SM31C-01 INVITED
Solar wind and IMF drivers during different modes of energy transfer
We report the results of a statistical investigation that relates various solar wind and IMF drivers to different modes of magnetospheric convection, including isolated substorms, periods of balanced magnetospheric convection (BRI) – more commonly termed steady magnetospheric convection (SMC), and individual sawtooth injections. We investigate the driver values as well as the 'steadiness' of the driver in order to determine the importance of magnitude, sign and stability of the drivers for the different modes of energy transport. It appears that the magnitude and direction of Bz is sufficient to describe the conditions for substorm occurrence , while BRIs and sawteeth require both magnitude and steadiness of certain drivers to specify the occurrence conditions. We find that BRI and sawteeth intervals show similar 'steadiness' values in the driving conditions, but greater magnitude of the drivers is found during sawteeth oscillations. We also find evidence that the magnitude of solar wind velocity may play a role in whether the magnetosphere enters a sawtooth mode or a BRI mode.
SM31C-02 INVITED
What determines and how to distinguish different magnetotail dynamical modes?
The question why the strongly driven magnetotail in similar conditions may respond via either loading/unloading substorm mode or via steady dissipation stays puzzling for many years. Here we discuss the essential differences between these states found so far, the possible physics which controls the different dynamics, as well as the set/combination of observed parameters. We briefly summarize the predictions of the most well-articulated scenario, in which the spatially-extended close flux tube region (plasma sheet) controls the global plasma transport mode via the mechanism of interchange-type motions, discuss how it can be tested using Themis and show the first results of corresponding Themis-based analyses. We also discuss the tail open magnetic flux as the global parameter which can help in characterizing the magnetotail state and identify the dynamical mode.
SM31C-03 INVITED
On the Three-Hour Periodicity of the Solar-Wind-Driven Magnetosphere
The level of geomagnetic activity is strongly linked to the parameters of the solar wind upstream of the Earth. The nature of the activity is also linked to the upstream solar wind: different regimes of solar-wind parameters result in different types of magnetospheric reaction. A common reaction has a 3-hr periodicity. This can take the form of periodic substorms for typical solar-wind parameters or global sawtooth oscillations for low-Mach number solar wind. The 3-hour length of the period is not a function of solar-wind parameters and the 3-hour length of the perio period is invariant to the level of geomagnetic activity driven by the solar wind. One of the outstanding questions in magnetospheric physics is why the magnetosphere displays a 3-hour periodicity (and sets the period at 3 hours).
SM31C-04 INVITED
Characteristics of Steady Magnetospheric Convection: A SuperDARN Perspective
Intervals of Steady Magnetospheric Convection (SMC) are loosely defined as times when convection in the magnetosphere is enhanced and there are no substorm signatures. There have been several quantitative definitions developed to detect SMC events. None of these methods, however, are based on observations of convection. SuperDARN is a useful tool for studying SMC, because it quantifies convection directly on a global scale. Previous SMC selection methods make use of ground based magnetometer responses to auroral electrojet currents. These magnetic observations are very useful, because they do not suffer from data gaps like radars do when the ionospheric propogation conditions are not ideal. The magnetometers have a disadvantage, however, in that they record a convolved response to both convection and conductivity. Previous SMC selection methods resulted in a strong seasonal bias in SMC occurrence due to seasonal changes in ionospheric conductivity. A new SMC selection criterion was developed to improve upon the previous criterion for enhanced convection. SuperDARN was used to evaluate the old and new selection methods. According to SuperDARN convection observations the new SMC selection criterion largely eliminated conductivity effects. Statistical studies reveal that the new SMC intervals have similar properties as events selected using traditional methods. Both SMC events sets have a moderate solar wind driver, enhanced convection, and stable polar cap size. Statistical studies also demonstrate that SuperDARN has good data coverage during SMC, unlike some other types of active magnetosphere phenomena. SuperDARN is therefore an excellent tool with which to study SMC.
