N. Østgaard

Interplanetary magnetic field control of the location of substorm onset and auroral features in the conjugate hemispheres

[1] During 2001 and 2002, when the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellite had its apogee in the Northern Hemisphere and the Polar spacecraft, owing to the apsidal precession of its orbit, reached higher altitudes in the Southern Hemisphere, the two spacecraft offered a unique opportunity to study the aurora in the conjugate hemispheres simultaneously. Owing to the large fields of view of the Polar Visible Imaging System (VIS) Earth camera and the IMAGE-FUV instruments, substorms and auroral features were imaged on a global scale in both hemispheres. We have identified five substorm onsets and several auroral features that can be unambiguously identified and compared in the two hemispheres. When mapped onto apex coordinates in the two hemispheres, we find that substorm onset locations and auroral features are usually not symmetric. The longitudinal displacement in one hemisphere compared with the other can be as much as 1.5 hours of local time (∼1500 km). For southward interplanetary magnetic field (IMF) the hemispherical asymmetry (AMLT) is strongly correlated with the IMF clock angle (θ C ) and a linear fit, ΔMLT = -0.017c C + 3.44, gives a correlation coefficient of 0.83 with a mean deviation of 0.4ΔMLT. These findings are interpreted as the magnetic tensions force acting on open magnetic field lines before reconnecting in the magnetotail. This can also be thought of as the IMF penetrating the magnetosphere.

Asymmetric auroral intensities in the Earth’s Northern and Southern hemispheres

It is commonly assumed that the aurora borealis (Northern Hemisphere) and aurora australis (Southern Hemisphere) are mirror images of each other because the charged particles causing the aurora follow the magnetic field lines connecting the two hemispheres. The particles are believed to be evenly distributed between the two hemispheres, from the source region in the equatorial plane of the magnetosphere. Although it has been shown that similar auroral features in the opposite hemispheres can be displaced tens of degree in longitude and that seasonal effects can cause differences in global intensity, the overall auroral patterns were still similar. Here we report observations that clearly contradict the common assumption about symmetric aurora: intense spots are seen at dawn in the Northern summer Hemisphere, and at dusk in the Southern winter Hemisphere. The asymmetry is interpreted in terms of inter-hemispheric currents related to seasons, which have been predicted but hitherto had not been seen.

Confining the angular distribution of terrestrial gamma ray flash emission

[1] Terrestrial gamma ray flashes (TGFs) are bremsstrahlung emissions from relativistic electrons accelerated in electric fields associated with thunder storms, with photon energies up to at least 40 MeV, which sets the lowest estimate of the total potential of 40 MV. The electric field that produces TGFs will be reflected by the initial angular distribution of the TGF emission. Here we present the first constraints on the TGF emission cone based on accurately geolocated TGFs. The source lightning discharges associated with TGFs detected by RHESSI are determined from the Atmospheric Weather Electromagnetic System for Observation, Modeling, and Education (AWESOME) network and the World Wide Lightning Location Network (WWLLN). The distribution of the observation angles for 106 TGFs are compared to Monte Carlo simulations. We find that TGF emissions within a half angle >30° are consistent with the distributions of observation angle derived from the networks. In addition, 36 events occurring before 2006 are used for spectral analysis. The energy spectra are binned according to observation angle. The result is a significant softening of the TGF energy spectrum for large (>40°) observation angles, which is consistent with a TGF emission half angle (<40°). The softening is due to Compton scattering which reduces the photon energies.

Estimating the capture and loss of cold plasma from ionospheric outflow

An important source of magnetospheric plasma is cold plasma from the terrestrial ionosphere. Low energy ions travel along the magnetic field lines and enter the magnetospheric lobes where they are convected toward the tail plasma sheet. Recent observations indicate that the field aligned ion outflow velocity is sometimes much higher than the convection toward the central plasma sheet. A substantial amount of plasma therefore escapes downtail without ever reaching the central plasma sheet. In this work, we use Cluster measurements of cold plasma outflow and lobe convection velocities combined with models of the magnetic field in an attempt to determine the fate of the outflowing ions and to quantify the amount of plasma lost downtail. The results show that both the circulation of plasma and the direct tailward escape of ions varies significantly with magnetospheric conditions. For strong solar wind driving with a southward interplanetary magnetic field, also typically associated with high geomagnetic activity, most of the outflowing plasma is convected to the plasma sheet and recirculated. For periods with northward interplanetary magnetic field, the convection is nearly stagnant, whereas the outflow, although limited, still persists. The dominant part of the outflowing ions escape downtail and are directly lost into the solar wind under such conditions.

Auroral electron distributions derived from combined UV and X-ray emissions

The Polar Ionospheric X-ray Imaging Experiment and the Ultraviolet Imager on board the Polar satellite provide the first simultaneous global scale views of the electron precipitation over a wide range of electron energies. By combining the results from these two remote sensing techniques we have developed a method to derive the electron energy distributions that reproduce the true electron spectra from 1 to 100 keV and that can be used to calculate the energy flux in the energy range from 100 eV to 100 keV. The electron energy spectra obtained by remote sensing techniques in three 5-min time intervals on July 9 and July 31, 1997, are compared with the spectra measured by low-altitude satellites in the conjugate hemisphere. In the energy range from 90 eV to 30 keV the derived energy flux is found to be 1.03±0.6 of the measured energy fluxes. The method enables us to present 5-min time-averaged global maps of precipitating electron energy fluxes with a spatial resolution of ∼700 km. The study shows that the combination of UV and X-ray cameras on a polar orbiting spacecraft enables comprehensive monitoring of the global energy deposition from precipitating electrons over the energy range that is most important for magnetosphere-ionosphere coupling.

