L. B. N. Clausen

Dynamics of the region 1 Birkeland current oval derived from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE)

[1]xa0The region 1 (R1) and region 2 current systems typically form concentric rings of field-aligned currents in the polar ionospheres; we term the inner ring the R1 oval. We apply an automated fitting scheme to field-aligned current densities provided by the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) and identify the latitude of maximum R1 current at all magnetic local times to yield the size of the R1 oval. We investigate the dynamics of the R1 oval size in response to geomagnetic activity for two cases corresponding to: repeated substorm activations with a minimally enhanced ring current; a significant ring current enhancement with multiple substorms. During the first event the dynamics of the R1 oval size reflected an expanding-contracting polar cap: during substorm growth phase dayside reconnection added open magnetic flux to the polar cap, expanding the R1 oval equatorward. Tail reconnection during the substorm expansion phase converted open into closed magnetic flux and the polar cap contracts as reflected by the poleward retreat of the R1 oval. During the period of enhanced ring current intensity the R1 oval grew to larger sizes during each substorm growth phase than it did during the other event, consistent with the suggestion that a stronger ring current stabilizes the magnetospheric tail to the onset of magnetic reconnection. The presented methodology allows AMPERE data to be condensed into a single parameter, the R1 oval size, which reflects magnetospheric dynamics and provides a convenient measure of the instantaneous magnetospheric system state in both hemispheres.

EMIC waves observed at geosynchronous orbit during solar minimum: Statistics and excitation

[1]xa0We identified 1875 wave events in magnetic field data from geosynchronous orbit. Most of these events were transverse with respect to the background magnetic field, left-hand polarized, and were observed in the post-noon magnetic local time sector at frequencies just below the helium gyrofrequency. Combined, these observations strongly suggest that most of these events are Electromagnetic Ion Cyclotron (EMIC) waves. Average wave amplitudes are presented, binned by frequency, geomagnetic activity and magnetic local time. The amplitude increases with increasing geomagnetic activity; increased activity also narrows the local time sector in which the waves are observed. A superposed epoch analysis of solar wind parameters and geomagnetic activity indices shows that 12 hours before wave onset the AE and Kp index increased, indicating storm and substorm activity that injects hot ion populations needed to drive the EMIC instability and providing ample time for those populations to drift into the post-noon local time sector. Just before wave onset a sudden enhancement in the AE index and the solar wind dynamic pressure are observed, indicating that a final perturbation of the magnetosphere is needed to excite EMIC wave growth. EMIC waves are thought to cause loss of relativistic particles in the radiation belt. Large solar wind densities have been associated with low flux of relativistic particles during the recovery phase of geomagnetic storms. We show that EMIC waves are preferentially generated during intervals of large solar wind density, indicating that such conditions drive EMIC waves which in turn cause enhanced loss of relativistic particles.

Large-scale observations of a subauroral polarization stream by midlatitude SuperDARN radars: Instantaneous longitudinal velocity variations

[1]xa0We present simultaneous measurements of flow velocities inside a subauroral polarization stream (SAPS) made by six midlatitude high-frequency SuperDARN radars. The instantaneous observations cover three hours of universal time and six hours of magnetic local time (MLT). From velocity variations across the field-of-view of the radars we infer the local 2D flow direction at three different longitudes. We find that the local flow direction inside the SAPS channel is remarkably constant over the course of the event. The flow speed, however, shows significant temporal and spatial variations. After correcting for the radar look direction we are able to accurately determine the dependence of the SAPS velocity on magnetic local time. We find that the SAPS velocity variation with magnetic local time is best described by an exponential function. The average velocity at 00 MLT was 1.2xa0km/s and it decreased with a spatial e-folding scale of two hours of MLT toward the dawn sector. We speculate that the longitudinal distribution of pressure gradients in the ring current is responsible for this dependence and find these observations in good agreement with results from ring current models. Using TEC measurements we find that the high westward velocities of the SAPS are - as expected - located in a region of low TEC values, indicating low ionospheric conductivities.

An examination of inter-hemispheric conjugacy in a subauroral polarization stream

During geomagnetically disturbed conditions the midlatitude ionosphere is subject to intense poleward directed electric fields in the dusk-midnight sector. These electric fields lead to the generation of a latitudinally narrow westward directed flow channel in the subauroral region called a subauroral polarization stream (SAPS). If the magnetic field lines are treated as equipotentials, electrodynamic events such as SAPS are expected to occur simultaneously at magnetically conjugate locations with similar features. In this paper we present simultaneous observations of a SAPS event in both hemispheres made by midlatitude SuperDARN radars with conjugate fields-of-view. We analyze the relation between the geomagnetic conditions and the characteristics of the channels such as latitudinal location, electric field, total potential variations across the channels, and Pedersen current. The results suggest a strong correlation between the strength of the ring current and the latitudinal location of the channel. An inter-hemispheric comparison of the characteristics of the channel indicates that the potential variations across the channels are similar while the electric fields, Pedersen currents and latitudinal widths of the channel exhibit differences that are consistent with equal potential variations. We attribute these differences to seasonal differences in ionospheric conductivity between the hemispheres and magnetic distortion effects in the inner magnetosphere.

