258 research outputs found
Giant negative magnetoresistance of spin polarons in magnetic semiconductors–chromium-doped Ti2O3 thin films
Epitaxial Cr-doped Ti2O3 films show giant negative magnetoresistance up to –365% at 2 K. The resistivity of the doped samples follows the behavior expected of spin (magnetic) polarons at low temperature. Namely, rho= rho0 exp(T0/T)p, where p = 0.5 in zero field. A large applied field quenches the spin polarons and p is reduced to 0.25 expected for lattice polarons. The formation of spin polarons is an indication of strong exchange coupling between the magnetic ions and holes in the system
Why torus-unstable solar filaments experience failed eruption?
To investigate the factors that control the success and/or failure of solar
eruptions, we study the magnetic field and 3-Dimensional (3D) configuration of
16 filament eruptions during 2010 July - 2013 February. All these events, i.e.,
erupted but failed to be ejected to become a coronal mass ejection (CME), are
failed eruptions with the filament maximum height exceeding . The
magnetic field of filament source regions is approximated by a potential field
extrapolation method. The filament 3D configuration is reconstructed from three
vantage points by the observations of STEREO Ahead/Behind and SDO spacecraft.
We calculate the decay index at the apex of these failed filaments and find
that in 7 cases, their apex decay indexes exceed the theoretical threshold
() of the torus instability. We further determine the
orientation change or rotation angle of each filament top during the eruption.
Finally, the distribution of these events in the parameter space of rotation
angle versus decay index is established. Four distinct regimes in the parameter
space are empirically identified. We find that, all the torus-unstable cases
(decay index ), have a large rotation angles ranging from . The possible mechanisms leading to the rotation and failed eruption
are discussed. These results imply that, besides the torus instability, the
rotation motion during the eruption may also play a significant role in solar
eruptions
The Initiation Mechanism of the First On-disk X-Class Flare of Solar Cycle 25
In this paper we study the initiation mechanism of the first on-disk X-class
eruptive flare in solar cycle 25. Coronal magnetic field reconstructions reveal
a magnetic flux rope (MFR) with configuration highly consistent with a filament
existing for a long period before the flare, and the eruption of the whole
filament indicates that the MFR erupted during the flare. However, quantitative
analysis shows that the pre-flare MFR resides in a height too low to trigger a
torus instability (TI). The filament experienced a slow rise before the flare
onset, for which we estimate evolution of the filament height using a
triangulation method by combining the SDO and STEREO observations, and find it
is also much lower than the critical height for triggering TI. On the other
hand, the pre-flare evolution of the current density shows progressive thinning
of a vertical current layer on top of the flare PIL, which suggests that a
vertical current sheet forms before the eruption. Meanwhile, there is
continuously shearing motion along the PIL under the main branch of the
filament, which can drive the coronal field to form such a current sheet. As
such, we suggest that the event follows a reconnection-based initiation
mechanism as recently established using a high-accuracy MHD simulation, in
which an eruption is initiated by reconnection in a current sheet that forms
gradually within continuously-sheared magnetic arcade. The eruption should be
further driven by TI as the filament quickly rises into the TI domain during
the eruption
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