505 research outputs found
Nonpotential magnetic fields at sites of gamma ray flares
The relation between the degree of nonpotentiality of photospheric magnetic fields and the occurrence of gamma ray flares is examined. The parameter delta phi (magnetic shear) and the strength of the magnetic field intensity are used as measures of the degree of nonpotentiality, where delta phi is defined as the angular difference between the observed direction of the transverse component of the photospheric field and the direction of the potential field prescribed by the distribution of measured photospheric flux. An analysis of the great flare of April 24 to 25, 1984 is presented as an example of this technique to quantify the nonpotential characteristics of the pre-flare magnetic field. For this flare, which produced a large gamma ray event, strong shear and high field strengths prevailed over an extended length of the magnetic neutral line where the flare occurred. Moreover, the flare began near the area of strongest measured shear (89 to 90 deg). Four other flaring regions were analyzed; one of these produced a moderate gamma ray event while the other three did not produce detectable gamma rays. For all four regions the flares were located in the area where the field was not nonpotential, regardless of the class of flare. The fields of the gamma ray flares were compared with those associated with the flares without gamma rays, and little distinction was found in the degree of magnetic shear. The major difference is seen in the extent of the sheared field: for gamma ray events, the field is sheared over a longer length of the neutral line
Evolution of Currents of Opposite Signs in the Flare Productive Solar Active Region NOAA 10930
Analysis of a time series of high spatial resolution vector magnetograms of
the active region NOAA 10930 available from SOT/SP on-board Hinode revealed
that there is a mixture of upward and downward currents in the two foot-points
of an emerging flux-rope. The flux emergence rate is almost the same in both
the polarities. We observe that along with an increase in magnetic flux, the
net current in each polarity increases initially for about three days after
which it decreases. This net current is characterized by having exactly
opposite signs in each polarities while its magnitude remains almost the same
most of the time. The decrease of net current in both the polarities is due to
the increase of current having a sign opposite to that of the net current. The
dominant current, with same sign as the net current, is seen to increase first
and then decreases during the major X-class flares. Evolution of non-dominant
current appears to be a necessary condition for a flare initiation. The above
observations can have a plausible explanation in terms of the superposition of
two different force-free states resulting in non-zero Lorentz force in the
corona. This Lorentz force then push the coronal plasma and might facilitate
the magnetic reconnection required for flares. Also, the evolution of the net
current is found to follow the evolution of magnetic shear at the polarity
inversion line.Comment: 24 pages, 6 figures, Submitted to Astrophysical Journa
A Search for Vector Magnetic Field Variations Associated with the M-Class Flares of 1991 June 10 in AR 6659
A careful analysis of a 6-hour time sequence of vector magnetograms of AR 6659, observed on 1991 June 10 with the MSFC vector magnetograph, has revealed only minor changes in the vector magnetic field azimuths in the vicinity of two M-class flares, and the association of these changes with the flares is not unambiguous. In this paper we present our analysis of the data which includes comparison of vector magnetograms prior to and during the flares, calculation of distributions of the rms variation of the azimuth at each pixel in the field of view of the active region, and examination of the variation with time of the azimuths at every pixel covered by the main flare emissions as observed with the H-alpha telescope coaligned with the vector magnetograph. We also present results of an analysis of evolutionary changes in the azimuth over the field of view of the active region
Acoustic Power Absorption and its Relation with Vector Magnetic Field of a Sunspot
The distribution of acoustic power over sunspots shows an enhanced absorption
near the umbra--penumbra boundary. Earlier studies revealed that the region of
enhanced absorption coincides with the region of strongest transverse potential
field. The aim of this paper is to (i) utilize the high-resolution vector
magnetograms derived using Hinode SOT/SP observations and study the
relationship between the vector magnetic field and power absorption and (ii)
study the variation of power absorption in sunspot penumbrae due to the
presence of spine-like radial structures. It is found that (i) both potential
and observed transverse fields peak at a similar radial distance from the
center of the sunspot, and (ii) the magnitude of the transverse field, derived
from Hinode observations, is much larger than the potential transverse field
derived from SOHO/MDI longitudinal field observations. In the penumbra, the
radial structures called spines (intra-spines) have stronger (weaker) field
strength and are more vertical (horizontal). The absorption of acoustic power
in the spine and intra-spine shows different behaviour with the absorption
being larger in the spine as compared to the intra-spine.Comment: 18 pages, 7 figures, In Press Solar Physics, Topical Issue on
Helio-and-Astroseismolog
Domination and Edge Domination in Trees
Let G=(V,E) be a simple graph. A set S⊆V is a dominating set if every vertex in V∖S is adjacent to a vertex in S. The domination number of a graph G, denoted by γ(G) is the minimum cardinality of a dominating set of G. A set D⊆E is an edge dominating set if every edge in E∖D is adjacent to an edge in D. The edge domination number of a graph G, denoted by γ′(G) is the minimum cardinality of an edge dominating set of G. We characterize trees with domination number equal to twice edge domination number.This work is supported by TATA-Realty and Infrastructure Limited.The authors thank the anonymous referees for their helpful and constructive comments leading to improvements in the presentation of the paper
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