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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.

Tenfjord P.,Birkeland Center for Space Science | Tenfjord P.,University of Bergen | Ostgaard N.,Birkeland Center for Space Science | Ostgaard N.,University of Bergen
Journal of Geophysical Research: Space Physics | Year: 2013

In this paper we describe and quantify the energy transfer, flow, and distribution. Our high-resolution data set covers 13 years of OMNI, SuperMAG, and Kyoto data. We employ what we consider to be the best estimates for energy sinks and relate these to SuperMAG indices for better coverage and spatial resolution. For the energy input, we have used the method of dimensional analysis that is presented in unit power and makes it appropriate for energy analysis. A cross-correlation analysis parameterizes the magnetospheric response on the solar wind parameters during a wide range of conditions, ranging from substorms and storms up to a decade. The determined functional form is then evaluated and scaled using superposed epoch analysis of geomagnetic storms, revealing that the effective area of interaction cannot be considered static. Instead, we present a dynamic area which depends to the first order on the cube of the interplanetary magnetic field Bz component. Also, we find that for longer time periods, this area must be increased compared to the area used for geomagnetic storms. We argue that some of the terms in the energy coupling function are contributory to describing magnetosheath conditions and discuss how our coupling function can be related to Maxwell stress components. Also, we quantify the relative importance of the different energy sinks during substorms, geomagnetic storms, and long time series and present the coupling efficiency of the solar wind. Our energy coupling functions are compared with the ε parameter and perform better for almost any event. Key Points A new dynamic energy coupling function to quantify energy transferEstimate energy transfer during substorms, storms up to a decade. ©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.

Hansen R.S.,University of Bergen | Hansen R.S.,Birkeland Center for Space Science | Ostgaard N.,University of Bergen | Ostgaard N.,Birkeland Center for Space Science | And 4 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

Up to a few years ago, terrestrial gamma ray flashes (TGFs) were only observed by spaceborne instruments. The aircraft campaign ADELE was able to observe one TGF, and more attempts on aircraft observations are planned. There is also a planned campaign with stratospheric balloons, COBRAT. In this context an important question that arises is what count rates we can expect and how these estimates are affected by the initial properties of the TGFs. Based on simulations of photon propagation in air we find the photon fluence at different observation points at aircraft and balloon altitudes. The observed fluence is highly affected by the initial parameters of the simulated TGFs. One of the most important parameters is the number of initial photons in a TGF. In this paper, we give a semi-analytical approach to find the initial number of photons with an order of magnitude accuracy. The resulting number varies over several orders of magnitude, depending mostly on the production altitude of the TGF. The initial production altitude is also one of the main parameters in the simulations. Given the same number of initial photons, the fluence at aircraft and balloon altitude from a TGF produced at 10 km altitude is 2-3 orders of magnitude smaller then a TGF originating from 20 km altitude. Other important parameters are altitude distribution, angular distribution and amount of feedback. The differences in altitude, altitude distribution and amount of feedback are especially important for the fluence of photons observed at altitudes less than 20 km, and for instruments with a low-energy threshold larger than 100 keV. We find that the maximum radius of observation in 14 km for a TGF with the intensity of an average RHESSI TGF is smaller than the results reported by Smith et al. (2011), and our results support the conclusion in Gjesteland et al. (2012) and Østgaard et al. (2012) that TGFs probably are a more common phenomenon than previously reported. ©2013. American Geophysical Union. All Rights Reserved.

Reistad J.P.,University of Bergen | Reistad J.P.,Birkeland Center for Space Science | Ostgaard N.,University of Bergen | Ostgaard N.,Birkeland Center for Space Science | And 3 more authors.
Journal of Geophysical Research: Space Physics | Year: 2013

