K. Oksavik

Van Allen probes, NOAA, GOES, and ground observations of an intense EMIC wave event extending over 12 h in magnetic local time

Although most studies of the effects of electromagnetic ion cyclotron (EMIC) waves on Earths outer radiation belt have focused on events in the afternoon sector in the outer plasmasphere or plume region, strong magnetospheric compressions provide an additional stimulus for EMIC wave generation across a large range of local times and L shells. We present here observations of the effects of a wave event on 23 February 2014 that extended over 8 h in UT and over 12 h in local time, stimulated by a gradual 4 h rise and subsequent sharp increases in solar wind pressure. Large-amplitude linearly polarized hydrogen band EMIC waves (up to 25 nT p-p) appeared for over 4 h at both Van Allen Probes, from late morning through local noon, when these spacecraft were outside the plasmapause, with densities ~5–20 cm−3. Waves were also observed by ground-based induction magnetometers in Antarctica (near dawn), Finland (near local noon), Russia (in the afternoon), and in Canada (from dusk to midnight). Ten passes of NOAA-POES and METOP satellites near the northern foot point of the Van Allen Probes observed 30–80 keV subauroral proton precipitation, often over extended L shell ranges; other passes identified a narrow L shell region of precipitation over Canada. Observations of relativistic electrons by the Van Allen Probes showed that the fluxes of more field-aligned and more energetic radiation belt electrons were reduced in response to both the emission over Canada and the more spatially extended emission associated with the compression, confirming the effectiveness of EMIC-induced loss processes for this event.

Space weather challenges of the polar cap ionosphere

This paper presents research on polar cap ionosphere space weather phenomena conducted during the European Cooperation in Science and Technology (COST) action ES0803 from 2008 to 2012. The main part of the work has been directed toward the study of plasma instabilities and scintillations in association with cusp flow channels and polar cap electron density structures/patches, which is considered as critical knowledge in order to develop forecast models for scintillations in the polar cap. We have approached this problem by multi-instrument techniques that comprise the EISCAT Svalbard Radar, SuperDARN radars, in-situ rocket, and GPS scintillation measurements. The Discussion section aims to unify the bits and pieces of highly specialized information from several papers into a generalized picture. The cusp ionosphere appears as a hot region in GPS scintillation climatology maps. Our results are consistent with the existing view that scintillations in the cusp and the polar cap ionosphere are mainly due to multi-scale structures generated by instability processes associated with the cross-polar transport of polar cap patches. We have demonstrated that the SuperDARN convection model can be used to track these patches backward and forward in time. Hence, once a patch has been detected in the cusp inflow region, SuperDARN can be used to forecast its destination in the future. However, the high-density gradient of polar cap patches is not the only prerequisite for high-latitude scintillations. Unprecedented high-resolution rocket measurements reveal that the cusp ionosphere is associated with filamentary precipitation giving rise to kilometer scale gradients onto which the gradient drift instability can operate very efficiently. Cusp ionosphere scintillations also occur during IMF B Z north conditions, which further substantiates that particle precipitation can play a key role to initialize plasma structuring. Furthermore, the cusp is associated with flow channels and strong flow shears, and we have demonstrated that the Kelvin-Helmholtz instability process may be efficiently driven by reversed flow events.

ESR mapping of polar‐cap patches in the dark cusp

We present the first ever measurement of the full thermal plasma properties, of an ionospheric patch in full darkness in the noon region where patches are believed to form. Further these data present the first experimental evidence for the Lockwood and Carlson class of mechanisms for forming patches by plasma injection. These data were possible only because of a new measurement capability we had to develop. We introduce the capability here because it crosses the high-speed threshold that now allows study of a broader class of mesoscale plasma flow-transients, which are thought to occur over time scales near 2 minutes vice 8-10 minutes. Cumulatively such transients may significantly drive global convection. We demonstrate both the validity of and need for our new measurement capability, by presenting a transient flow reversal sweeping across a 500 by 1000 km area, with initial reversal in 4 minutes, and recovery within 6 minutes.

