SM41B-01
Geosynchronous Magnetic Field Variations Associated With the Passage of Interplanetary Shocks or Solar Wind Discontinuities
It has been reported that geosynchronous magnetopause crossings (GMCs) are more frequently observed in the prenoon sector than in the postnoon sector. Since the GMCs indicate that the magnetopause is compressed to within geosynchronous orbit, previous studies suggested that the magnetopause is asymmetric with respect to local noon during extreme solar wind conditions. Motivated by this suggestion, we investigate geosynchronous magnetic field variations when sudden commencements (SC)/sudden impulses (SI), which are associated with the passage of interplanetary shocks or solar wind discontinuities, are observed on the ground. From a statistical analysis of the geosynchronous magnetic field responses to SC/SI events from 1997 to 2005, we find that the magnetic field enhancement in the prenoon sector is larger than that in the postnoon sector over dayside local times. This asymmetry of the geosynchronous magnetic field amplitude with respect to local noon cannot be explained by the solar wind aberration effect due to the motion of the Earth around the Sun. Using solar wind and ground magnetic field data, we examine what causes the asymmetry.
SM41B-02
Nonlinear Electrostatic Solitary Waves Observed in Connection with the 24 August 2005 Super-Substorm
Nonlinear electrostatic solitary waves (ESWs) are often observed in the magnetotail and sometimes are clearly seen to be associated with substorm processes. We present Cluster Wideband Data (WBD) from three of the four Cluster satellites showing the presence of ESWs at the onset in the main phase of the so-called super- substorm on 24 August 2005. Although ESWs are present in the observations for the two hours on either side of this onset, the ones that are observed at main phase onset are generally 1) more numerous, 2) of larger electric field amplitude, and 3) trending toward shorter time scales. In addition, there are significant differences in the detection of ESWs across the three Cluster satellites, which we discuss. For example, Cluster 1 detects enhanced ESWs for a much longer period of time after onset than Cluster 4. The Cluster satellites are approximately 6,000 km apart at this time. We compare and contrast the ESW observations from 24 August 2005 with those from another substorm during which Cluster was in the magnetotail and magnetic reconnection was observed. We briefly discuss the possible generation mechanisms for the ESWs associated with the main phase onset of the 24 August 2005 super-substorm based on data from other Cluster instruments and from various other satellites, thus providing a link to the super-substorm itself.
SM41B-03
Multiple Cusps under Northward IMF Conditions: Observations and MHD Simulations Compared
Multiple cusps have been observed by the Cluster spacecraft when the satellites were traveling outbound on Mar 21 and Mar 22, 2001. We conclude that multiple cusps are a temporal effect which is due to the change of the solar wind azimuthal flow (the wind sock effect). The cusp properties derived from two MHD codes, Open GGCM and BATSRUS, have been compared with the Cluster observations. We find that although the simulated magnetospheres are smaller than the real magnetosphere, the simulated magnetic fields and the amplitude of the model-derived plasma parameters (velocity, density, temperature and thermal pressure) agree well with Cluster observations. The MHD code successfully simulated the responses of the cusp positions to the solar wind azimuthal flow. This event is under northward IMF condition. The Geotail spacecraft observed the cold dense plasma when it crossed the dusk flank of the magnetotail. MHD code successfully simulated the formation of the cold sense plasma sheet on the flanks.
SM41B-04
Measures of Geo-effectiveness in Storms
Geomagnetic storms are produced by solar wind disturbances causing large currents to flow throughout the magnetosphere. These currents are the magnetosphere's response to the solar wind electric field and the rate of the interplanetary magnetic field's reconnection with the magnetosphere. To gauge the geo-effectiveness of a storm, or the magnetosphere's response to the storm, we consider the ratio of the ring current injection rate (RCIR) to measures of the solar wind input. Burton et al. [1975] called this parameter α, using VBs as the solar wind input. We calculate three versions of α: α1 is the Burton et al. [1975] parameter, α2 is the ratio of the RCIR to the Newell et al. [2007] universal coupling function, α3 is the ratio of the RCIR to a measure of the dayside reconnection rate recently proposed by Borovsky. Using each of these values of α, we rank 100 storms with Dst < -75 nT, between 1995 and 2005. The top 10% and lowest 10% of storms are examined in detail to determine what characteristics they might have in common.
