Hemispheric Distribution of Subsurface Kinetic Helicity and Its Variation with Magnetic Activity

We study the hemispheric distribution of the kinetic helicity of subsurface flows in the near-surface layers of the solar convection zone and its variation with magnetic activity. We determine subsurface flows with a ring-diagram analysis applied to Global Oscillation Network Group (GONG) Dopplergrams and Dynamics Program data from the Michelson Doppler Imager (MDI) instrument onboard the Solar and Heliospheric Observatory (SOHO). We determine the average kinetic helicity density as a function of Carrington rotation and latitude. The average kinetic helicity density at all depths and the kinetic helicity, integrated over 2?–?7 Mm, follow the same hemispheric rule as the current/magnetic helicity proxies with predominantly positive values in the southern and negative ones in the northern hemisphere. This holds true for all levels of magnetic activity from quiet to active regions. However, this is a statistical result; only about 55 % of all locations follow the hemispheric rule. But these locations have larger helicity values than those that do not follow the rule. The average values of helicity density increase with depth for all levels of activity, which might reflect an increase of the characteristic size of convective motions with greater depth. The average helicity of subsets of high magnetic activity is about five times larger than that of subsets of low activity. The solar-cycle variation of helicity is thus mainly due to the presence or absence of active regions. During the rising phase of cycle 24, locations of high magnetic activity at low latitudes show a weaker hemispheric behavior compared to the rising phase of cycle 23.     

Global Solar Free Magnetic Energy and Electric Current Density Distribution of Carrington Rotation 2124

Solar eruptive phenomena, like flares and coronal mass ejections (CMEs), are governed by magnetic fields. To describe the structure of these phenomena one needs information on the magnetic flux density and the electric current density vector components in three dimensions throughout the atmosphere. However, current spectro-polarimetric measurements typically limit the determination of the vector magnetic field to only the photosphere. Therefore, there is considerable interest in accurate modeling of the solar coronal magnetic field using photospheric vector magnetograms as boundary data. In this work, we model the coronal magnetic field for global solar atmosphere using nonlinear force-free field (NLFFF) extrapolation codes implemented to a synoptic maps of photospheric vector magnetic field synthesized from the Vector Spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) as boundary condition. Using the resulting three-dimensional magnetic field, we calculate the three-dimensional electric current density and magnetic energy throughout the solar atmosphere for Carrington rotation 2124 using our global extrapolation code. We found that spatially, the low-lying, current-carrying core field demonstrates a strong concentration of free energy in the active-region core, from the photosphere to the lower corona (about 70 Mm). The free energy density appears largely co-spatial with the electric current distribution.     

H α Observations of 8 June, 2004 Venus Transit

The cosmic event of Venus transit across the solar disk occurred on 8 June, 2004. The previous such event was witnessed about 122 years ago on 6 December, 1882. We observed this rare transit in H _α 6563 Å line-center from Udaipur Solar Observatory (USO) using both the full-disk and small field-of-view solar telescopes. In the earlier historical transits, a “black-drop” effect was observed in white light images, during the contact phases. The transit of 8 June, 2004 provided a unique opportunity to observe this effect, for the first time, in H _α. We report that the “black-drop” effect is present in H _α also, as in the white light observations made by the ground-based Global Oscillation Network Group (GONG) instrument and the space-borne Transition Region and Coronal Explorer (TRACE) satellite. We did not observe any noticeable “aureole” (atmospheric glow) around Venus during the ingress or egress phases. We have compared the H _α images with the multi-wavelength data obtained from the TRACE satellite.     

Magnetic Field Structures in a Facular Region Observed by THEMIS and <Emphasis Type="Italic">Hinode</Emphasis>

We study the space – time distributions of intensity fluctuations in 2 – 3 hour sequences of multi-spectral, high-resolution, high-cadence, broad-band filtergram images of the Sun made by the SOT – FG system aboard the Hinode spacecraft. In the frequency range 5.5<f<8.0 mHz both G-band and Ca ii H-line oscillations are suppressed in the presence of magnetic fields, but the suppression disappears for f>10 mHz. By looking at G-band frequencies above 10 mHz we find that the oscillatory power, both at these frequencies and at lower frequencies, lies in a mesh pattern with cell scale 2 – 3 Mm, clearly larger than normal granulation, and with correlation times on the order of hours. The mesh pattern lies in the dark lanes between stable cells found in time-integrated G-band intensity images. It also underlies part of the bright pattern in time-integrated H-line emission. This discovery may reflect dynamical constraints on the sizes of rising granular convection cells together with the turbulence created in strong intercellular downflows.     

