SM23B-01 INVITED 13:40h
Steady Magnetospheric Convection: A Review
On occasion, solar wind energy enters Earth's magnetosphere yet the common discrete energy-dissipation events known as magnetospheric substorms fail to occur. During these times, the magnetotail assumes a configuration where earthward of about 12 Re the tail remains in a stretched tail-like state with a thin current sheet similar to the substorm growth phase. At the same time the more distant tail attains a more relaxed configuration with a thick plasma sheet, weak lobe field and enhanced northward Bz, similar to the substorm recovery phase. Simultaneously, (1) auroral zone currents remain strong and assume a two cell DP 2 convection pattern; (2) the auroral oval is wide and optically active, particularly at its poleward and equatorward edges; (3) polar cap area remains constant and energetic particle boundaries are stable, (4) earthward plasma flow persists near the center of the tail as implied by the name steady magnetospheric convection (SMC) except that it occurs on a time scale of minutes and the flow remains bursty. These small scale flows in the tail correspond to auroral streamers that form near the poleward boundary of the oval and propagate equatorward in a few minutes time. Although SMC events have some substorm-like characteristics, such as Pi2's, particle injections and region 1-type field aligned currents with their associated westward ionospheric currents, such phenomena occur on much shorter time and spatial scales and with much smaller amplitudes than actual substorms. Modeling the global magnetic field for several specific SMC events suggest that a minimum in the equatorial tail field Bz magnitude exists near 12 Re which may correspond to the one known equilibrium field configuration that can avoid the pressure catastrophe that may correspond to substorms. This unique field configuration may permit the return of magnetic flux to the dayside that allows the persistence of the steady state field configuration.
SM23B-02 14:02h
State of the Magnetotail: Steady and Bursty Magnetospheric Convection
Geotail and Cluster observations are used to examine the state of the Earth's magnetotail. In this paper we are particularly interested about the plasma sheet convection and the tail "stress level". Tail static pressure (magnetic + thermal pressure) is used to characterize different convection modes, and earthward bulk velocity is used to identify burstyness of the plasma sheet. Four basic convection modes are identified: loading, unloading, steady magnetospheric convection (SMC) and continuous magnetospheric dissipation (CMD). Bulk ion velocity is used to characterize the nature of the convection in the plasma sheet during each of these states. Bursty bulk flows (BBFs) were found to be basic building blocks of the all the tail states. When solar wind drives magnetosphere over several substorm cycle (> 8 hours) the plasma sheet was observed to be highly active and non-steady. The tail stress level was studied by using magnetic and plasma measurements from four Cluster spacecraft. Tail total current was computed, and x-component of the J cross B was used as a "tail stress index". Examples of slightly and strongly stretched magnetotail will be presented and the validity of the tail stress index will be evaluated.
SM23B-03 14:17h
Steady Magnetospheric Covnection Events as Measured by Polar UVI
Periods of enhanced magnetospheric convection without substorm signatures have been called Steady Magnetospheric Convection events (SMC). We have recently started investigating SMCs under the hypothesis that during these events dayside and nightside reconnection rates will balance. When this occurs the poleward auroral boundary, as measured by Polar UVI images, should remain steady. If the boundary is steady for 3 hours or more and we see no substorm signatures, the reconnection rates are fairly well balanced and we have an SMC. .We have used this methodology to pick out SMC during all seasons. Initial results show that SMCs can occur during many levels of storm time activity but mostly happen during weaker activity (avg. Dst ~-50). Our solar wind and IMF drivers thus far concur with past research with an average IMF Bz of -5 nT. We have also found various degrees of balance between the nightside and dayside reconnection rates, in that some of our events show a slow loading of the tail, up to 8 hours, before a substorm occurs. One of our most interesting results is that not all of our SMCs start substorms. We will look at the differences in the solar wind/IMF drivers and the initial conditions of the magnetosphere that lead up the initiation of the substorm and non-substorm SMCs.
SM23B-04 INVITED 14:32h
Global Sawtooth Oscillations of the Magnetosphere
During geomagnetic storms, geosynchronous-orbit satellites observe that the magnetosphere of the Earth will sometimes undergo global sawtooth oscillations. Global sawtoth oscillations are characterized by a periodic stretching and collapse (crash) of the dipole magnetic field at nearly all local times and by a periodic decline and rapid recovery of energetic-particle fluxes at all local times. The rapid recovery of the fluxes can be dispersionless over a very large range of local times. During sawtooth oscillations, the stretching of the field lines can be so great that the lobe is encountered at the equator at geosynchronous orbit. At the time of sawtooth crash, there can be a jump in the magnitude of the Dst index. Statistically, the solar wind that drives sawtooth oscillations has a low number density and a strong southward IMF, with normal to fast speeds, and with low-levels of upstream MHD turbulence. This "sawtooth solar wind" is a low-Mach-number wind that creates an unusual magnetosheath flow with extremely low beta values.
