SA44A-01 INVITED
Separating Sources of Storm-Time Electrodynamics Using Physical Models and Observations
Interpretation of observations of the electrodynamic response to geomagnetic storms is relatively unambiguous during the first two to three hours of the onset of an event. During these early hours, penetration electric fields dominate. After this time, other processes begin to influence the response, such as the disturbance dynamo. Modeling real events reveals that dynamo processes can cause an electrodynamic response at the equator within two to three hours of storm onset. Confirmation of the modeling results can only be achieved by observing both the electrodynamic response and the time evolution of the dynamics, which is the cause of the disturbance dynamo. The ionospheric response to a storm at mid and low latitude is further complicated by a range of additional physical processes, in particular changes in ion recombination rates due to altered neutral thermospheric composition. To separate the influence of electrodynamic and the neutral atmosphere requires, in addition, knowledge of the neutral composition response.
SA44A-02 INVITED
Science and Measurement of Global Ionospheric Storms
Recent observations are revealing surprising features of global ionospheric storms that defy simple explanation. We review new and recently published results that define new science questions regarding these storms. Geomagnetic storms have been studied for decades and possible physical mechanisms for the observed ionospheric variability have been proposed. The increased availability of data from numerous Global Positioning System receivers has revealed new qualtitative and quantitative behavior that earlier observations did not resolve. This has led to new science questions ranging from fundamental physics to collective behavior of the ionosphere as part of the larger geospace "system." Analysis of multiple storms reveals new patterns of commonality across multiple events, but also variations in response that are not easily explained. Certain aspects of the global dynamics are a manifestation of long-range electromagnetic forces at work on planetary scales. Energy and mass coupling between the ionosphere and magnetosphere are significant, suggesting that understanding geospace cannot be achieved by focusing on a single regime (such as ionosphere alone, or magnetosphere alone). The interface between the magnetosphere and the solar wind is clearly important also and influenced by ionospheric dynamics. Observations show that spatial and temporal dynamics encompass all scales (from minutes to days, from tens of meters to planetary scale). The neutral thermosphere dynamics strongly influences the storm time response, but understanding of the many possible responses is incomplete. The wide variety of phenomena exhibited during geospace storms has renewed the debate about basic physical processes that might be operating. Not surprisingly, fundamental physical mechanisms are being reexamined such as: shielding of electric fields by the conducting ionosphere, and the role of electric fields in plasma drift. Recent modeling work suggests that the role of the complex plasma-neutral interaction needs to be far better understood. It is clear that new observations are needed to resolve these significant and fundamental questions. New experimental and observational methods offer great promise for significant new insights. We will discuss recent community dialog oriented towards meeting the challenges.
SA44A-03 INVITED
SuperDARN Hokkaido HF radar observation of subauroral ionospheric convection during storms
SuperDARN Hokkaido HF radar, capable of measuring the subauroral ionospheric plasma convection especially during storms, has been in continuous operation since the beginning of December 2006. Dayside merging flow, overshielding flow, and SAPS/SAID have been successfully observed during storms so far. As an excellent example of Hokkaido radarfs contributions, we report the first two-dimensional observation of a dynamic variation of convection flow reversal in subauroral postmidnight sector during the storm main phase on 29 January 2007. The flow reversal region is extended over 20 deg in longitude and 5 deg in latitude, lasting for about 10-15 min, and the maximum flow speed is about 0.5-1.0 km/s. The flow reversal structure is reasonably reproduced by the ring current simulation coupled with the ionosphere, suggesting that it is produced by the region 2 field-aligned current associated with the ring current enhancement during the storm main phase. The dynamic variation of the flow reversal structure is interpreted as a transient eastward extension of the elongated dusk convection cell to the postmidnight and equatorward of the dawn cell, associated with the variation of the ring current whose structure is controlled by the interplanetary magnetic field and solar wind dynamic pressure. It is suggested that the ring current variation is highly coupled with the interplanetary parameters and is much more complicated than ever thought.
