C. R. Clauer

A statistical study of BRIs (SMCs), isolated substorms, and individual sawtooth injections

[1]xa0There are many similarities and differences in the solar wind drivers during three of the main modes of convection in the magnetosphere (isolated substorms, global sawtooth oscillations, and steady magnetospheric convection (SMC) events, which we term here balanced reconnection intervals (BRI)). Thus, this investigation utilizes statistical analysis to compare the solar wind and interplanetary magnetic field (IMF) drivers and their steadiness (standard deviation divided by the mean) during these three different event types. By including the steadiness of the drivers, the importance of magnitude, sign, and stability of the drivers for the different modes can be investigated. A series of histograms with each mode plotted over top of 6 years of background is used to measure the deviation of the mode drivers from the nominal data, and also allows for a comparison between each event type. We found that the magnitude and direction of Bz are the dominate driver for substorms, while BRIs and sawteeth require both magnitude and steadiness of certain drivers to occur. Both BRIs and sawteeth show similar steadiness in their drivers, while the magnitude of the drivers is much stronger for the sawtooth oscillations. Also included in this study are the substorms that initiate BRIs. The solar wind and IMF drivers for the initiating substorms are similar to drivers for BRIs; thus initiating substorms of BRIs are different from isolated substorms and may play a role in preconditioning the magnetosphere for BRIs.

Investigation of the period of sawtooth events

[1]xa0A sawtooth event is identified as a series of energetic particle flux variations, i.e., rapid increases following slow decreases, from geosynchronous satellites observations. The saw blade shape is prominent in the proton flux with energy range between 50 and 400 keV. Though sawtooth events are described as large-amplitude oscillations with periods between 2 and 4 h on the basis of case studies, their period has not been accurately determined by previous studies. In this study, we have identified 111 sawtooth event intervals, which include 438 individual teeth, from January 1998 to December 2007. We find the average period is 179.6 min. However, the period shows large variations from event to event, even from tooth to tooth in the same sawtooth interval. The variability is independent with the length of the period and the number of oscillations, so we conclude that sawtooth events are quasi-periodic events instead of periodic events.

Statistical maps of geomagnetic perturbations as a function of the interplanetary magnetic field

[1]xa0Mappings of geomagnetic perturbations are shown for different combinations of the solar wind velocity, interplanetary magnetic field (IMF), and dipole tilt angle (season). Average maps were derived separately for the northward, eastward, and vertical (downward) components of the geomagnetic disturbances, using spherical cap harmonics in least error fits of sorted measurements. The source data are obtained from 104 ground-based magnetometer stations in the Northern Hemisphere at geomagnetic latitudes over 40° during the years 1998 through 2001. Contour maps of statistical fits are shown along-side scatter plots of individual measurements in corrected geomagnetic apex coordinates. The patterns are consistent with previous mappings of ionospheric electric potential. Interestingly, the vertical component of the magnetic perturbations closely resembles maps of the overhead, field-aligned currents, including the Northward IMF configuration. The maximum and minimum values from the statistical mappings are graphed to show their changes as a function of southward IMF magnitude, solar wind velocity, and seasons. It is expected that this work will lead to better advance predictions of the geomagnetic perturbations that are based on real-time IMF measurements.

The nonlinear response of the polar cap potential under southward IMF: A statistical view

[1]xa0We report the results of an investigation into the effect of solar wind properties on the saturation of the polar cap potential (CPCP) during periods of strongly southward IMF. We use propagated solar wind data to search for periods between 1998 and 2007 when the interplanetary electric field is stable for more than 50 min and placed further conditions on the availability of SuperDARN and DMSP velocity data. CPCP values are calculated from these data sets and various fits of the polar cap potential to the interplanetary electric field (IEF) are compared. It is found that the trend is nonlinear, with a square root function fitting better than a straight line, and that the CPCP does not appear to exhibit asymptotic behavior. The nonlinearity of the CPCP is then correlated with various interplanetary parameters to test the various models of polar cap potential saturation. It is also found that the deviation of the CPCP from a linear fit has statistically significant correlation with solar wind Alfvenic Mach number and no significant correlation with solar wind dynamic pressure.

Polar cap electric field saturation during interplanetary magnetic field Bz north and south conditions

[1] We report the results of an investigation of the saturation of the polar cap electric field during periods of large northward and southward interplanetary magnetic field (IMF). While it has been demonstrated that saturation can occur for both northward and southward IMF, a direct comparison between the two regimes during saturated driving has not been performed. We use solar wind measurements to search for events between 1998 and 2007 when the IMF is stable for 50 min. The selected intervals are binned according to interplanetary electric field (-V sw × B). SuperDARN Doppler radar velocity vectors from high-latitude antisunward looking beams are averaged to determine the approximate polar cap electric field. Results show that sunward convection under northward IMF is stronger in summer than in winter, but that antisunward convection under southward IMF exhibits the opposite seasonal behavior. One explanation is that, as the earth tilts near solstice, lobe reconnection is less effective in the winter hemisphere.

