Mesoscale structure of a morning sector ionospheric shear flow region determined by conjugate Cluster II and MIRACLE ground-based observations

Amm, O.; Aikio, Anita; Bosqued, J. M.; Dunlop, M. W.; Fazakerley, A. N.; Janhunen, P.; Kauristie, Kirsti; Lester, M.; Sillanpää, I.; Taylor, Matt; Vontrat-Reberac, A.; Мурсула, К.; André, Mats

doi:10.5194/angeo-21-1737-2003

Cited by 15 publications

(21 citation statements)

References 38 publications

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“…However, one should remember that the ionospheric current reversal boundary may deviate from the MCRB even by several degrees (Amm et al, 2003). In this study we found that in the presence of the Harang discontinuity during substorm periods, the MRCB and the OCB do not agree well.…”

Section: Discussioncontrasting

confidence: 41%

“…By itself, this is not a determination of the PCB location, nor of the OCB location. However, this boundary depends on the ionospheric current system and on the ionization produced by the auroral precipitation, so that the motion of this boundary can be expected to mimic that of the OCB, and in some circumstances, the boundaries may well match each other (Amm et al, 2003;Aikio et al, 2006Aikio et al, , 2008.…”

Section: Methodsmentioning

confidence: 99%

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Comparison of the open-closed field line boundary location inferred using IMAGE-FUV SI12 images and EISCAT radar observations

Hubert¹,

Aikio

Amm

et al. 2010

Ann. Geophys.

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Abstract.We compare the location of the polar cap boundary (PCB) determined using two different techniques, and use them as proxies for the open-closed field line boundary (OCB). Electron temperatures from observations of the EIS-CAT radar facility are used to estimate the latitude of the PCB along the meridian of the EISCAT VHF beam. The second method utilizes global images of proton aurora obtained by the IMAGE satellite FUV SI12 instrument. These methods are applied to three different intervals. In two events, the agreement between the methods is good and the mean of the difference is within the resolution of the observations. In a third event, the PCB estimated from EISCAT data is located several degrees poleward of that obtained from the IMAGE FUV SI12 instrument. Comparison of the reconnection electric field estimated from the two methods shows that highresolution measurements both in time and space are needed to capture the variations in reconnection electric field during substorm expansion. In addition to the two techniques introduced above to determine the PCB location, we also use a search for the location of the reversal of the east-west component of the equivalent current known as the magnetic convection reversal boundary (MCRB). The MCRB from the MIR-ACLE magnetometer chain mainly follows the motion of the polar cap boundary during different substorm phases, but differences arise near the Harang discontinuity.

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Section: Discussioncontrasting

confidence: 41%

Section: Methodsmentioning

confidence: 99%

Comparison of the open-closed field line boundary location inferred using IMAGE-FUV SI12 images and EISCAT radar observations

Hubert¹,

Aikio

Amm

et al. 2010

Ann. Geophys.

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“…In ground magnetograms the 23:00-24:00 UT period is only slightly disturbed, maximum horizontal negative deflections are about 140 nT in the region equatorward of the equatorward arc, indicating WEJ flow in that area (Amm et al, 2003). So, in the Scandinavian sector the event starting at 23:24 UT appears as a pseudo-breakup, as discussed already in Sect.…”

Section: Cluster Electric Field and Potential Observationsmentioning

confidence: 57%

“…In this study, data from three spacecraft (C1, C2 and C4) are available, since instruments at Cluster 3 were switched on later. The meso-scale structures of the same Cluster pass were studied by Amm et al (2003) and according to that study the part of the orbit studied here (geographic latitudes below 70 • ) was located in the region of the large-scale westward electrojet (WEJ), equatorward of the convection reversal and the openclosed field line boundary, which was situated 3-4 • poleward of the convection reversal. In this paper, we use coordinated ground-based and satellite observations to study the evolution of two auroral arcs.…”