A comparison of magnetic field measurements and a plasma‐based proxy to infer EMIC wave distributions at geosynchronous orbit

[1]xa0Wave-particle interactions are fundamental to the dynamics of the outer radiation belt. Electromagnetic ion cyclotron (EMIC) waves can resonate with energetic electrons, causing pitch angle diffusion and scattering of the electrons into Earths atmosphere. These waves act locally; thus, accurately measuring their spatial and temporal distributions is critical to understanding their contribution to radiation belt electron losses. Using Los Alamos National Laboratory Magnetospheric Plasma Analyzer data from geosynchronous orbit, we examine a plasma-based proxy for enhanced EMIC wave growth during a set of 52 relativistic electron flux dropout events. This proxy is compared to in situ wave measurements from the GOES satellites, also at geosynchronous orbit, for single-wave events as well as a superposed epoch statistical analysis. The proxy is extended to calculate an amplitude for the inferred waves, to enable a more quantitative comparison to the in situ GOES EMIC measurements. Signatures of EMIC waves are present in both the proxy and the direct wave observations at similar local times as well as epoch times. The waves are most prevalent in the afternoon sector, with enhanced occurrences beginning half a day before the onset of the dropouts and peaking in the day following. We see agreement in occurrence between the proxy and waves both statistically and in individual instances. This study demonstrates the powerful applications of plasma data to infer wave distributions in space.

A survey of plasma irregularities as seen by the midlatitude Blackstone SuperDARN radar

[1]xa0The Super Dual Auroral Radar Network (SuperDARN) is a chain of HF radars that monitor plasma dynamics in the ionosphere. In recent years, SuperDARN has expanded to midlatitudes in order to provide enhanced coverage during geomagnetically active periods. A new type of backscatter from F region plasma irregularities with low Doppler velocity has been frequently observed on the nightside during quiescent conditions. Using three years of data from the Blackstone, VA radar, we have implemented a method for extracting this new type of backscatter from routine observations. We have statistically characterized the occurrence properties of the Sub Auroral Ionospheric Scatter (SAIS) events, including the latitudinal relationships to the equatorward edge of the auroral oval and the ionospheric projection of the plasmapause. We find that the backscatter is confined to local night, occurs on ≈70% of nights, is fixed in geomagnetic latitude, and is equatorward of both the auroral region and the plasmapause boundary. We conclude that SAIS irregularities are observed within a range of latitudes that is conjugate to the inner magnetosphere (plasmasphere).

First radar observations in the vicinity of the plasmapause of pulsed ionospheric flows generated by bursty bulk flows

[1]xa0Recent expansion of the SuperDARN network to mid-latitudes and the addition of a new high-time resolution mode provides new opportunities to observe mid-latitude ultra-low frequency waves and other ionospheric sub-auroral features at high temporal resolution. On 22 February 2008, the Blackstone SuperDARN radar and THEMIS ground magnetometers simultaneously observed substorm Pi2 pulsations. Similarities in measurements from the Blackstone radar and a magnetometer at Remus suggest a common generating mechanism. Cross-phase analysis of magnetometer data places these measurements at the ionospheric projection of the plasmapause, while fine spatial and temporal details of the radar data show evidence of field line compressions. About 1 min prior to ground Pi2 observation, 2 Earthward-moving Bursty Bulk Flows (BBFs) were observed by THEMIS probes D and E in the near-Earth plasma sheet. We conclude that the first 2 pulses of the Pi2s observed at Blackstone and Remus result from compressional energy generated by BBFs braking against the magnetospheric dipolar region.

ULF wave characteristics at geosynchronous orbit during the recovery phase of geomagnetic storms associated with strong electron acceleration

[1]xa0We identified 17 geomagnetic storms between January 2007 and December 2008. Using particle measurements, each storm is classified as a high-flux event or a low-flux event. High-flux events are those storms associated with enhanced flux of relativistic electrons at geosynchronous orbit, and low-flux events show no such enhancement. We study the characteristics of ultralow frequency waves between 0 and 80 mHz during the recovery phase of the high-flux storms using magnetic field data. By determining the wave propagation direction and magnetic perturbation direction, we are able to calculate how the power is distributed between toroidal, poloidal, and compressional wave modes. We find that on the nightside most wave power is compressional at all frequencies below 80 mHz. On the dayside compressional wave power dominates the lowest frequencies below 20 mHz; toroidal waves carry most energy between 20 and 50 mHz, and poloidal waves are associated with higher power levels between 50 and 80 mHz. This suggests that on the nightside electrons are predominantly transported and energized by mechanisms involving the compressional wave mode; on the dayside, electron energization is achieved by acceleration mechanisms involving toroidal waves and mechanisms dependent on poloidal and compressional waves.