activity

Abstract. We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°λ-78°λ. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71°λ by the time of the expansion phase onset. A westward electrojet, centred at 65.4°λ, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1°λ at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the "distant" neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.

The response of ionospheric convection in the polar cap to substorm activity

<strong class="journal-contentHeaderColor">Abstract.</strong> We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°&#x03BB;-78°&#x03BB;. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71°&#x03BB; by the time of the expansion phase onset. A westward electrojet, centred at 65.4°&#x03BB;, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1°&#x03BB; at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the "distant" neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.

The association between giant pulsations (Pgs) and the auroral oval

Abstract. Two features of giant pulsations (Pgs) which still require an explanation are firstly, why Pgs occur mainly in the early morning sector (i.e. 03:00-07:00 MLT) and not at other times of day, and secondly, why Pgs occur preferentially in a narrow latitudinal band (approximately 63°-68° geomagnetic latitude). Using statistics from 34 Pg events observed by the EISCAT magnetometer cross, a comparison has been made between the location of the Pg resonant field lines and the equatorward edge of the auroral oval. The majority of these Pg events appear to occur just poleward of this boundary. Using these results, an explanation of the two features of Pgs as detailed above is made. This explanation involves the interaction of protons, which may be responsible for the Pg events, with the inner edge of the plasma sheet or with its ionospheric equivalent, the equatorward edge of the auroral oval.

Combined measurements of EISCAT and the EISCAT magnetometer cross to study Ω bands

A combination of incoherent scatter radar data and data from a magnetometer array complement each other in an ideal way. The magnetometer results help to distinguish between temporal and spatial fluctuations in the radar data, and the radar results provide a means to separate magnetometer‐inferred equivalent currents into their Hall and Pedersen components. The technique is used to study Ω bands and leads to a better understanding of the ionospheric current structure associated with this phenomenon. In extending previous models we propose a pair of counterrotating source free ionospheric current vortices driven by field‐aligned currents, an upward current centered in the luminous part of the Ω band and a downward current in the dark part with its center about 400 km west of the upward current.

The association between giant pulsations (Pgs) and the auroral oval

<strong class="journal-contentHeaderColor">Abstract.</strong> Two features of giant pulsations (Pgs) which still require an explanation are firstly, why Pgs occur mainly in the early morning sector (i.e. 03:00-07:00 MLT) and not at other times of day, and secondly, why Pgs occur preferentially in a narrow latitudinal band (approximately 63°-68° geomagnetic latitude). Using statistics from 34 Pg events observed by the EISCAT magnetometer cross, a comparison has been made between the location of the Pg resonant field lines and the equatorward edge of the auroral oval. The majority of these Pg events appear to occur just poleward of this boundary. Using these results, an explanation of the two features of Pgs as detailed above is made. This explanation involves the interaction of protons, which may be responsible for the Pg events, with the inner edge of the plasma sheet or with its ionospheric equivalent, the equatorward edge of the auroral oval.

An ionospheric travelling convection vortex event observed by ground‐based magnetometers and by Viking

A transient ionospheric travelling convection vortex (ITCV) event was recorded by the EISCAT magnetometer cross in northern Scandinavia on April 21, 1986 around 8:40 MLT. Simultaneously, the near‐conjugate satellite VIKING observed strong magnetic and electric field variations on closed magnetic field lines. The signatures seen by the spacecraft are consistent with a passage through two oppositely directed field‐aligned current tubes. The total upward and downward currents turned out to be matched within a region of ∼ 1000 km in the ionosphere. Because of the close spatial and temporal proximity of both observations we interpret the field‐aligned currents as being associated with the ITCV. This is the first time that in‐situ measurements of the currents connecting the ITCV to the source region close to the magnetopause have been published. Both the current density and the dimensions of the current tubes could be estimated here.

Simultaneous observation of the poleward expansion of substorm electrojet activity and the tailward expansion of current sheet disruption in the near‐Earth magnetotail

In this paper we present observations of a substorm that occurred on June 7, 1985. The data consist of energetic ion and magnetic field data from AMPTE/CCE, magnetometer data from the ground stations of the EISCAT magnetometer cross, STARE radar data, and Pi 1 data from Sodankylä. CCE was in the Finnish local time sector, and the model field line threading CCE came down near the center of the EISCAT magnetometer cross. Furthermore, CCE was located near the neutral sheet and observed the disruption of the cross‐tail current in situ. At 2209 UT, Sodankylä registered a Pi 1 burst and the EISCAT stations recorded the onset of a negative bay. Simultaneously, CCE observed the onset of current sheet disruption and ion energization. The energetic ions observed during this initial burst had gyrocenters earthward of CCE. An intensification of activity occurred at 2212 UT, and it was observed essentially simultaneously on the ground and in space. It consisted of an intensification of Pi 1 activity, an intensification of the westward electrojet poleward of the initial breakup latitude (as determined from magnetometer and radar data), and magnetic turbulence and a burst of energetic ions at CCE in the magnetotail. Significantly, this second burst was composed only of ions with gyrocenters tailward of the satellite, and the magnetic turbulence was considerably weaker than that observed during the initial current disruption. This strongly suggests that the current disruption region had moved tailward of CCE. The event provides a direct observational link between the tailward expansion of active regions in the near‐Earth magnetotail and the poleward expansion of ionospheric electrojet activity during substorms.

