SM43B-1323
Irregular Magnetic Pulsations – Generation, Propagation, and Transformation
Low frequency magnetic signals are commonly observed during periods of enhanced Magnetospheric activity. Both Pi1B and Pi2 magnetic pulsations have been shown to correlate strongly with the onset of substorms in the magnetotail, and some authors have considered them manifestations of the same phenomena. However, recent analyses [Arnoldy, 1998; Posch 2007] have concluded that Pi1B and Pi2 signals most likely arise from different physical processes. In this paper, we present the results of theoretical and numerical investigations into the Pi1B and Pi2 phenomena and attempt to reconcile certain recent observations [e.g. Kepko, 2002; Lessard, 2006] with the existing theoretical framework.
SM43B-1324
Solar Wind and IMF Control of the Recovery Phase of Substorms
The IMF and solar wind dynamic pressure can affect the onset location and phenomenon such as transpolar arcs and the overall substorm process. It is unclear, however, if the IMF, solar wind density and/or dynamic pressure have any effect on the substorm recovery phase. Using the IMAGE FUV WIC and SI-12 instruments a selection of 138 isolated substorms was determined. Using Weimer-mapped ACE IMF and solar wind measurements, the controlling effect on various aspects of the recovery phase global auroral evolution are examined. In particular, the effect of IMF By component is of interest due to its significant effect on transpolar arcs and onset location. The results clearly show that there is no significant effect of the IMF on the recovery phase. There is a tendency for the solar wind density and dynamic pressure to effect the recovery phase process as manifested in the global aurora. In particular, the proton aurora as measured with the SI-12 instrument is more heavily influenced by the density and pressure than the largely electron aurora measured with the WIC instrument.
SM43B-1325
Revisiting the Bimodal Response of the Magnetosphere – Truth or Fiction
The bimodal response model of the magnetosphere was proposed to explain how substorm current systems respond to solar wind coupling. The model is simple. It suggests that the magnetospheric system responds on two different time scales, to a motional electric field impulse imposed across magnetosphere the by the solar wind. The original work, published over 20 years ago, examined the linear impulse response of the auroral electrojet AE indices to the solar wind input parameter VBs calculated by using IMP-8 observations. The VBs-AL filters were found to exhibit two response peaks, one centered at an ~20-min time lag, the other at ~70-min. The two peaks were interpreted as the superimposed responses of the convection current system driven directly by dayside reconnection (the 20-min peak) and the substorm current wedge driven by the release of energy stored in the magnetotail following formation of an x-line in the near-Earth tail (70-min peak). The data set analyzed contained only ~73 days of data. This is small compared to the magnitude of data based compiled over the last decade from Wind and ACE. We have repeated the original study utilizing nearly 10 years of solar wind observations from ACE. The wealth of data allows one to study long-term variations in magnetospheric response, characterized by the AE indices and the polar cap PC index. Changes in response character with season also will be addressed.
SM43B-1326
A Study of Single and Multiple Onset Substorms
A good indicator of substorm expansion phase onset is a well-defined increase and/or energization of the HYDRA electron flux measured onboard POLAR when the satellite is on the night side in the central region of the near earth plasmasheet. This signature is usually, but not always, accompanied by a dipolarization of the magnetic field. Another clear indicator of expansion phase onset is a well-defined increase in the z-component of the magnetic field which is indicative of dipolarization on the night side at geostationary orbit. Substorm events for this study were selected using these two indicators. 34 expansion phase onsets were found using the HYDRA instrument and 119 onsets were found using GOES 10 satellite data. For event selection the GSM coordinates of POLAR were constrained as follows: -9 < x < -7, -2 < y < 2, -1 < z < 1 in units of earth radii. The GOES 10 location was subject to the requirement that the satellite was located within 3 hours either side of local midnight. As expected these onset times were found to be closely correlated with the onset of ground-based auroral zone enhanced Pi2 activity and magnetic bays. Multiple onset substorms were distinguished from single onset events by observing the occurrence of one or more additional subsequent Pi2 intensifications and negative bays corresponding to enhancements of the westward electrojet. For several events, when data was available, auroral brightenings at the equatorward edge of discrete arcs as observed by the FUV experiment onboard the IMAGE spacecraft were also found to be closely correlated with not only the initial Pi2 intensification but also with subsequent Pi2 intensifications. The ratio of multiple onset to single onset substorms was found to be 2.3:1. Using Pi2 and IMAGE FUV data it was found that the initial onset of a multiple onset substorm usually corresponds to Pi2 intensifications and auroral brightening signatures at a lower auroral zone latitude than for a single onset event. In addition the time interval between Pi2 intensifications and/or auroral brightenings for the multiple onset events was found to have a range of 8 to 30 min with an average of 19 min. 63 percent of the events selected by increases in the z-component of the magnetic field at the GOES 10 satellite were found while the satellite was located in the pre-midnight sector. The triggering of substorms was also studied using OMNI Solar Wind data. It was found that 54 percent of the substorm events were consistent with external solar wind triggering.