SM23B-05 14:54h
Sawtooth Events During Steadily Depressed Dst
A series of sawtooth oscillations were observed during a magnetic cloud event on October 21-22, 2001. The storm main phase was driven by the cloud sheath region behind the interplanetary shock. The cloud proper had IMF Bz close to zero at its leading edge and negative at the trailing edge. The quasi-steady oscillations causing sawtooth-like behavior in the particle injections were observed during the negative IMF Bz period at the trailing edge of the cloud. The particle injections were recorded at multiple locations by the LANL spacecraft, and they were associted with strong dipolarizations of the nightside magnetic field, as recorded by the Polar spacecraft at 8 Re in the premidnight sector. During that time, ground-based observations showed quasi-continuous electrojet activity at a level exceeding 1000 nT in the AL index. The Dst index stayed at about -150 nT, showing no further enhancement even though the IMF Bz was negative, indicating that there was a rough balance of the energy input and dissipation processes. On the other hand, each of the sawtooth oscillations were seen as a positive deflection in the SYM-H index and a strong enhancement in the ASY-H index. These factors indicate that the sawtooth events were associated with field dipolarizations together with strongly asymmetric ring current formation. We discuss the global solar wind - magnetosphere - ionosphere coupling and the details of magnetospheric and ionospheric processes during the sawtooth events during this storm.
SM23B-06 15:09h
Periodic magnetospheric substorms and the effect of solar wind
Periodic magnetospheric substorms can last for 6-10 cycles with a nearly constant period of ~3 hours. The characteristics of the substorms include periodic magnetic reconnection onsets in the near tail between X = -20 and -30 Re, sawtooth-like injections of energetic charged plasma fluxes from the tail into the inner magnetosphere, periodic intensifications of the auroral electrojet, and other periodic variations in the magnetosphere and ionosphere. An outstanding problem is whether the period of the substorms is determined by the solar wind or by the magnetosphere. We will show that periodic substorms can occur when the IMF is continuously southward or fluctuating between southward and northward. The period of the substorms during fluctuating IMF is nearly the same as that during continuously southward IMF. The substorms have a narrow spectrum with a strong peak at periods of ~3 hours, while the solar wind has a broad spectrum and does not have the same peak as the substorms. Large sudden changes in the solar wind (northward IMF turning or solar wind pressure impulses) after an interval of southward IMF do not necessarily trigger substorm onsets. Periodic variations in some geomagnetic indices (such as Dst and polar cap index) during periodic substorms are caused by the substorms but not by solar wind pressure fluctuations. The observations show that the period of substorms is not controlled either directly by fluctuations in the solar wind or indirectly by the energy transferred to the magnetosphere from the solar wind. We suggest that a sudden change in the solar wind can trigger a substorm onset if and only if the magnetosphere has reached a state conducive to the generation of substorms. Substorms will occur every ~3 hours, no matter whether the IMF is continuously southward or fluctuating between southward and northward. The period of the substorms is determined by some internal processes of the magnetosphere.
SM23B-07 15:24h
Investigation of Sawtooth activity duing the Halloween Storms of 2003
We present here a simulation based study of possible sawtooth activity during the magnetic storms during the periods of October 29 to November 1, 2003. Our initial simulation of this period using the Lyon-Fedder-Mobarry (LFM) global MHD code showed indications of sawtooth activity after 1800 of October 29, consistent with observations from geosynchronous orbit. We present here the results of new simulations we have performed to address possible problems of our initial simulation due to uncertainties in the solar wind observations and relatively low spatial resolution used. We have used a synthesis of the ACE and GEOTAIL plasma data, we have using the Lyon-Fedder-Mobarry (LFM) global MHD code to simulate both the magnetosphere and ionosphere, with progressively larger grid resolutions. We also have performed a simulation using the CMIT model, in which the ionosphere of the LFM is replaced by the NCAR Thermosphere Ionosphere Nested Grid (TING). We will investigate closely the plasma and magnetic field configurations in the inner magnetosphere for all the simulations to identify the presence of sawtooth activity. We will compare our results with the available magnetospheric and ionospheric observations, in particular observations from geosynchronous satellites. [This work is supported by NSF grant ATM-0120950].