Magnetic Reconnection: Theory and Observations IV
Presiding: T G Forbes, University of New Hampshire; J T Gosling, Los Alamos National Laboratory
SM34A-01 15:30h
Computing the reconnection rate at the Earth's magnetopause using two spacecraft observations
A new multi-spacecraft technique is introduced which, under some restrictive assumptions and conditions, provides a snapshot of the reconnection inflow velocity into the magnetosphere and an estimate of the distance from the spacecraft to the reconnection site. The two quantities are not obtained independent of one another and additional, independent information is needed to separate them. This new technique is applied to Cluster spacecraft observations at the Earth's magnetopause. Additional Cluster observations and observations from the IMAGE spacecraft are used as independent information to provide an estimate of the distance from the spacecraft to the reconnection site for the event. From this distance estimate and the new multi-spacecraft technique, it is concluded that component reconnection was probably occurring at the magnetopause and that the local inflow velocity was significantly less than 0.1 VA.
SM34A-02 15:45h
The Location of the reconnection line for southward IMF
One of the major outstanding questions about magnetic reconnection is where reconnection will occur at the magnetopause. There are two scenarios discussed in the literature, a) anti-parallel reconnection where shear angles between the magnetospheric field and the IMF are near 180 degrees, and b) component reconnection where shear angles are as low as 50 degrees. One popular component reconnection model is the tilted neutral line model. Both reconnection scenarios have a profound impact on the location of the X-line and plasma transfer into the magnetosphere. We have analyzed about 80 northern cusp crossings by the Polar satellite during southward IMF conditions. Measurements from the Toroidal Imaging Mass-Angle Spectrograph (TIMAS) onboard the Polar spacecraft are used to estimate the distance to the reconnection line by using the low-velocity cutoffs of the precipitating and mirrored magnetosheath populations in the cusp. Our analysis revealed that the occurrence of anti-parallel or tilted X-line (component) reconnection depends on the clock angle of the IMF. The reconnection lines are located at the anti-parallel reconnection sites for southward IMF conditions and small angles of the IMF By component. However, an increase in the By component increases the probability to encountner a tilted X-line.
SM34A-03 16:00h
Dual Lobe Reconnection During Northward IMF
Cluster and Polar high latitude observations provide evidence that dual lobe reconnection during northward IMF is common, and that the magnetopause boundary layers are primarily on closed magnetic field lines at these times. This contrasts with the open MSBL/LLBL model of Fuselier et al. (JGR, 1995) which used low latitude observations to deduce the structure of the magnetopause. The primary difficulty with the low latitude observations is the unambiguous identification of the magnetopause due to the lack of strong magnetic shear during northward IMF. The open MSBL/LLBL model also requires some unknown mechanism to heat the electrons at the magnetopause even during low shear conditions that fail to heat the ions. This paper presents an alternative model of the low shear magnetopause. Dual lobe reconnection is favored because lobe reconnection requires a plasma depletion layer so the magnetosheath flow is sub-Alfvenic. The depletion layer initially forms at the sub-solar region after the field line has draped, and spreads toward both lobes simultaneously. Sub-solar draping assures that the magnetic shear in both lobes is about the same leading to nearly identical conditions in both hemispheres. Reconnection will not be simultaneous, but is favored to occur in both hemispheres due to similar conditions. Cluster observations will be presented to support this model.
SM34A-04 16:15h
Near Simultaneous Observations of the Magnetopause by the Polar and Cluster Satellites
On 2 May 2003 from 00:00 - 02:30 UT, both the Cluster and Polar satellites encountered the magnetopause around 9:00 MLT. At this time, the Cluster satellites were in the mid-latitude northern hemisphere while Polar was in the southern hemisphere near the equator. Both the cold ion and the electric field data gathered by instruments on Polar revealed strong oscillations with a period of 4.5 minutes. The four Cluster spacecraft also observed the oscillating magnetopause as evidenced by their multiple encounters with the magnetopause. In the meantime, FTEs (flux transfer events) were observed almost simultaneously by electric and magnetic field probes on both Polar and Cluster. This event not only provides an excellent opportunity to study the motion, structure, fields and particle properties of the magnetopause at a large scale, but also helps us to better understand the global properties of FTEs. Data from multi-instruments of Polar and Cluster are utilized for the detailed analysis of this event and will be shown in this presentation.
