SM42A-01 10:20h
The geomagnetic seasons in the auroral zone: dayside magnetopause reconnection versus solar illumination
There are three mechanisms that are proposed to explain the geomagnetic seasons. The most widely accepted mechanism is the Russell-McPherron (R-M) effect, which attributes the dependence to seasonal changes in an external driver of geomagnetic activity, the dayside magnetopause reconnection rate. The second mechanism results from a dipole tilt modulation of dayside magnetopause reconnection that depends on season. The third process is based on seasonal changes in solar generated conductivity. Using data from auroral zone magnetometers over a full solar cycle, we show that the R-M effect does not explain any of the seasonal changes in either the amplitude of the auroral zone geomagnetic field or its variability. The seasonal changes are shown to be too large to be caused by the dipole tilt mechanism. The auroral zone magnetometer data in the eastward electrojet region are shown to be consistent with seasonal changes in solar--generated conductivity. In the midnight sector and westward electrojet region, geomagnetic activity variations are consistent with the solar insulation effect except for a hemisphere--dependent Spring--Fall asymmetry.
SM42A-02 10:35h
Semiannual Variation of Geomagnetic Activity: Protons or Photons?
The cause(s) of the semiannual variation (SAV) of geomagnetic activity is a problem of long standard ($\sim$100 years). The various mechanisms put forward can be divided into 'excitations' and 'modulations'. Using 45 years of the am-index, we show that the SAV is a modulation of existing activity. The modulation is a function of the angle between the Earth's dipole moment and either (1) the aberrated solar wind velocity and/or (2) the sun-Earth line, causing both time of year and time of day (UT) variations. Here we examine the correlation of geomagnetic activity with directions (1) and (2). Mechanisms involving interaction between the solar wind and the magnetopause would correlate best with direction (1). Mechanisms involving ionospheric conductance would correlate best with direction (2).
SM42A-03 10:50h
Seasonal Variations of the Intensities of Large-scale Field-aligned Currents
In this study we examine seasonal variations of the intensities of large-scale field-aligned currents (FACs) with a focus on their dependence on the dipole orientation. The study is based on ~185,000 FAC crossings we identified from nearly 19 years_Eworth of magnetic field data from DMSP-F7 and F12-15 spacecraft. It is found that the average dayside FAC intensity is larger in the summer hemisphere than in the winter hemisphere by a factor of 2-3. This is consistent with the results of previous studies, and it can be attributed to the annual variation of the ionospheric conductance. It is also found that the annual variation of the dayside R2 current intensity is smaller in amplitude than that of the dayside R1 current intensity. In contrast, the annual variation of the nightside FAC intensity is complicated, which will be examined in more detail in terms of the ionospheric condition at the footpoints of FACs. It is also inferred from the dependence on the dipole clock angle that the FAC tends to be more intense around the equinoxes than around the solstices, which is inferred to reflect the fact that geomagnetic activity tends to be higher around the equinoxes.
SM42A-04 11:05h
Comparison of Iridium Observations of Birkeland Currents Associated With Two Magnetic Cloud Events With MHD Simulations
We present global observations of Birkeland currents associated with two magnetic clouds passing by the Earth's magnetosphere. During the first event on 19--21 March 2001, the interplanetary magnetic field (IMF) is initially oriented southward with a negative $B_\mathrm{y}$ component. After a sign reversal of $B_\mathrm{y}$, the IMF turns increasingly northward over the course of 36 hours. For the second event on 17--19 August 2003, the IMF is initially directed due northward and subsequently turns strongly southward while sustaining a negative IMF $B_\mathrm{y}$ component. Following a brief interval of positive $B_\mathrm{y}$, the $B_\mathrm{y}$ component turns negative once again, and the IMF transitions beyond its initial northward direction toward positive $B_\mathrm{y}$ within a 24-hour period. The events were specifically chosen because of the slow transitions in the IMF orientation and because of the differences in the IMF rotation between the two events. The transitions are slow enough to derive global maps of Birkeland currents from the 70 satellites of the Iridium constellation. The Birkeland current distributions observed by Iridium show characteristic variations in response to changes in IMF orientation, and the most equatorward latitude of the Birkeland currents expands from $60^\circ$ for northward IMF to $40^\circ$ during southward IMF orientation. MHD simulations, driven by solar wind conditions observed during the magnetic cloud events, exhibit variations similar to the ones observed by Iridium. We compare the Iridium Birkeland current distributions obtained during the two events with the MHD simulation results and examine the temporal development of the Birkeland currents between observations and simulations.