How the IMF By induces a By component in the closed magnetosphere ‐ and how it leads to asymmetric currents and convection patterns in the two hemispheres

We acknowledge the use of NASA/GSFC’s Space Physics Data Facility for OMNI data. Simulation results have been provided by the Community Coordinated Modeling Center at Goddard Space Flight Center through their public Runs on Request system (http://ccmc.gsfc.nasa.gov). The CCMC is a multiagency partnership between NASA, AFMC, AFOSR, AFRL, AFWA, NOAA, NSF, and ONR (Paul-Tenfjord-032514-1). We thank the AMPERE team and the AMPERE Science Center for providing the Iridium-derived data products. This study was supported by the Research Council of Norway/CoE under contract 223252/F50.

Global‐scale electron precipitation features seen in UV and X rays during substorms

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) and the ultraviolet imager (UVI) onboard the Polar satellite have provided the first simultaneous global-scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung and the auroral ultraviolet (UV) emissions. While the UV images respond to the total electron energy flux, which is usually dominated by electron energies below 10 keV, the PIXIE, 9.9–19.7 keV X-ray images used in this study respond only to electrons of energy above 10 keV. Previous studies by ground-based, balloon, and space observations have indicated that the patterns of energetic electron precipitation differ significantly from those found in the visible and the UV auroral oval. Because of the lack of global imaging of the energetic electron precipitation, one has not been able to establish a complete picture. In this study the development of the electron precipitation during the different phases of magnetospheric substorms is examined. Comparisons are made between the precipitation patterns of the high-energy (PIXIE) and low-energy (UVI) electron populations, correlated with ground-based observations and geosynchronous satellite data. We focus on one specific common feature in the energetic precipitation seen in almost every isolated substorm observed by PIXIE during 1996 and which differs significantly from what is seen in the UV images. Delayed relative to substorm onsets, we observe a localized maximum of X-ray emission at 5–9 magnetic local time. By identifying the location of the injection region and determining the substorm onset time it is found that this maximum most probably is caused by electrons injected in the midnight sector drifting (i.e., gradient and curvature drift) into a region in the dawnside magnetosphere where some mechanism effectively scatters the electrons into the loss cone.

Persistent global proton aurora caused by high solar wind dynamic pressure

[1] Global images of the proton aurora taken with the SI-12 camera onboard the IMAGE satellite reveal a very direct relationship between the solar wind dynamic pressure and the intensity of the global proton aurora. We show that an increase in dynamic pressure leads to an immediate and persistent increase in proton precipitation, also when the increase is slow. When the dynamic pressure decreases, the proton aurora diminishes. Five events during geomagnetic quiet times, with mostly northward IMF, have been selected in order to characterize the proton aurora caused exclusively by high dynamic pressure and establish important criteria that the dynamic pressure-induced precipitation mechanism(s) must satisfy. We also present measurements during southward IMF and show that the combined effect of high solar wind dynamic pressure and southward IMF produces intense global proton aurora. Some of the characteristics are: (1) The aurora is global, with peak intensities at midnight and flanks. (2) A dawn/dusk asymmetry shows that the precipitation originates from magnetospheric protons that have undergone gradient/curvature drift. (3) The time delay between ground magnetic signatures of a change in the solar wind dynamic pressure and a change in global proton aurora is short (� 2 minutes). Our observations indicate that the precipitation mechanism(s) behind the proton aurora during high dynamic pressure is directly connected to the compression of the magnetosphere, both at the flanks and nightside.

Cause of the localized maximum of X‐ray emission in the morning sector: A comparison with electron measurements

The Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the Polar satellite has provided the first global scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung. While other remote sensing techniques like ultraviolet (UV) and visible imaging sense emissions that are dominantly produced by low-energy electrons ( 2-10 keV electrons by wave-particle interaction into the loss cone is the main mechanism for this precipitation.

Interhemispheric observations of emerging polar cap asymmetries

[1] In this paper we use simultaneous global UV images of the aurora in the two hemispheres to study differences in the polar cap boundary location. We show that the northern and southern auroral ovals circumvent the same amount of magnetic flux, providing additional evidence that the poleward boundary of the aurora coincides with the open/closed field line boundary. During a period of significant flux closure, large asymmetries in the polar cap boundaries developed between the hemispheres. The asymmetry was strongest in the regions where the polar caps contracted the most, suggesting that emerging interhemispheric polar cap asymmetries is an intrinsic phenomenon during substorm expansions, when magnetic flux closes rapidly in the tail. Utilizing the prolonged surveillance of the open/closed boundary location, we show that the growing asymmetries can be accounted for by differences in the ionospheric convection in the two hemispheres. The observations suggest that the differences in convection were due to seasonal differences between the hemispheres, and that the summer hemisphere responded more promptly to changes in magnetospheric convection than the winter hemisphere.