SA44A-04 INVITED
First two-dimensional observations of overshielding by the SuperDARN Hokkaido radar
Two-dimensional ionospheric plasma flows possibly caused by overshielding were observed for the first time. The observations were made by the mid-latitude SuperDARN Hokkaido radar in Japan during a major magnetic storm on December 15, 2006. The magnetosphere was exposed continuously to a southward interplanetary magnetic field (IMF) for several hours during the main phase of the storm. Immediately following the subsequent northward turning of the IMF, an anti-sunward plasma flow was observed for about 14 min in the pre-dusk sector at magnetic latitudes of 50-60 deg, reaching a maximum line-of-sight speed of 70-80 m/s. These features are consistent with a simulation of coupling between the ring current and the ionosphere associated with an overshielding condition. Within 1 h of the first observation, a similar anti-sunward flow was observed during a period of southward-oriented IMF. However, the simulation cannot account for the anti-sunward flow in this case. It is suggested that this second overshielding-like condition is attributable to a sudden contraction of the polar cap associated with the substorm, or to a sudden strengthening of the inertial current converted from the abrupt injection of magnetospheric ions. Thus, the cause of the overshielding is probably not as simple as previously thought. Specific view on future needs from the magnetosphere-ionosphere modeling perspective will be discussed.
SA44A-05
Effects of Ionospheric Trough on the Relationship Between the Ring Current and SAPS
Subauroral polarization streams (SAPS), are usually associated with geomagnetically disturbed times and considered as a manifestation of magnetosphere and ionosphere (M-I) coupling. Previous research results using radar and satellite measurements have revealed many features of the SAPS events. However, the generation mechanism of SAPS is not without dispute. In this paper, we focus on the effects of subauroral trough conductance on the attributes of SAPS and the evolution of the coupled magnetosphere and ionosphere system through the comprehensive ring current model (CRCM), which includes the coupled electrodynamics of the inner magnetosphere and ionosphere with a self-consistent description of the electric field. Our numerical analysis indicates that low conductance at the subauroral latitudes (due to mid-latitude trough) is critical to the large amplitude of SAPS. The model results are generally in good agreement with common characteristics of SAPS and provide some insights into one possible generation mechanism from a coupled system perspective.
SA44A-06
Dynamics of American Sector Mid and Low Latitude Ionospheric and Thermospheric Response During the November 2004 Superstorm
Interplanetary electric field (IEF) penetration into the inner magnetosphere and plasmasphere can occur during intense geomagnetic storms, enhancing eastward electric fields over the sunlit ionosphere. Such events can serve as triggers for complex ionosphere-magnetosphere feedback mechanisms which increase ionospheric convection and neutral winds both locally and globally. We present a study of ionospheric dynamics and physical drivers during several events contained within the November 9 - 11, 2004 superstorm. This event was marked by excellent coverage from the full American sector incoherent scatter radar chain at Jicamarca, Arecibo, Millstone Hill, and Sondrestrom, which allows wide latitude diagnostics of E and F region electric fields, plasma densities and temperatures, and neutral wind vectors. We also employ CARISMA magnetometer chain observations, DMSP SSIES topside ion drifts, and GPS derived total electron content (TEC) maps to place the radar data in context. Penetrating eastward electric fields were seen from Millstone Hill equatorwards to Jicamarca for over 15 hours on November 9 and 10, accompanied by very low TEC values and a plasmasphere boundary layer midway between Millstone Hill and Arecibo. Equatorward neutral wind surges of ~ 300 m/s were driven locally by substorms at Millstone Hill and Arecibo, with associated dynamo effects creating prompt electric fields and large F layer downdrafts. Substorm timing during three separate events as identified from CARISMA magnetometers is consistent with Millstone Hill observed neutral wind surges. We also present results from TIMEGCM model runs and compare predictions of ionospheric conditions along the radar chain to gain insight into the complex physical drivers during this superstorm event.
SA44A-07
Parameterization of Ionosphere/Inner-Magnetosphere Electrodynamic Interactions for Ionospheric Wind Dynamo Modeling
Electrodynamic interaction of hot plasma in the inner magnetosphere with the conducting ionosphere produces the phenomenon of steady-state shielding, which reduces the transmission of electric fields across the latitudes of region-2 field-aligned currents. This affects the penetration of magnetospheric electric fields to the low-latitude ionosphere, and influences the distribution of ionospheric electric fields generated by the ionospheric wind dynamo. These effects can be simulated by representing the magnetosphere as a combination of equivalent magnetospheric Hall and east-west Pedersen conductances. In this study we examine how the equivalent magnetospheric conductances might be expected to vary with different magnetospheric and ionospheric parameters, and we investigate the response of modeled ionospheric electric fields and region-2 currents to the equivalent magnetospheric conductances.