SM31C-05
Solar wind plasma controlling magnetospheric coupling efficiency
A wealth of new observational evidence add to the earlier understanding of solar wind - magnetosphere coupling the importance of solar wind speed and density. Statistical analysis shows that for the same electric field, events with higher solar wind speed cause larger AE disturbance than those with lower speed and higher magnetic field. Analysis of abrupt solar wind density changes shows that the ionospheric activity is lower for lower values of the density, and that the response is near-simultaneous with the arrival of the front at the magnetopause. Global MHD simulations confirm these results: The reconnection efficiency at the magnetopause is a strong function of the solar wind speed, and responds to a lesser extent also to changes in the solar wind density. Furthermore, series of simulation runs confirm the higher activity during higher solar wind speed, and the role of dynamic pressure in driving the magnetotail activity. Lastly, both observational and simulation evidence suggest that the coupling efficiency is higher for lower level of solar wind driving such that a smaller percentage of incident energy is processed in the magnetotail and auroral ionosphere during strong magnetic activity. Here we use both observational and simulation results to extract the processes via which the solar wind plasma parameters affect the level of activity both in the magnetotail and in the auroral ionosphere.
SM31C-06
Distortions of the Inner Magnetosphere Magnetic Field during storms, saw-tooth oscillations and Steady Magnetospheric Convection events
We present a quantitative investigation of the processes controlling the inner magnetospheric magnetic field topology during several dynamical states of the Earth's magnetosphere, namely, geomagnetic storms, sawtooth oscillations and steady magnetospheric convection (SMC) events. For this analysis we use our event-oriented model, a unique tool to provide a realistic representation of the magnetospheric magnetic field during disturbed times for specific events. The output from this model is an input to the Inner Magnetosphere Particle Transport and Acceleration model (IMPTAM). This model traces ions and electrons with arbitrary pitch angles in the drift approximation in time-dependent magnetic and electric fields taking into account the loss processes. We model two storm events, one moderate with Dst drop of - 150 nT on November 6-7, 1997 and one intense with Dst drop of - 250 nT on October 21-23, 1999, two saw-tooth events, on October 22, 2001 and April 18, 2002, and two SMC events on February 3-4, 1998 and May 5, 1998. With this modeling we quantify which of the major current systems cause what aspects of the magnetic field distortion during magnetospheric dynamical states. With our physics-based numerical models we will determine the flow of plasma leading to the current systems that dominate the magnetic field distortion of the magnetosphere. In addition, the contribution of temporal variations of plasma dynamics (e.g., dispersed and dispersionless injections) to magnetic field distortions will be investigated. Furthermore, the detailed comparison between sawtooth oscillations and steady magnetosphric convection (SMC) events in terms of their observational features and model outputs will be conducted. By the analysis of sawtooth oscillations and steady magnetospheric convection (SMC) events a fundamental question whether the sawtooth events form a new class of magnetospheric events or whether they are simply quasi-periodic storm-time substorms will be addressed.
SM31C-07
Preliminary evidence for enhanced solar wind-magnetosphere energy transfer during periods of Alfvénic IMF
During the past winter, campaigns were set up with the Sondrestrom and Poker Flat incoherent scatter radars to provide dayside and nightside radar observations of ionospheric flows simultaneously with nightside plasma sheet flow observations from multiple THEMIS spacecraft. We have compared observations obtained during periods of large amplitude Alfvénic magnetic field fluctuations within the solar wind, which occur primarily within high-speed solar wind streams, with observations during periods of relatively steady southward and northward IMF. This comparison provides evidence that the average flow speed within the dayside and nightside ionosphere and within the nightside plasma sheet is substantially enhanced when the Alfvénic magnetic fluctuations impinge upon the magnetosphere relative to flows under steady southward and northward IMF conditions. Observations during a period of Alfvénic magnetic fluctuations without high-speed solar wind indicate that the enhanced flows are at least in part directly due to the Alfvénic magnetic fluctuations and are not solely due to the high-speed solar wind. While these results require further corroboration, if true, they would suggest that, in addition to the well-known contribution from the direction and magnitude of the IMF and from solar wind dynamic pressure, Alfvénic magnetic fluctuations contribute directly to the strength of solar wind magnetosphere- ionosphere coupling. A speculative possibility for why this might occur is resonance between the Alfvénic IMF fluctuations and the natural oscillation frequencies of the magnetosphere.