Reversed flow events in the winter cusp ionosphere observed by the European Incoherent Scatter (EISCAT) Svalbard radar

[1] High-resolution fast azimuth sweeps by the European Incoherent Scatter (EISCAT) Svalbard radar provide an unparalleled opportunity to study small-scale flow disturbances in the cusp ionosphere. Observations from 11 days of the winter cusp ionosphere of highresolution ion flow data have been analyzed. Transient channels of reversed plasma flow appear to be a regular feature of the cusp, and they were seen in 16% of 767 analyzed EISCAT Svalbard Radar (ESR) scans. We introduce a new descriptive term, reversed flow events (RFEs), for this class of events. RFEs are defined as longitudinally elongated segments of transiently enhanced ion flow in the direction opposite to the background flow. RFEs typically occurred near the cusp inflow region in association with enhancements in the polar cap convection observed by the Super Dual Auroral Radar Network (SuperDARN). Their lifetime was found to be � 19 min on average. Their longitudinal dimension typically exceeded the ESR field of view (>400–600 km), and ranged from � 50 to 250 km in latitude. The occurrence rate of RFEs appears independent of the BZ and BY component polarity of the interplanetary magnetic field (IMF), and RFEs occurred for clock angles between 40 and 240. RFE ion flow was in 95% of the cases documented to oppose the magnetic tension force, and RFEs cannot be interpreted in terms of newly opened flux. RFEs formed one by one and never simultaneously in pairs. To explain these observations, we propose an asymmetric version of the Southwood (1987) twin cell flux transfer event model to account for significant IMF BY, in which only the poleward cell located on open field lines develops.

Day‐night coupling by a localized flow channel visualized by polar cap patch propagation

We present unique coordinated observations of the dayside auroral oval, polar cap, and nightside auroral oval by three all-sky imagers, two Super Dual Auroral Radar Network (SuperDARN) radars, and Defense Meteorological Satellite Program (DMSP)-17. This data set revealed that a dayside poleward moving auroral form (PMAF) evolved into a polar cap airglow patch that propagated across the polar cap and then nightside poleward boundary intensifications (PBIs). SuperDARN observations detected fast antisunward flows associated with the PMAF, and the DMSP satellite, whose conjunction occurred within a few minutes after the PMAF initiation, measured enhanced low-latitude boundary layer precipitation and enhanced plasma density with a strong antisunward flow burst. The polar cap patch was spatially and temporally coincident with a localized antisunward flow channel. The propagation across the polar cap and the subsequent PBIs suggests that the flow channel originated from dayside reconnection and then reached the nightside open-closed boundary, triggering localized nightside reconnection and flow bursts within the plasma sheet.

GPS scintillation and irregularities at the front of an ionization tongue in the nightside polar ionosphere

In this paper we study a tongue of ionization (TOI) on 31 October 2011 which stretched across the polar cap from the Canadian dayside sector to Svalbard in the nightside ionosphere. The TOI front arrived over Svalbard around 1930 UT. We have investigated GPS scintillation and irregularities in relation to this TOI front. This is the first study presenting such detailed multi-instrument data of scintillation and irregularities in relation to a TOI front. Combining data from an all-sky imager, the European Incoherent Scatter Svalbard Radar, the Super Dual Auroral Radar Network Hankasalmi radar, and three GPS scintillation and total electron content (TEC) monitors in Longyearbyen and Ny-Alesund, we observe bursts of phase scintillation and no amplitude scintillation in relation to the leading gradient of the TOI. Spectrograms of 50Hz phase measurements show highly localized and variable structuring of the TOI leading gradient, with no structuring or scintillation within the TOI or ahead of the TOI.