A simple procedure for estimating distributed daily evapotranspiration using landsat-TM data

A procedure to estimate distributed daily evapotranspiration (ET) using remotely sensed data is presented. Landsat-TM data for a part of the Western Yamuna Canal command (Haryana) has been used for model application. The model utilizes the surface reflectance in visible, infrared and thermal bands to generate surface albedo, surface temperature and leaf area index and thus surface energy fluxes to determine distributed daily ET. Result reveals a reasonable estimate of distributed daily ET. For well-watered crop, average ET by the proposed model is estimated as 1.8 mm/d, whereas using Penmen-Monteith equation it is calculated as 1.9 mm/d. The error involved in estimating ET by the proposed model is calculated about 5%, which is quite acceptable for most applications. The proposed procedure is also found computationally simple and can also be applied on current Landsat ETM+ data.     

Comparative Study of LiNbO3 and Servo Controlled Air Gap Fabry-Perot Etalons for Solar Application

In this note, we compare the LiNbO₃ and Piezo-Electrically(PE) servo controlled air gap Fabry-Perot etalons for observing the Sun. An identical test setup was used to evaluate the instrumental parameters of the two etalons. It is found that for the etalons with similar finesse the advantage of using PE Etalons is tunability over entire Free Spectral Range. On the other hand, the LiNbO₃ etalons have advantage of wider acceptance angle. This revised version was published online in July 2006 with corrections to the Cover Date.     

High-resolution Observations of H$\alpha$ Spectra with a Subtractive Double Pass

High-resolution imaging spectroscopy in solar physics has relied on Fabry–Pérot interferometers (FPIs) in recent years. FPI systems, however, become technically challenging and expensive for telescopes larger than the 1 m class. A conventional slit spectrograph with a diffraction-limited performance over a large field of view (FOV) can be built at much lower cost and effort. It can be converted into an imaging spectro(polari)meter using the concept of a subtractive double pass (SDP). We demonstrate that an SDP system can reach a similar performance as FPI-based systems with a high spatial and moderate spectral resolution across a FOV of \(100^{\prime\prime} \times100^{\prime \prime}\) with a spectral coverage of 1 nm. We use H\(\alpha\) spectra taken with an SDP system at the Dunn Solar Telescope and complementary full-disc data to infer the properties of small-scale superpenumbral filaments. We find that the majority of all filaments end in patches of opposite-polarity fields. The internal fine-structure in the line-core intensity of H\(\alpha\) at spatial scales of about 0\(.\!\!^{\prime \prime }\)5 exceeds that in other parameters such as the line width, indicating small-scale opacity effects in a larger-scale structure with common properties. We conclude that SDP systems in combination with (multi-conjugate) adaptive optics are a valid alternative to FPI systems when high spatial resolution and a large FOV are required. They can also reach a cadence that is comparable to that of FPI systems, while providing a much larger spectral range and a simultaneous multi-line capability.     

Simultaneous Stokes-V diagnostic of a Sunspot using Mg b and Fe I lines

Simultaneous observations of Stokes profiles in photospheric Fei (630.15 nm and 630.25 nm) and chromospheric Mgi b ₁ and b ₂ (518.4 nm and 517.3 nm) lines over a sunspot are presented. Observations were carried out using the Advanced Stokes Polarimeter of HAO/NSO, VTT, SacPeak, U.S.A. The Stokes-V amplitude asymmetries for these lines are analyzed. The values of amplitude asymmetry in Mgb lines are negative in disk-center-side penumbra while they are positive in limb-side penumbra. This trend is similar in nature to photospheric Fei line observations. Further, the spatial distribution of Stokes-V asymmetry is analyzed using Net Circular Polarization (NCP) maps. The chromospheric and photospheric NCP maps are different in many aspects. These observations with longitudinal magnetic field, estimated using weak field approximation, are discussed in this paper.     

Acoustic Power Absorption and its Relation to Vector Magnetic Field of a Sunspot

The distribution of acoustic power over sunspots shows an enhanced absorption near the umbra – penumbra boundary. Previous 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.