Solar Wind-Magnetosphere Interactions I
Presiding: M Spasojevic, Stanford University; C R Clauer, University of Michigan
SM42A-01 10:30h
Validation of the Space Weather Modeling Framework for Northward IMF Conditions
We have simulated the magnetosphere and the ionosphere for an extended period of northward interplanetary magnetic field (IMF) conditions that occurred between 17:00 UT October 22 2003 and 00:00 UT October 24 2003. The Space Weather Modeling Framework (SWMF) is run with the coupled global magnetosphere (BATSRUS), inner magnetosphere (RCM) and ionosphere electro-dynamics (Ridley) components. The simulation results are compared with the magnetic field measurements of the GOES 10, GOES 12, Polar, Wind and Geotail satellites, and with the density, temperature, magnetic field and velocity measured by the Cluster satellites. We examine the effects of the grid resolution, Joule heating, resistivity, and coupling with the Inner Magnetosphere. It is found that the coupling with the RCM significantly improves the agreement between the observed and simulated magnetic fields near the Earth. In order to better match the far-tail Wind observations, resistivity needs to be added to the simulation. This indicates that the numerical resistivity in the code is most likely too low in the reconnection sites. In addition, it is shown that the amount of Joule heating in the reconnection site has a strong influence on the density and temperature of the plasma sheet. This series of simulations also serve as a validation of the SWMF.
SM42A-02 10:45h
Dependence of the Occurrence of Low Latitude Pc3 Geomagnetic Pulsations on Solar Wind Velocity
Energy for the Earth's magnetospheric processes is provided by solar wind. Pc3 Geomagnetic pulsations are quasi-sinusoidal variations in the Earth's Magnetic field in the period range 10-45 seconds. The magnitude of these pulsations ranges from fraction of a nT(nano Tesla) to several nT. These pulsations can be observed in a number of ways. However the application of ground based manetometer arrays has proven to be one of the most successful methods of studying the spatial structure of hydromagnetic waves in the Earth's magnetosphere. With few exceptions, the Pc3 studies undertaken in the past have been confined to middle and high latitudes. The spatial and temporal variations observed in Pc3 occurrence are of vital importance because they provide evidence which can be directly related to wave generation mechanisms both inside and external to the magnetosphere. At low latitudes (L < 3) wave energy predominates in the Pc3 band and the spatial characteristics of these pulsations have received little attention in the past. An array of four low latitude induction coil magnetometers was established in south-east Australia over a longitudinal range of 17 degrees at L=1.8 to 2.7 for carrying out the study of the effect of the solar wind velocity on these pulsations. Digital dynamic spectra showing Pc3 pulsation activity over a period of about six months have been used to evaluate Pc3 pulsation occurrence. Pc3 occurrence probability at low latitudes has been found to be dominant for the solar wind velocity in the range 400-700 Km/sec. The results suggest that solar wind controls Pc3 occurrence through a mechanism in which Pc3 wave energy is convected through the magnetosheath and coupled to the standing oscillations of magnetospheric field lines.
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SM42A-03 11:00h
Evidence of Effective Shielding in the Inner Magnetosphere?
Within the inner magnetosphere, the large scale, solar wind imposed electric field can be significantly modified by internal magnetospheric coupling processes. One of these processes is shielding which acts to reduce the strength of the convection field at low latitudes. The Region-2 field aligned currents, driven by pressure gradients in the ring current/plasma sheet region, can close via a horizontal current across the ionosphere. This closure current in turn generates an electric field that opposes the main convection field. Observations and theoretical models have shown that shielding takes a finite time to be established (on the order of a half hour) and changes in the imposed electric field or other magnetospheric parameters can cause it be ineffective for extended periods. We present a seemingly rare observation from 10 Jul 2001 when it appears that the inner magnetosphere was effectively shielded from the solar wind imposed convection field for an extended period. IMAGE EUV observations show that despite a prolonged period (>12 hours) of southward IMF, a plasmaspheric plume did not form. DMSP field-aligned current observations support the shielding mechanism and suggest that a substorm at ~19UT disrupted the shielding layer and allowed the convection field to penetrate the inner magnetosphere.
