220 research outputs found
Flat microwave spectra seen at X-class flares
We report peculiar spectral activity of four large microwave bursts as obtained from the Solar Arrays at the Owens Valley Radio Observatory during observations of X-class flares on 1990 May 24 and 1991 March 7, 8, and 22. Main observational points that we newly uncovered are: (1) flat flux spectra over 1–18 GHz in large amounts of flux ranging from 10² to 10⁴ s.f.u. at the maximum phase, (2) a common evolutionary pattern in which the spectral region of dominant flux shifts from high frequencies at the initial rise to low frequencies at the decaying phase, and (3) unusual time profiles that are impulsive at high frequencies but more extended at lower frequencies.
In an attempt to elucidate these new properties, we carry out the model calculations of microwave spectra under assumptions of gyrosynchrotron mechanism and a dipole field configuration to reproduce the observational characteristics. Our results are summarized as follows. First, a flat microwave spectrum reaching up to 10²–10⁴ s.f.u. may occur in a case where a magnetic loop is extended to an angular size of ∼(0.7–7.0) × 10⁻⁷ sterad and contains a huge number (N(E > 10 keV) ∼ 10³⁶– 10³⁸) of nonthermal electrons with power-law index δ ∼ 3–3.5 over the entire volume. Second, the observed spectral activity could adequately be accounted for by the shrinking of the region of nonthermal electrons to the loop top and by the softening of the power-law spectrum of electrons in a time scale ranging 3–45 min depending on the event. Third, the extended microwave activity at lower frequencies is probably due to electrons trapped in the loop top where magnetic fields are low. Finally, we clarify the physical distinction between these large, extended microwave bursts and the gradual/post-microwave bursts often seen in weak events, both of which are characterized by long-period activity and broadband spectra
Evidence for Solar Tether-cutting Magnetic Reconnection from Coronal Field Extrapolations
Magnetic reconnection is one of the primary mechanisms for triggering solar
eruptive events, but direct observation of its rapid process has been of
challenge. In this Letter we present, using a nonlinear force-free field
(NLFFF) extrapolation technique, a visualization of field line connectivity
changes resulting from tether-cutting reconnection over about 30 minutes during
the 2011 February 13 M6.6 flare in NOAA AR 11158. Evidence for the
tether-cutting reconnection was first collected through multiwavelength
observations and then by the analysis of the field lines traced from positions
of four conspicuous flare 1700 A footpoints observed at the event onset. Right
before the flare, the four footpoints are located very close to the regions of
local maxima of magnetic twist index. Especially, the field lines from the
inner two footpoints form two strongly twisted flux bundles (up to ~1.2 turns),
which shear past each other and reach out close to the outer two footpoints,
respectively. Immediately after the flare, the twist index of regions around
the footpoints greatly diminish and the above field lines become low lying and
less twisted (~0.6 turns), overarched by loops linking the later formed two
flare ribbons. About 10% of the flux (~3x10^19 Mx) from the inner footpoints
has undergone a footpoint exchange. This portion of flux originates from the
edge regions of the inner footpoints that are brightened first. These rapid
changes of magnetic field connectivity inferred from the NLFFF extrapolation
are consistent with the tether-cutting magnetic reconnection model.Comment: 6 pages, 5 figures, accepted to the Astrophysical Journal Letter
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