SM41B-05
Response of Polar Magnetospheric Density to High Solar Wind Dynamic Pressure
Understanding the sources and transport of the magnetospheric plasma is a fundamental research topic in magnetospheric physics and is crucial to space weather forecasting. The ultimate plasma sources of the magnetosphere are the solar wind and ionosphere. The polar magnetosphere (above 2000-3000 km altitude in the polar cap) is often the pathway for the solar wind and/or ionosphere plasma to access other regions of the magnetosphere. Therefore, the plasma density in the polar magnetosphere and its variations are strongly modulated by variations in the solar wind/interplanetary magnetic field (IMF) and ionospheric ion outflows. Based on this fact, we may extract useful information about the plasma sources of the magnetosphere by investigating the plasma density variations in the polar cap. In this study we present the electron density measurements from IMAGE RPI in the polar magnetosphere and show that a solar wind dynamic pressure enhancement can increase the plasma density in the polar cap. The plasma density increase in the polar magnetosphere in the case examined here is shown to be most possibly associated with the enhancement of the solar wind density although there might be a contribution from the ionospheric ion outflows. The IMF By is larger than the IMF Bz in magnitude and the polar cap convection is anti-sunward in general. The present observation indicates that the direct entry of the solar wind through the cusp has significant effects on the plasma density in the polar magnetosphere.
SM41B-06
Interaction of Interplanetary Shocks with the Earth's Magnetosphere: Observations and Global MHD Simulations Compared During the Nov 9, 2002 Event
It is widely known that interplanetary shocks cause pressure pulses and launch a variety of waves into the magnetosphere. Our analysis of several interplanetary shocks reveals that the disturbance speeds are higher in the magnetosphere than in the solar wind and gradually increase from the dayside to the nightside magnetotail. On November 9th, 2002, observations of two consecutive fast forward interplanetary shocks were provided by several widely separated satellites: ACE, Wind, Genesis, SOHO, and Geotail in the solar wind, GOES series spacecraft, Polar, and Cluster in the magnetosphere. In addition, the shock propoagation was monitored by networks of ground magnetometers. This rich set of multipoint observations allows us to monitor in detail the propagation of the disturbance from the solar wind to the near-Earth's magnetosphere. In order to resolve causes for the unusual double onset structures observed at geostationary orbit and to verify our disturbance speed estimations, we have used the GUMICS-4 global MHD code to simulate the entire sequence of events. The simulation gives us a global picture of the shock -- magnetosphere interaction, which allows us to match many features with the satellite observations and helps to interpret to satellite measurements in different parts of the magnetosphere. We demonstrate the capabilities and discuss the constraints of the simulation model to describe such rapidly propagating dynamic events.
SM41B-07
Solar wind pressure as a cause for persistent proton precipitation
Recent studies have demonstrated that the intensity of the proton aurora increases at all magnetic local times following a solar wind shock. The proton aurora intensity has been shown to be higher when the shock is preceded by a period with southward IMF, than for northward IMF. The magnitude of the solar wind pressure after the shock is believed to be of minor importance. We use global satellite images to study the proton precipitation in relation to the solar wind dynamic pressure. The study is focused on times when the geomagnetic activity is low, and the majority of the observed aurora unambiguously can be attributed to the solar wind pressure. The new finding in our study is that the intensified nightside proton aurora is persistent, and lasts as long as the pressure is high. When the pressure drops, the proton aurora diminishes within few minutes. The behavior of the electron dominated global aurora during these events will also be investigated.
SM41B-08
Magnetospheric responses to periodic variations in the solar wind dynamic pressure
Before we investigate magnetospheric responses to various solar wind changes including IMF and dynamic pressure, it is essential to understand fundamental properties of the magnetospheric responses for simple solar wind variations. For this purpose, a global MHD model is a unique and powerful tool because we can assign any solar wind variations in the model [e.g., Tanaka, 1995]. In the present study, we investigate the magnetospheric response to periodic variations in the solar wind dynamic pressure with various time scales. Through the simulation. we obtain the following results. EThe magnetosheath is a buffer of the magnetosphere against short-period solar wind dynamic pressure variations. EThe shorter-period variations, the MHD-wave process is dominant in the magnetosphere. On the other hand, for the longer-period one, the magnetospheric process is associated with slowly varying convection system in the magnetosphere-ionosphere system. The criterion between the shorter and longer period is about 5 minutes. EFor shorter-period variations, spectral characteristics of the ground magnetic variations reflect those of the solar wind variation. Note that the field line resonances must be superposed on these spectral characteristics. EFor longer-period variations, the spectral characteristics of the ground magnetic variations are different from those of the solar wind variations. The ground magnetic variations have higher-harmonic spectral components of the original one of the solar wind variations. This is independent of the field line resonance mechanism.