SM42A-04 11:15h
Characterizing the April 18, 2002 Storm-time Sawtooth Events using Ground Magnetic Data
Global periodic sawtooth oscillations begin during the main phase and dominate for over 12 hours the magnetic storm activity on April 18, 2002. We examine the individual sawtooth events and determine that their characteristics are consistent with large, periodic sutstorm disturbances, however, these disturbances are different from typical isolated substorms in a few important ways. First, they are global. The extent of the substorm current wedge, measured using the midlatitude local time magnetic disturbance profile, is approximately 8 hours of local time, compared with 4 hours for typical isolated substorms. The injection region observed in energetic particle flux data at geostationary orbit extends across the entire tail and past the dawn and dusk terminators into the dayside in contrast to the more typical injection region that is limited to a few hours of MLT in the tail. In addition, they appear to be quasi-periodic, however they are driven by a steady solar wind driver. Thus, they do not conform to the "loading - unloading" substorm paradigm. Such driving conditions have been thought to produce Steady Magnetospheric Convection events, however, it appears that they may also produce sawtooth oscillations. It remains to be determined what parameters/conditions control the development of SMC or sawtooth oscillations during periods of steady magnetospheric driving, such as the magnetic cloud that is driving the April 18, 2002 activity reported here.
SM42A-05 11:30h
Global Observations of the Effect of a Sustained North-South Oscillation in the IMF
This paper presents a preliminary study of what appears to be the first IMF event initially identified as interesting on the basis of ground based atmospheric electricity data. On Jan 2, 1993, the electric field mill and air-earth current meters at South Pole Station detected a sustained oscillation in the earth-ionosphere potential at South Pole with a quasi-period of 1 hour and duration of more than 8 hours. Similar oscillations were found in both local and conjugate magnetometer data. This event was recorded all over the earth and by satellite. In addition, the event occurred during the PPB and ELBBO balloon campaigns. Periodic electric field fluctuations associated with the event were observed on most of the balloons. The solar wind and IMF conditions were only available from GEOTAIL, which was located in the magnetosheath at x~-120 Re. The global electrodynamics of the event was modeled using plots generated by AMIE, (Assimilative Mapping of Ionospheric Electrodynamics). The convection pattern variations during the event were a combination of periodic positive turnings of the convection pattern, and periodic intensifications of the electric potential. The auroral energy flux was observed to intensify periodically during the same period, with a major peak at about 1100 UT.
SM42A-06 11:45h
Identification and Decoupling of DP1 and DP2 Current Systems
Severe controversy about the storm-substorm relationship still exists as shown in recent studies. This is not only because of the complexity of the phenomenon, but also the limitation of measurements from the ground and space. For example, in the high latitude region of the northern hemisphere, magnetometers distributed along the auroral oval are used to create AU, AL and AE indices. Changes in the indices are the manifestation of the auroral electrojet variation contributed by both the near midnight substorm current wedge (also called DP1 current) and the ionospheric Hall current (also called DP2 current). The two current systems have different response to the solar wind and magnetosphere/ionosphere conditions. By studying only these indices, one is unable to determine when the DP1 current system is developed and dominant, as well as unable to elucidate which portion of the electrojet, the DP1 or DP2 current, is more important to the ring current intensification. To clarify this situation, efforts have been put to distinguish and decouple the DP1 and DP2 currents. In this paper we use the method of natural orthogonal components (MNOC) to quantitatively determine DP1 and DP2 separately, and therefore to distinguish high latitude ionospheric current patterns. We have studied the current patterns and DP1/DP2 current densities during magnetic clouds and very disturbed interplanetary magnetic field conditions. Correlations between DP1/DP2 currents and geomagnetic storm intensity have been discussed as well.