Section: Introductionmentioning

confidence: 99%

Temporal evolution of two auroral arcs as measured by the Cluster satellite and coordinated ground-based instruments

et al. 2004

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Abstract. The four Cluster s/c passed over Northern Scandinavia on 6 February 2001 from south-east to north-west at a radial distance of about 4.4 R E in the post-midnight sector. When mapped along geomagnetic field lines, the separation of the spacecraft in the ionosphere was confined to within 110 km in latitude and 50 km in longitude. This constellation allowed us to study the temporal evolution of plasma with a time scale of a few minutes. Ground-based instrumentation used involved two all-sky cameras, magnetometers and the EISCAT radar. The main findings were as follows.Two auroral arcs were located close to the equatorward and poleward edge of a large-scale density cavity, respectively. These arcs showed a different kind of a temporal evolution. (1) As a response to a pseudo-breakup onset, both the up-and downward field-aligned current (FAC) sheets associated with the equatorward arc widened and the total amount of FAC doubled in a time scale of 1-2 min. (2) In the poleward arc, a density cavity formed in the ionosphere in the return (downward) current region. As a result of ionospheric feedback, a strongly enhanced ionospheric southward electric field developed in the region of decreased Pedersen conductance. Furthermore, the acceleration potential of ionospheric electrons, carrying the return current, increased from 200 to 1000 eV in 70 s, and the return current region widened in order to supply a constant amount of return current to the arc current circuit.Evidence of local acceleration of the electron population by dispersive Alfvén waves was obtained in the upward FAC region of the poleward arc. However, the downward accelerated suprathermal electrons must be further energised below Cluster in order to be able to produce the observed visible aurora.Correspondence to: A. T. Aikio (anita.aikio@oulu.fi) Both of the auroral arcs were associated with broad-band ULF/ELF (BBELF) waves, but they were highly localised in space and time. The most intense BBELF waves were confined typically to the return current regions adjacent to the visual arc, but in one case also to a weak upward FAC region. BBELF waves could appear/disappear between s/c crossings of the same arc separated by about 1 min.

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“…Amm et al, 2000). In the event discussed by Amm et al (2003), the PCB measured by Cluster ion instrument was located 3-4 • of latitude poleward of the MCRB in the midnight sector. No systematic study of the relationship between the poleward boundary of the electrojets (MCRB) and the PCB has been conducted to our knowledge.…”

Section: Introductionmentioning

confidence: 99%

Method to locate the polar cap boundary in the nightside ionosphere and application to a substorm event

et al. 2006

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Abstract. In this paper we describe a new method to be used for the polar cap boundary (PCB) determination in the nightside ionosphere by using the EISCAT Svalbard radar (ESR) field-aligned measurements by the 42-m antenna and southward directed low-elevation measurements by the ESR 32m antenna or northward directed low-elevation measurements by the EISCAT VHF radar at Tromsø. The method is based on increased electron temperature (T e ) caused by precipitating particles on closed field lines. Since the Svalbard field-aligned measurement provides the reference polar cap T e height profile, the method can be utilised only when the PCB is located between Svalbard and the mainland. Comparison with the Polar UVI images shows that the radar-based method is generally in agreement with the PAE (poleward auroral emission) boundary from Polar UVI.The new technique to map the polar cap boundary was applied to a substorm event on 6 November 2002. Simultaneous measurements by the MIRACLE magnetometers enabled us to put the PCB location in the framework of ionospheric electrojets. During the substorm growth phase, the polar cap expands and the region of the westward electrojet shifts gradually more apart from the PCB. The substorm onset takes place deep within the region of closed magnetic field region, separated by about 6-7 • in latitude from the PCB in the ionosphere. We interpret the observations in the framework of the near-Earth neutral line (NENL) model of substorms. After the substorm onset, the reconnection at the NENL reaches within 3 min the open-closed field line boundary and then the PCB moves poleward together with the poleward boundary of the substorm current wedge. The poleward expansion occurs in the form of individual bursts, which are separated by 2-10 min, indicating that the reconnection in the magnetotail neutral line is impulsive. The poleward expansions of the PCB are followed by latitude dispersed intensifications inCorrespondence to: A. T. Aikio (anita.aikio@oulu.fi) the westward electrojet with high latitudes affected first and lower latitudes later. We suggest that reconnection bursts energize plasma and produce enhanced flows toward the Earth. While drifting earthward, part of the plasma population precipitates to the ionosphere producing latitude-dispersed enhancements in the WEJ.

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Mesoscale structure of a morning sector ionospheric shear flow region determined by conjugate Cluster II and MIRACLE ground-based observations

Cited by 15 publications

References 38 publications

Comparison of the open-closed field line boundary location inferred using IMAGE-FUV SI12 images and EISCAT radar observations

Comparison of the open-closed field line boundary location inferred using IMAGE-FUV SI12 images and EISCAT radar observations

Temporal evolution of two auroral arcs as measured by the Cluster satellite and coordinated ground-based instruments

Method to locate the polar cap boundary in the nightside ionosphere and application to a substorm event

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