SM34A-05 16:30h
Multiple Observations of Electron Diffusion Regions by the Electric Field Experiment on the Polar Satellite
During dayside magnetopause crossings by the Polar Satellite in 2001-2003, hundreds of observations were made of ~100 millisecond duration, ~50 mV/m electric fields having parallel components. These events existed in current filaments that contained important density variations and major conversions of electromagnetic energy. They are interpreted as multiple electron diffusion regions existing simultaneously inside the magnetopause. Statistical properties of these events are described.
SM34A-06 16:45h
Electron Demagnetization and Collisionless Magnetic Reconnection in β e ≪ 1 Plasmas: Theory and Observations
Abrupt, intense bipolar and unipolar electric spikes with E > 100mV/m surveyed over 3 years of Polar data (Mozer et al 2005) have been organized to answer the limited question: can they be involved in the local demagnetization of thermal electrons? We determine a lower bound on the electric strength sufficient to cause non-gyrotropic effects on the electron pressure tensor of the form E>E*=B{we}/{8cℑ}, where B is the ambient magnetic field strength, we=√2kTe/me, c is the speed of light, and ℑ is an electron velocity space weighted average displacement along the electric field while transiting the layer (assumed localized with a scale Δ x=a ρe, where ρe is the electron thermal gyroradius). The variation of ℑ as a function of a for equal mean energy Maxwellian and more typical κ distributions seen in the Earth's magnetosphere provides strong evidence that the surveyed electric field spikes are generally smaller than E* (assuming ≈ 1), although 23% (n=57) exceed E* . Only 11% (n=6) of the bipolar class exceed E* ; the frequency of occurrence distribution for the bipolar class of spikes is peaked at 0.1E*. The unipolar occurrence is flat below E*, but has a significant 26% subgroup (n=51) that exceed E* . While E* does not depend on the ambient density, the occurrence distribution of all demagnetizingevents is well organized by the ratio ℜ=λDe/ρe=Ωce/‰pe, residing almost exclusively in the regime ℜ <1. Spikes with E < E* generally occur with ℜ >1 . All the electrostatic spike events surveyed occur in the regime 10-8≤β e≤3×10-2. The demagnetizing events of either class occupy the more restricted low beta regime 10-3≤β e≤3×10-2. Because these demagnetizing events occur in β e ≪ 1 they would not, however, be considered unmagnetized at current channels as thin as the electron skin depth, de, since for such current channels ρe ≡ β e-1/2de ≪ de. As a group the subset of unipolar events with E > E* are consistently understood as sites where the electron pressure tensor could become deformed from cylindrical symmetry by electric field enhancement in layers with scale sizes up to the local thermal electron's gyroradius. Such a deformation is critical for a viable mechanism that supports collisionless reconnection. After selecting events as demagnetizing based on the size of the relevant forces and work done, the geophysical locale of their detection has been investigated. Previously, all E spikes in this survey were found near the invariant latitudes Λ of the earth's magnetic cusps but at all magnetic local times. The demagnetizing events identified here via E* are strongly organized at magnetic local noon (with a secondary, much shallower maximum at local magnetic midnight), occur preferentially at orbit apogee, and without significant preference for the magnetic latitude of the spacecraft. These geophysical organizations are consistent with the demagnetizing E spikes as indices of ongoing, collisionless reconnection in low β e regimes at the earth's subsolar magnetopause. The identification of this sub-class of electric spikes at low β e with E>E* widens the observed venues in the E and B fields where topology changing departures from ideal MHD should be anticipated in collisionless astrophysical plasmas.