Observations of a giant pulsation across an extended array of ground magnetometers and on auroral radar

It has often been suggested that energetic protons are involved in the generation of giant pulsations (Pgs). A Pg event occurring in the early morning of 29 December 1987 has been observed by magnetometers in Scandinavia, the Faroe Islands and Iceland. The Pg has also been observed by the SABRE coherent radar system. The azimuthal drift velocity of the Pg disturbance region across the available longitudinal extent has been measured using the magnetometer data and the radar data have been used to estimate the ionospheric convection electric field. The energies of protons that would drift at this velocity in the region of the magnetosphere where the Pg occurred have been deduced to be ∼ 10–20 keV, depending on the pitch angle of the protons. Using azimuthal wavenumbers calculated on the EISCAT magnetometer cross and the SABRE radar for this event, the energies of protons that would satisfy the bounce resonance condition for this Pg have been calculated to be ∼ 5–18 keV. The Pg is thought to be driven by protons associated with a substorm particle injection that occurred on 28 December 1987. The energies of protons that would drift westward from this injection region and reach the Pg disturbance region have been calculated to be ∼ 11–24 keV. This study therefore presents good evidence to suggest that the bounce resonance interaction with protons is the mechanism responsible for the Pg and that these protons originated from a substorm particle injection.

Statistical studies of giant pulsations (Pgs): Harmonic mode

Thirty-four Pg events have been observed by the EISCAT magnetometer cross. For each of these events the equatorial mass density at the time of the event has been estimated for both a dipole and a more realistic magnetospheric field model. By comparing these results with those of previous studies it has been deduced that Pgs are second harmonic rather than fundamental standing wave oscillations. This is consistent with the bounce resonance mechanism being the driving mechanism for Pgs. The Pg source region in the equatorial plane has also been accurately located.

Multipoint observations of a small substorm

In this paper we present multipoint observations of a small substorm which occurred just after 0110 UT on April 25, 1985. The observations were made by spacecraft (AMPTE CCE, AMPTE IRM, DMSP F6, and DMSP F7), ground auroral stations (EISCAT magnetometer cross, Syowa, Narssarssuaq, Great Whale River, and Fort Churchill), and mid‐latitude stations (Furstenfeldbruck, Toledo, and Argentine Island). These data provide us with a broad range of observations, including the latitudinal extent of the polar cap, visual identification of substorm aurorae and the magnetic perturbations produced directly beneath them, in situ magnetic field and energetic particle observations of the disruption of the cross‐tail current sheet, and observations concerning the spatial expansion of the current disruption region from two radially aligned spacecraft. The DMSP data indicate that the event took place during a period when the polar cap was relatively contracted, yet the disruption of the current sheet was observed by CCE at 8.56 RE. We have been able to infer a considerable amount of detail concerning the structure and westward expansion of the auroral features associated with the event, and we show that those auroral surges were located more than 10° equatorward of the boundary between open and closed field lines. Moreover, we present evidence that the current sheet disruption observed by CCE in the neutral sheet was located on field lines which mapped to the westward traveling surge observed directly overhead of the ground station at Syowa. Furthermore, the observations strongly imply that disruption of the cross‐tail current began in the near‐Earth region and that it had a component of expansion which was radially antisunward.

A model for the electric fields and currents during a strong Ps 6 pulsation event

On April 21, 1985, an intense Ps 6 pulsation event was observed with the EISCAT radar and the EISCAT magnetometer cross. These measurements serve as a reference for a new electrostatic model for the ionospheric conductances, electric fields, and currents of the auroral structures associated with the pulsations, whose auroral signatures are the Ω bands. All parameters are essentially derived from the input field‐aligned current distribution. By varying this distribution and a few free parameters in the relation between the conductances and the upward current, the model is adjusted to the data. We find that by a rearrangement of the upward current from a one‐dimensional sheet configuration to tonguelike poleward extensions the observed event is reproduced in a satisfactory way. Compared to previous works, the Hall current is modulated in a different, less symmetric way, and considerably lower field‐aligned current densities are required.