Storm time equatorial belt – an “image” of RC behavior

[1] During geomagnetic storms a well defined belt of trapped protons and ENAs (energetic neutral atoms) is observed around geomagnetic equator at low L-values. Their source is RC (ring current) protons existing at larger L-values. Through charge exchange with the geocorona RC protons become ENAs and if subjected to a new charge exchange become trapped protons. From low latitude particle observations at four different local times we follow; the RC injection region, the drift of RC-particles through the evening/afternoon into the morning sector, the RC-asymmetry and convection loss to the dayside during the storm initial and main phase, and its development into a symmetric RC in the recovery phase of the storm. INDEX TERMS: 2778 Magnetospheric Physics: Ring current; 2720 Magnetospheric Physics: Energetic particles, trapped; 2730 Magnetospheric Physics:Magnetosphere— inner; 2788 Magnetospheric Physics: Storms and substorms. Citation: Soraas, F., K. Oksavik, K. Aarsnes, D. S. Evans, and M. S. Greer, Storm time equatorial belt – an ‘‘image’’ of RC behavior, Geophys. Res. Lett., 30(2), 1052, doi:10.1029/ 2002GL015636, 2003.

Optical and particle signatures of magnetospheric boundary layers near magnetic noon: Satellite and ground‐based observations

In this paper we present a set of satellite and ground-based observations suggesting that energetic magnetospheric electrons cannot be used as an unambiguous discriminator between open and closed field lines on the dayside. Using two data sets from the Defense Meteorological Satellite Program (DMSP) F13 and NOAA 12 satellites flying through dayside Type 1 cusp aurora (both close in time and space), we reach two apparently incompatible conclusions. Cusp/mantle precipitation, stepped cusp signatures, and antisunward convection in the DMSP F13 data set strongly suggest open magnetic field lines. On the other hand, NOAA 12 observed a mixture of magnetosheath and isotropic energetic particles. Trapped energetic electrons are traditionally regarded as being on closed flux. However, in addition to earlier proposed trapping on open field lines, we suggest that transmission lines connecting merging sites near the cusp in the Southern Hemisphere with the northern auroral ionosphere can be several tens of RE long. Alfven wave transit times of several minutes may make it impossible to determine from satellite measurements in the ionosphere whether magnetic field lines threading low-latitude boundary layer (LLBL) plasmas are open or closed. New research tools will be needed to unify understanding of complementary particle measurements from the DMSP and NOAA satellites.

Scintillation and loss of signal lock from poleward moving auroral forms in the cusp ionosphere

We present two examples from the cusp ionosphere over Svalbard, where poleward moving auroral forms (PMAFs) are causing significant phase scintillation in signals from navigation satellites. The data were obtained using a combination of ground-based optical instruments and a newly installed multiconstellation navigation signal receiver at Longyearbyen. Both events affected signals from GPS and Global Navigation Satellite System (GLONASS). When one intense PMAF appeared, the signal from one GPS spacecraft also experienced a temporary loss of signal lock. Although several polar cap patches were also observed in the area as enhancements in total electron content, the most severe scintillation and loss of signal lock appear to be attributed to very intense PMAF activity. This shows that PMAFs are locations of strong ionospheric irregularities, which at times may cause more severe disturbances in the cusp ionosphere for navigation signals than polar cap patches.

Severe and localized GNSS scintillation at the poleward edge of the nightside auroral oval during intense substorm aurora

In this paper we study how GPS, GLONASS, and Galileo navigation signals are compromised by strong irregularities causing severe phase scintillation (σϕ>1) in the nightside high-latitude ionosphere during a substorm on 3 November 2013. Substorm onset and a later intensification coincided with polar cap patches entering the auroral oval to become auroral blobs. Using Global Navigation Satellite Systems (GNSS) receivers and optical data, we show severe scintillation driven by intense auroral emissions in the line of sight between the receiver and the satellites. During substorm expansion, the area of scintillation followed the intense poleward edge of the auroral oval. The intense auroral emissions were colocated with polar cap patches (blobs). The patches did not contain strong irregularities, neither before entering the auroral oval nor after the aurora had faded. Signals from all three GNSS constellations were similarly affected by the irregularities. Furthermore, two receivers spaced around 120km apart reported highly different scintillation impacts, with strong scintillation on half of the satellites in one receiver and no scintillation in the other. This shows that areas of severe irregularities in the nightside ionosphere can be highly localized. Amplitude scintillations were low throughout the entire interval.