Relationship between sawtooth events and magnetic storms

[1]xa0The relationship between sawtooth events and magnetospheric substorms has been discussed extensively. However, the relationship between sawtooth events and magnetic storms has not been systematically examined. Using the sawtooth event list and magnetic storm list from January 1998 to December 2007, we investigate whether sawtooth events are storm time phenomena and whether there is a dependence on the strength and phase of storms. We have found that most of sawtooth events occur during storm time. Nevertheless, there are still 6 sawtooth events (5.4% of total events) that occur during nonstorm intervals. Sawtooth events also tend to occur during intense storms, with an occurrence rate of 63.5%. Sawtooth events can initiate during any stage of storms, however 55.9% of sawtooth events occur during the storm main phase through the time the ring current reaches its maximum strength. Therefore we conclude that sawtooth events are very often but not necessarily storm time phenomena. And not all storms contain sawtooth events. We suggest most sawtooth events occur during a special subset of storms that have just the right driving conditions to set intense, periodic, near-tail magnetic reconnection bursts.

Magnetospheric sawtooth events during the solar cycle 23

[1]xa0The Earths magnetosphere and ionosphere have a variety of responses to different solar activities. As a result, there are several types of magnetosphere response modes; one of which is termed sawtooth injection events. Currently, it is still unclear whether sawtooth events occurrence has a solar cycle dependence. Partially, this is due to a lack of an event list which covers a full solar cycle. In this research, we have extended our original event list to cover the solar cycle 23, which now includes 126 events from 1996 to 2007. Sawtooth events have been grouped into three categories based on their solar wind drivers: interplanetary coronal mass ejections, stream interaction regions, and others. Then we examine in detail whether sawtooth events occurrence has dependencies on solar cycle and season. We also test a hypothesis that sawtooth events occurrence is related to a threshold of total eroded magnetic flux at the magnetopause regardless of solar wind structures. This paper suggests understanding sawtooth events and other convection modes from solar wind magnetic flux or energy aspect rather than from structure aspect, which is an important idea to understand the complicated magnetosphere dynamics.

The relationship between sawtooth events and O+ in the plasma sheet

In order to study the relationship between sawtooth events and the composition of the plasma sheet, we perform a superposed epoch analysis (SEA) of the O+ concentration inside the near-Earth plasma sheet during sawtooth events and substorms sorted by different geomagnetic storm phases, using Cluster/Composition Distribution Function data. The SEA shows that the O+ content increases during sawtooth growth phase, regardless of storm phase, and reaches 20% around the onset of dipolarization. For storm main phase events, the plasma sheet O+ concentration during sawtooth events is only slightly higher than that observed during substorm events. However, for storm recovery phase and nonstorm time events, there is significantly more O+ within the plasma sheet during sawtooth events than during substorm events. No difference is found in the comparison between the O+/H+ density ratio changes during the first tooth and the subsequent teeth in a series of a sawtooth interval. Hence, there is no evidence to support the hypothesis that due to the higher O+ inside the plasma sheet, subsequent teeth will lead to a closer near-Earth X line and then a wider magnetic local time response. Finally, despite the association between sawtooth events and high O+/H+ ratio, there are times when the O+/H+ density ratio is high in the plasma sheet but no sawtooth event is observed, and there are sawtooth events when the O+/H+ ratio is low. This indicates that enhanced O+ is neither a necessary nor a sufficient condition but is likely one of many factors that play a role in triggering sawtooth events.

Conjugate observations of traveling convection vortices associated with transient events at the magnetopause

Traveling convection vortices (TCVs) are generally produced by field-aligned currents (FACs) at high latitudes associated with transient changes of the magnetopause. This paper presents multipoint conjugate observations of transient events at the magnetopause measured in space and on the ground. The transient events showing radial fluctuation of the magnetopause in association with sudden increases in solar wind dynamic pressure were detected by both the Time History of Events and Macroscale Interactions during Substorms and the Geostationary Operational Environmental Satellite spacecraft. Geomagnetic signatures seen as TCVs in response to the transient events were observed by the ground magnetometer array in Greenland and Canada and their conjugate locations in Antarctica including recently developed Antarctic magnetometers, mostly located along the 40° magnetic meridian. This new conjugate network provides a unique opportunity to observe geomagnetic field signatures over a relatively large region in both hemispheres. This study focuses mainly on the spatial and temporal features of the TCVs in the conjugate hemispheres in relation to the transient events at the magnetopause. The TCV events are characterized by their single or twin vortex, of which the centers are located approximately at 72°–76° magnetic latitude, propagating either dawnward or duskward away from local noon. While interhemispheric conjugacy is expected with an assumption that TCV signatures are created by FACs directed in both hemispheres, our observations suggest that there might be more complex mechanisms contributing the asymmetrical features, perhaps due to field line mapping and/or conductivity differences.

Interhemispheric observations of dayside convection under northward IMF

[1]xa0The reverse convection potential and electric field under northward interplanetary magnetic field (IMF) has been shown to saturate at a larger value in the summer than in the winter. Previous studies of reverse convection under northward IMF have suggested that in the winter hemisphere, much of the reconnection involves “internal reconnection” with over-draped field lines from the summer hemisphere. We present a case study in which reverse convection was simultaneously visible in the northern and southern hemispheres near the northern winter solstice. We use SuperDARN radar velocity measurements to demonstrate that the sunward (reverse) convection along the noon meridian is significantly faster in the summer hemisphere. We also use DMSP energetic particle data to demonstrate that in the winter hemisphere, some of the convection circulates on closed field lines, which is a signature of internal reconnection. These results suggest that internal reconnection is much less effective in transmitting the interplanetary electric field (IEF) into the ionosphere.