1,874 research outputs found
Variations in p-mode Parameters and Sub-surface Flows of Active Regions with Flare Activity
We examine the characteristic properties of photospheric p-modes and
sub-photospheric flows of active regions (ARs) observed during the period of
26-31 October 2003. Using ring diagram analysis of Doppler velocity data
obtained from the Global Oscillations Network Group (GONG), we have found that
p-mode parameters evolve with ARs and show a strong association with flare
activity. Sub-photospheric flows, derived using inversions of p-modes, show
strong twist at the locations of ARs, and large variation with flare activity.Comment: 7 pages, 3 figures, to be appear in the ASI Conference Serie
Activity Related Variations of High-Degree p-Mode Amplitude, Width and Energy in Solar Active Regions
We study the properties of high degree p-mode oscillations in flaring and
dormant ARs and compare them with those in corresponding quiet regions (QRs) to
find the association of mode parameters with magnetic and flare related
activities. Our analysis of several flaring and dormant ARs, showed strong
association of mode amplitude, width and energy with magnetic and flare
activities although their changes are combined effects of foreshortening,
filling factor, magnetic activity, flare activity, and measurement
uncertainties. We find that the largest decrease in mode amplitude and
background power of an AR are caused by the angular distance of the AR from the
solar disc centre. After correcting the mode parameters for foreshortening and
filling factor, we find that the mode amplitude of flaring and dormant ARs are
smaller than in corresponding QRs, and decreases with increasing MAI suggesting
a larger mode power suppression in ARs with stronger magnetic fields. The mode
widths in ARs are larger than in corresponding QRs and increase with MAI,
indicating shorter lifetimes of modes in ARs than in QRs. The largest decrease
(increase) in mode amplitude (mode width) of dormant ARs is found in the five
minute frequency band. The average mode energy of both the flaring and dormant
ARs are smaller than in their corresponding QRs, decreasing with increasing
MAI. But the average mode energy decrease rate in flaring ARs are smaller than
in dormant ARs. Also, the increase in mode width rate in dormant (flaring) ARs
is followed by decrease (increase) in amplitude variation rate. Furthermore,
inclusion of mode corrections for MAI shows that mode amplitude and mode energy
of flaring ARs increase with FI while mode width shows an opposite trend. The
increase (decrease) in mode amplitude (width) is larger in the five minute and
higher frequency band. Increase in width variation rate is ..
Enhanced conduction band density of states in intermetallic EuTSi (T=Rh, Ir)
We report on the physical properties of single crystalline EuRhSi and
polycrystalline EuIrSi, inferred from magnetisation, electrical transport,
heat capacity and Eu M\"ossbauer spectroscopy. These previously known
compounds crystallise in the tetragonal BaNiSn-type structure. The single
crystal magnetisation in EuRhSi has a strongly anisotropic behaviour at 2 K
with a spin-flop field of 13 T, and we present a model of these magnetic
properties which allows the exchange constants to be determined. In both
compounds, specific heat shows the presence of a cascade of two close
transitions near 50 K, and the Eu M\"ossbauer spectra demonstrate that
the intermediate phase has an incommensurate amplitude modulated structure. We
find anomalously large values, with respect to other members of the series, for
the RKKY N\'eel temperature, for the spin-flop field (13 T), for the spin-wave
gap ( 20-25 K) inferred from both resistivity and specific heat data,
for the spin-disorder resistivity in EuRhSi ( Ohm.cm) and
for the saturated hyperfine field (52 T). We show that all these quantities
depend on the electronic density of states at the Fermi level, implying that
the latter must be strongly enhanced in these two materials. EuIrSi
exhibits a giant magnetoresistance ratio, with values exceeding 600 % at 2 K in
a field of 14 T.Comment: 6 pages, 8 figure
- …