We have investigated a data set of 19 h of simultaneous global conjugate auroral imaging from space. The data set consists of 10 sequences with durations from 1 to 5 h during active geomagnetic conditions (average AE ∼ 400 nT). We have identified 15 features (including two presented earlier) of auroral forms that appear mainly in one hemisphere, and we define this as non-conjugate aurora. Three generator mechanisms has been suggested for producing interhemispheric currents and non-conjugate aurora: (1) Hemispherical differences in solar wind dynamo efficiency due to interplanetary magnetic field (IMF) Bx and dipole tilt angle leading to asymmetric region 1 currents in the two polar hemispheres, (2) interhemispheric currents induced by the penetration of IMF By into the closed nightside magnetosphere, and (3) hemispheric differences in ionospheric conductivity controlled by the dipole tilt angle inducing interhemispheric currents on closed field-lines. We want to find out if our observations are consistent with these mechanisms. Our analysis shows that five features were consistent with the IMF By penetration mechanism, seven features consistent with the solar wind dynamo mechanism, three features consistent with the conductivity mechanism, and two features could not be explained by any of the three suggested mechanisms. Because two features were consistent with two different mechanisms, the numbers add up to 17 although the total number of features is 15. The analysis also shows the expected correlation between the magnitude of the longitudinal shift of conjugate points, ΔMLT, and the occurrence of non-conjugate aurora consistent with the By mechanism. Key Points Our observations are consistent with the three suggested generator mechanisms Observed By asymmetries occurs during DeltaMLT > 1 hour SW dynamo observations consistent with statistical studies related to IMF Bx ©2013. American Geophysical Union. All Rights Reserved.

Nesse Tyssoy H.,Birkeland Center for Space Science | Nesse Tyssoy H.,University of Bergen | Stadsnes J.,Birkeland Center for Space Science | Stadsnes J.,University of Bergen | And 3 more authors.
Geophysical Research Letters | Year: 2013

Protons in the energy range 1-20 MeV deposit most of their energy in the middle atmosphere (60-100 km). Knowledge of their magnetic latitudinal and local time distribution is crucial for determining their effect on the chemistry and dynamics in the atmosphere. Using POES 16-19 and METOP02 satellites, we investigate the latitudinal cutoff boundaries and the energy deposition during the January 2012 solar proton event. The dayside cutoff latitudes show high correlation with the Dst index even when Dst turns positive, leading to an abrupt poleward movement of more than 5°. In the same time interval, the nightside cutoff latitudes move equatorward resulting in vastly asymmetric energy deposition into the atmosphere on the dayside and nightside. The differences are sustained for almost a day in the middle atmosphere at 65° corrected geomagnetic latitude. These features cannot be taken into account by applying the frequently used GOES particle data. Key Points Distribution of solar protons in latitude and local time during the Jan 2012 SPE Strong day-night asymmetries in the proton energy deposition into the atmosphere One can map SPEs using polar orbiting POES, but not the geostationary GOES data. © 2013. American Geophysical Union. All Rights Reserved.

Kleinknecht N.H.,Norwegian University of Science and Technology | Espy P.J.,Norwegian University of Science and Technology | Espy P.J.,Birkeland Center for Space Science | Hibbins R.E.,Norwegian University of Science and Technology | Hibbins R.E.,Birkeland Center for Space Science
Journal of Geophysical Research: Atmospheres | Year: 2014

The zonal wave components 1 and 2 were extracted from the meridional wind along the latitude band of 51-66°N for the years 2000-2008 using eight Super Dual Auroral Radar Network (SuperDARN) radars spanning longitudes from 25°E to 150°W. Each extracted zonal component represents the superposition of all temporal periods with that zonal structure and indicates the total planetary wave energy available with that wave number. The Hovmöller diagrams show stationary as well as eastward and westward traveling planetary waves propagating in the background wind. The method used to detect the zonal planetary wave components in the SuperDARN data are detailed and validated using UK Meteorological Office data, which allows the evolution of S1 and S2 planetary wave energy between the stratosphere and mesosphere to be assessed. The climatology of zonal wave number 1 and 2 planetary wave activity in the mesosphere-lower thermosphere (MLT) is presented and compared to the activity in the stratosphere. The MLT climatology of the mean wind anomalies shows stronger planetary wave activity during winter and weaker activity during summer with enhancement around midsummer and autumn equinox. The climatology of the mean wind displays similar amplitudes apart from very strong S1 planetary wave amplitudes during summer. In addition planetary wave activity during winters with major and minor stratospheric warming events are examined and contrasted. Key Points Planetary wave zonal structure derived from network of SuperDARN meteor winds Robustness of extracting each zonal component compared to current techniques Mesospheric planetary waves used as possible precursor of stratospheric warmings ©2014. American Geophysical Union. All Rights Reserved.

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.

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.

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