SM43B-1327
On the relationship between the occurrence of substorm injections and interplanetary parameters during the declining phase of solar cycle 23
We have examined the relationship between magnetospheric substorms identified by LANL particle injections and daily interplanetary parameters observed by ACE and Geotail during the second half (from July to December) of 2003, which is the declining phase of solar cycle 23. From a statistical study of the relationship between substorms and interplanetary parameters, the following observational results are obtained: (1) Substorm injection occurrence is very well associated with high-speed stream geomagnetic activity and the correlation coefficient between daily substorm injection occurrence and daily median solar wind speed is ~0.7, implying that solar wind speed itself strongly modulates substorm injection; (2) The average of negative IMF Bz is not responsible for the increase in injections with solar wind speed; and (3) There is the evidence that IMF Bz triggering might be important to substorm injection occurrence. In addition, we tested if the substorms in our study are triggered with several types of northward triggering criteria (growth phase duration time, average Bz during the growth phase, and increase value of Bz after northward turning). We found that the correlation coefficients between the tested IMF Bz triggers and substorm injection occurrence range from 0.60 to 0.80, implying that the tested types of northward turning are responsible for a large fraction of substorms and thus are a significant contributor to the increase in onsets with increasing solar wind speed.
SM43B-1328
Relation of Substorm Onset to Local AL index
In this study we investigate the relation of auroral substorm onset to the sharp decrease in the local AL (IL) index during substorms. With a database of over 4200 onsets determined from auroral images we have statistically examined the timing between the auroral substorm onset and the sharp decrease in the IL index, as determined with data from the IMAGE magnetometer network. From the database of onsets, 58 substorms were determined to be within 6° of the central meridian of the IMAGE ground array. Our superposed epoch median curve shows that IL index begins to sharply decrease 4 minutes before the auroral onset, which is twice the 2 minute resolution of the auroral imager. An analysis of the superposed epoch median curves of the SOPA particle data for the LANL spacecraft closest to the auroral onset meridian indicates that both the electron and ion injections begin about 3 to 4 minutes before the auroral onset. The location of the electron injection seems to be in the 01 to 04 MLT sector while the proton injection appears to be in the 22 to 01 MLT sector. These statistical results support the idea that dispersionless particle injection and field aligned and ionospheric currents of the substorm current wedge begin to flow before the auroral onset. http://www.igpp.ucla.edu/jweygand
SM43B-1329
Variations of Substorm Recovery Time Scales
Previous work [Chua et al., 2004] computed the substorm recovery time scale for over three hundred substorms observed by the Polar Ultraviolet Imager (UVI). When sorted according to season, the substorm recovery times were well ordered by whether or not the nightside auroral region was sunlit: substorms occurring in the winter and equinox periods had similar recovery time scales which were both roughly a factor of two longer than that for summer when the auroral oval was sunlit. These results strongly suggest that simultaneous auroral intensifications in the northern and southern hemispheres develop differently during solstice conditions. We expect the auroral breakup in the dark (winter) hemisphere to be more intense and longer lived than that observed in the sunlit (summer) hemisphere. This also implies that more energy is deposited by electron precipitation in the winter hemisphere than in the summer one during a substorm. Here we extend this previous work by including a similar number of substorms observed by IMAGE Far Ultraviolet Imager (FUV) as well as simultaneous, conjugate auroral substorm observations by Polar UVI and the IMAGE FUV. The observed hemispheric asymmetry and non-conjugacy of auroral substorms is consistent with the suppression of discrete aurora in sunlight and highlights the importance of ionospheric conductivity plays in global-scale dynamics of the aurora and in magnetosphere-ionosphere coupling. http://sprg.ssl.berkeley.edu/matt/AGU2007/