SM42A-05 11:20h
Hemispheric Asymmetries in the Dayside Aurora
Using global auroral images from Polar UVI in the southern hemisphere and IMAGE FUV in the north, we have analyzed the asymmetries in the dayside aurora for a five-month period. The observed asymmetries were related to the interplanetary magnetic field (IMF). Initial results suggest that the direction of the IMF plays a major role in influencing the asymmetrical behavior of the dayside aurora. When the IMF is southward and a significant dawnward component exists, there is an enhancement in the afternoon aurora in the northern hemisphere. If there is a significant duskward component to the IMF, then an enhancement in the afternoon aurora in the southern hemisphere is observed. Additionally, when the ratio of the magnitudes of the Y and Z components of the IMF are greater than about 2, distinct structure (i.e., a string of pearls configuration) is seen in the hemisphere with enhanced emission. These results are in general agreement with models and observations of the IMF influence on ionospheric convection. A strong Y component of the IMF can lead to a strong shear flow at the dayside convection reversal boundary. Strong flow shear can lead to strong field aligned currents (i.e., elelctron precipitation for upward current) in this same region. If the flow shear exceeds some threshold, an instability (e.g., a Kelvin-Helmholtz instability) can develop leading to discrete structures.
http://sprg.ssl.berkeley.edu/matt/AGU04/
SM42A-06 11:35h
The Effects of IMF Orientation and Dipole Tilt on the Configuration of the Magnetosphere
The response of the magnetosphere to changes in the interplanetary magnetic field (IMF) is determined by two factors: The extent to which the interplanetary magnetic field and the geomagnetic field are antiparallel and the relative velocity of the reconnected field lines in the direction perpendicular to the magnetic field. The first factor determines where the reconnection occurs while the second determines how the reconnected field lines move. We have used a three-dimensional global magnetohydrodynamic simulation of the interaction between the solar wind and magnetosphere to carry out a systematic investigation of the effects of the orientation of the IMF and dipole tilt on magnetospheric structure and dynamics. The combination of dipole tilt and finite IMF By and Bz leads to complex structures in the magnetosphere with no symmetry planes. Competition between antiparallel reconnection and the relative velocity of reconnected field lines determines the location of dayside reconnection. It frequently suppresses to occur in the subsolar region because the geomagnetic field is not weakest there. The reconnection maximizes where the fields are antiparallel. The addition of the Bx component to the IMF adds dawn-dusk and north-south asymmetry to the dayside magnetic reconnection. Open field lines on the dusk side are relatively straight. They increase the lobe magnetic pressure and compress the plasma sheet. Open field lines on the dawn side are bent sharply and decrease the lobe magnetic pressure. As a result tail reconnection occurs preferably on the dusk side. This tendency is largest when the solar wind Alfve'n Mach number is low. The dawn-dusk and north-south asymmetries cause an inclined plasma sheet, rotation of the magnetotail and asymmetric plasma flows in the tail.
SM42A-07 11:50h
A Statistical Test of the Assumptions Used to Form a Minimal Substorm Model.
A minimal model for the evolution of the global dynamical state of the magnetotail during the substorm, involving only three simple rules has been developed. The model considers the general state of the magnetospheric system rather than concentrate on the physical nature of the substorm instability. When driven by a real solar wind power input, the minimal substorm model produces a probability distribution of times between substorm onsets that compares favourably with the distribution of 1001 inter-substorm intervals found by Borovsky et al. from observation. In this paper we examine the validity of the assumptions behind the model, specifically that the integrated solar wind input between two contiguous substorm onsets is proportional to the solar wind input at the time of the first substorm. We do this by comparing the integrated solar wind input between pairs of substorm onsets with the solar wind input at the time of the first substorm onset for a set of observed substorm pairs.
SM42A-08 12:05h
The Connection Between Transpolar Arcs and the Recovery Phase of Substorms
Contrasting the well observed and much discussed growth and expansion phases, the recovery phase has received much less attention and little is known about it. Conventional wisdom says that the recovery phase is the relaxation of the magnetosphere back to a quiet auroral configuration. Optically, the recovery phase appears to start when the auroral bulge stops moving poleward but observations indicate that the recovery phase has considerable activity. Measurements have established the basic relationships between auroral recovery phase features and magnetotail regimes such as the double oval phenomenon. Observations obtained using the FUV instruments on the IMAGE satellite show that transpolar arcs (TPA's) are often associated with the recovery phase of substorms. These TPA's are controlled by the interplanetary magnetic field (IMF), in particular the By component. Examples of recovery phases emphasizing the timing of the TPA's compared to onset and the TPA's relationship to the IMF will be the focus of this presentation.