Geomagnetic effects of the Hall and Pedersen current flowing in the auroral ionosphere

Correlation studies are made on relationships between ionospheric currents calculated from EISCAT radar data (ion drift, electron density and temperature of ions and electrons), and ground magnetic variations observed at seven stations of the EISCAT magnetometer cross. The data used are 5‐min averaged values taken every 10 min during 24 hours on July 28–29, 1987, when the eastward and westward electrojet were well developed in the afternoon and early morning, respectively. Two coordinate systems called B and E frame are used. The B frame represents the ordinary geomagnetic coordinates where the geomagnetic north and east are taken as x and y axis, respectively. In the E frame, the direction of the horizontal electric field in the ionosphere is parallel to the x axis so that the Hall current always flows in the positive y direction and the Pedersen current in the positive x direction. The correlations in both frames are examined for the x and y component of the ionospheric currents and the corresponding magnetic variation on the ground. The correlation is highest for the y (Hall) current and x magnetic field in the E frame and lowest for the x (Pedersen) current and y magnetic field in the E frame. More detailed quantitative analysis is made by taking into account the dependence on the direction, latitudinal location and nonuniform distribution of the current. The contribution of the Hall current was estimated as 0.56 nT/(A/km), when its center is above the radar station. The Pedersen current also makes contributions of 0.14–0.20 nT/(A/km). The results of this analysis also show that the nighttime level of a quiet day is a suitable choice of the base value for the geomagnetic variations in the auroral zone.

Pulsating auroral forms and their association with geomagnetic giant pulsations

On the night of 29/30 October 1987 a giant pulsation (Pg) was recorded on the EISCAT magnetometer cross and the U.K. Sub-Auroral Magnetometer Network (SAMNET), accompanied by a variation in auroral luminosity with an identical periodicity of 77 s. Coincident all-sky image data showed that the auroral emissions arose from a series of pulsating patches (which exhibited lifetimes of the order 5–15 s) whose occurrence was modulated with a 77 s periodicity. At several times during this event, a new type of pulsating auroral patch was observed. These data provide a unique example of the auroral precipitation signature during a Pg event.

Multisatellite and ground‐based observations of transient ULF waves

A unique alignment of the Active Magnetospheric Particle Tracer Explorers (AMPTE) CCE and Viking satellites with respect to the EISCAT Magnetometer Cross has provided an opportunity to study transient ULF pulsations associated with variations in solar wind plasma density observed by the IMP 8 satellite. These observations were acquired during a relatively quiet period on April 24, 1986, during the Polar Region and Outer Magnetosphere International Study (PROMIS) period. An isolated 4‐mHz (4‐min period) pulsation was detected on the ground which was associated with transverse magnetic field oscillations observed by Viking at a ∼2‐RE altitude above the auroral zone and by CCE at ∼8‐RE in the equatorial plane on nearly the same flux tube. CCE detected a compressional oscillation in the magnetic field with twice the period (∼10 min) of the transverse waves, and with a waveform nearly identical to an isolated oscillation in the solar wind plasma density measured by IMP 8. We conclude that the isolated 10‐min oscillation in solar wind plasma density produced magnetic field compression oscillations inside the magnetosphere at the same frequency which also enhanced resonant oscillations at approximately twice the frequency that were already present. The ground magnetic field variations are due to ionospheric Hall currents driven by the electric field of the standing Alfvén waves. The time delay between surface and satellite data acquired at different local times supports the conclusion that the periodic solar wind density variation excites a tailward traveling large‐scale magnetosphere wave train which excites local field line resonant oscillations. These oscillations diminish immediately after the passage of the wave train. We conclude that these transient magnetic field variations are not associated with magnetic field reconnection or flux transfer events.

Magnetometer and incoherent scatter observations of an intense Ps 6 pulsation event

In the morning sector of 21 April 1985, during the recovery phase of a geomagnetic storm, a Ps 6 pulsation event was recorded by the EISCAT magnetometer cross in northern Scandinavia. Simultaneously, the EISCAT incoherent scatter radar measured E- and F-region plasma parameters with a latitudinal scanning program. Electric fields and height-integrated Hall and Pedersen conductivities are derived. Two-dimensional patterns of these quantities are constructed for one Ps 6 period. The conductance patterns closely resemble the typical auroral forms of eastward drifting Ω bands with low and high conductances at the northern and southern edges of the scanned area, respectively. From the equatorward region a tongue of high ionization extends poleward into the dark area. The location of the maximum southward current is slightly displaced towards the west from the centre of the conductance tongue. The east-west disturbance electric field points towards the tongue; the north-south fields are enhanced outside and reduced inside the high conductance region. As has been previously suggested, the observations can be explained with a model which superposes currents caused by conductance variations and electric fields. Both effects need to be taken into account for this event. The current structures move within a few degrees in the direction of the background E× B drift, but their speed is about 15% lower than the average F-region plasma drift.

Ground observations of kinetic Alfvén waves

Ground observations of locally confined, very intense, drifting current systems by the EISCAT magnetometer cross in correlation with GEOS 2 measurements will be explained in terms of kinetic Alfvén waves. In the ground based magnetograms the events are characterized by strong pulsations with amplitudes in the horizontal component up to 1000 nT and periods of about 300s and longer. They occur in the evening hours adjacent to the poleward side of the Harang discontinuity with the onset of a substorm. At the same time the inner edge of the plasma sheet passes the GEOS 2 position, magnetically conjugate to ground stations. The common features of four events during Nov and Dec 1982 will be discussed.