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Milan S.E.,University of Leicester | Milan S.E.,Birkeland Center for Space Science
Journal of Geophysical Research: Space Physics | Year: 2013

We present a simple mathematical model of the region 1 and 2 Birkeland current system intensities for differing dayside and nightside magnetic reconnection rates, consistent with the expanding/contracting polar cap paradigm of solar wind-magnetosphere-ionosphere coupling. The current intensities are shown to be dependent on the cross-polar cap potential, which is the average of the dayside and nightside reconnection rates. Current intensities are expected to maximize on the dayside or the nightside when magnetopause or magnetotail reconnection dominates. Current intensities are also dependent on the ionospheric conductance, the width of the merging gaps, the width of the ionospheric convection return flow region, and on the size of the polar cap. Key PointsBirkeland current intensities estimated from day- and nightside reconnectionCurrents depend on ionospheric conductance and cross-polar cap potentialDependence on expanding/contracting polar cap examined ©2013. American Geophysical Union. All Rights Reserved.


Milan S.E.,University of Leicester | Milan S.E.,Birkeland Center for Space Science
Astrophysics and Space Science Proceedings | Year: 2015

The Dungey (Phys. Rev. Lett. 6:47–48, 1961) open model of the magnetosphere, and especially its time-dependent form, the expanding/contracting polar cap (ECPC) paradigm, has provided an important theoretical framework within which to understand solar wind-magnetosphere-ionosphere coupling. This paper reviews the evidence supporting the open and ECPC models, and discusses developments that have arisen in the last 20 years, concentrating on the contributions made by measurements of the ionospheric convection pattern and global auroral imagery. Various magnetospheric phenomena are discussed within the context of the open model, including substorms, geomagnetic storms, steady magnetospheric convection, sawtooth events, cusp auroral spots, and transpolar arcs. The review concludes with a discussion of avenues for future research in the field of solar wind-magnetosphere-ionosphere coupling. © Springer International Publishing Switzerland 2015.


de Wit R.J.,Norwegian University of Science and Technology | Hibbins R.E.,Norwegian University of Science and Technology | Hibbins R.E.,Birkeland Center for Space Science | Espy P.J.,Norwegian University of Science and Technology | Espy P.J.,Birkeland Center for Space Science
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2015

A new generation all-sky SKiYMET meteor radar, optimized to measure high-frequency gravity wave momentum flux, was installed in Trondheim, Norway (63.4°N, 10.5°E), and has been providing near-continuous measurements since September 2012. Using the system's first full calendar year of observations the seasonal cycle of gravity wave momentum flux and forcing in the mesopause region is studied. The vertical flux of zonal momentum is observed to change from westward to eastward with increasing altitude in winter, and from eastward to westward in summer. This vertical divergence results in westward gravity wave forcing in winter, and eastward forcing in summer. It is shown that the seasonal cycle in gravity wave momentum flux and forcing can be interpreted in terms of selective filtering of a uniform spectrum of vertically propagating GWs between the surface and the mesopause region. © 2014 Elsevier Ltd.


De Wit R.J.,Norwegian University of Science and Technology | Hibbins R.E.,Norwegian University of Science and Technology | Hibbins R.E.,Birkeland Center for Space Science | Espy P.J.,Norwegian University of Science and Technology | And 2 more authors.
Journal of Geophysical Research: Atmospheres | Year: 2013

Zonal-wind measurements obtained between October 2001 and July 2011 with the SKiYMET meteor radar located at Ascension Island (8°S, 14°W) have been used to study the interannual variability at meteor ablation altitudes (approximately 78-100 km) and its coupling to the stratospheric quasi-biennial oscillation (QBO). An upper mesospheric QBO (MQBO) with a period of 27.5 months has been detected throughout the observational period. The MQBO is found to be out-of-phase with the stratospheric QBO (SQBO) at 15-20 hPa and in-phase compared to 70 hPa, whereas no significant zero time-lag correlation exists between the long-term mesospheric zonal winds and the SQBO at 40-50 hPa. The MQBO magnitude is found to be 4.1±0.7 m/s at 88 km. No significant change in MQBO magnitude is found throughout the altitude range under consideration. It was found that the MQBO signal is mainly carried around the March equinox, although the MQBO signal is present throughout most of the year, although less pronounced, at the lower altitudes as well. No observational evidence was found that the MQBO, between approximately 78-100 km, plays a role in the interhemispheric ducting of the quasi-16 day wave. Key Points Interannual variability in meteor radar equatorial zonal winds is presented An MQBO with a period of 27.5 months and a magnitude of 4.1+/-0.7 m/s is found Vertical and interhemispheric coupling of GW and PW is discussed ©2013. American Geophysical Union. All Rights Reserved.


Soraas F.,University of Bergen | Soraas F.,Birkeland Center for Space Science | Laundal K.M.,University of Bergen | Laundal K.M.,Birkeland Center for Space Science | And 2 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

We report two instances of nightside subauroral proton precipitation observed during a geomagnetic storm: (1) An arc at è 50° magnetic latitude (MLAT), which extended roughly from midnight to 6 magnetic local time (MLT), and (2) A corotating spot at 51.5° MLAT, postmidnight. The proton precipitation was observed by both the IMAGE SI-12 proton aurora (Doppler-shifted Lyman- α) imager, and low-altitude Polar-orbiting Operational Environmental Satellites (POESs) that serendipitously traversed both the arc and the spot, providing in situ particle measurements. The Lyman-alpha emission and the particle observations match closely. The particle measurements showed that the energies of the precipitating protons extended to several hundred keV, but no enhancements in the protons below 20 keV were observed. Cluster observations showed that the ring current contained protons with energies extending from tens of eV to several hundred keV; thus, the particles are scattered into the atmosphere from an existing reservoir. Outside the luminous regions, but at the same magnetic latitude, the POES satellites observed localized regions with enhanced proton fluxes outside, but close to, the loss cone. This shows that the protons along a large region in MLT are subjected to pitch angle scattering. Model calculations of the plasmasphere show that the arc and the proton precipitation are located just inside the plasmapause. The intensity and longitudinal extension of the arc are modulated by the dynamical pressure in the solar wind. In the dusk sector, ground stations recorded electromagnetic ion cyclotron waves associated with localized proton precipitation just inside the plasmapause. This suggests that the proton precipitation is caused by wave/particle interaction. Key points Sub-auroral proton aurora is reported, with coincident in-situ measurements The pitch angle scattering extends beyond the luminous regions The location of the precipitation suggests a relation to EMIC waves ©2013. American Geophysical Union. All Rights Reserved.

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