Whistlers as a diagnostic technique in the lower ionosphere

We have taken a plasma model of ionosphere which is quite different from the earlier model employed by several workers to study the whistler as a diagnostic technique in the ionosphere. In contrast to the earlier model, we have considered an ionosphere that includes negative ionic species and due to which the mathematical technique loses the basic assumptions chosen earlier to derive the group travel time. We have shown that the negative ion has a significant role and without its contribution, the method to diagnose the ionospheric parameters may lead to an error. We have discussed also how to uset()= ₀ ^h dh/v _g as a diagnostic tool in determining the ionospheric parameters.     

Characteristics of damping of the pulses in the Sun

This paper is a sequel to our earlier paper on the mathematical modelling in determining the rotational frequency and the density of an ionized medium. The technique is based on the measurement of the group travel time for a wave propagating in a rotating ionized medium and finally a simple approximate formula determines the rotation and the density of the medium. The present paper calculates the damping of the pulse-waves in the rotating Sun and leads to a mathematical development to estimate more physical parameters of the solar system.     

Thermal effect on the pulses of the rotating Sun

In this paper, a theoretical investigation is undertaken of the group travel-time for the Sun's pulses to travel from the source of the pulse to the solar surface. For mathematical simplicity, we consider a simple ionized model of the Sun that includes the thermal effect and rotation with uniform angular velocity. The expression for the group travel time gives two terms: the term arising from the linear theory which varies inversely to (2-)^1/2 and the term introduced by the thermal effect which is inversely proportional to (2-)^7/2. The thermal effect variation has been shown and an estimation of the temperature of the medium has been made. Furthermore, the velocity distribution and the amplitude of the magnetic field of the wave, arising from the damping of ionized particles, have been calculated.     

The Sun as a system of elementary particles

No unusual observation of the Sun is described here. No new theory is proposed. The paper — based on known facts of solar physics — is a modest attempt to interpret the Sun as a selfgravitating system of about 10⁵⁷ nucleons and electrons. These elementary particles are endowed with strong, electromagnetic, weak and gravitational interactions. Origin of the Sun, its evolution, structure and physiology are consequences of the four interactions.     

Sun as a star: Rotation rates from the Ca K-index

Daily measurements for 18 months made at Tucson of the Ca K-index in integrated sunlight have been used to derive solar rotation rates. Power spectral analysis shows that one can obtain a well defined value of solar rotation rate in a period of six months, provided the data are fairly continuous. One might suppose it is possible to study the variation of solar (or stellar) rotation rate with time, a variation arising from a combination of differential rotation and shifts in the active latitude. A comparison between our observed rates and the prevailing dominant activity zones does not support this supposition, at least for the interval studied. Rather, our rates seem to depend on the circumstances of sampling and active region birth and decay.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

The eruption of a small filament in the quiet Sun

We analyzed multi-wavelength observations of the eruption of a small-scale filament on the quiet Sun. The filament first became thicker, then broke into two, and eventually underwent a partial eruption with possible rotating motion. The eruption was followed by a small flare with three bright kernels on either side of the eruptive section in Hα and a small coronal dimming near one end of this section in EUV and soft X-ray. On the photosphere, MDI magnetograms show the flux emergence or motions and cancellation between opposite polarities before and during the filament eruption. We find that this small-scale filament shows the similar characteristics as the previous findings in the large-scale filament eruptions on the multi-wavelength, indicating the common nature.     

Short-periodic oscillations of the magnetic field of the Sun as a star

lCorrelation analysis applied to recordings of the magnetic field and velocity of the Sun as a star reveals oscillations close to 300 s. The power spectrum of these oscillations is discussed.     

DynaMICCS: The Sun as an Integrated Magnetic System

After the observations of SoHO, there is a real need to pursue global solar observations from the core to the corona and to put new constraints on the dynamical processes which act inside and outside the Sun. The DynaMICCS mission proposed to ESA in the framework of COSMIC VISION 2015–2025 has this objective. We concentrate here on the motivations to look for the magnetism of the solar radiative zone.     

Helioseismology as a diagnostic of the solar interior

Helioseismology has given us a unique window into the solar interior. Helioseismic data have enabled us to study the internal structure and dynamics with unprecedented detail. This has also allowed us to use the Sun as a laboratory to study the basic properties of stellar matter. We describe how helioseismology is used to determine solar structure and what we have learned about the Sun so far. We also describe how knowledge of the solar structure can be used to constrain the physics inputs.     

The multiple modes of interaction of the solar wind with a comet as it approaches the Sun

A quasi-steady 1-D hydrodynamic model, with mass addition, has been used to study the various modes of interaction of the solar wind with a medium-bright, H₂O-dominated comet (such as P/Halley) approaching the Sun.At large heliocentric distances (d 5 AU) the solar wind penetrates unimpeded on to the surface. As the comet moves further in, mass loading of the solar wind by heavy ions from the fledgling cometary atmosphere causes it to slow down, thereby causing a significant enhancement of the interplanetary field. Still further in at d 3.14 AU, as the mass loading reaches a critical value, a collision-less standing shock is formed in the solar wind upstream of the nucleus. As d decreases further, the distance of this shock from the nucleus increases. The cometary atmosphere becomes dense enough to stand off the solar wind ahead of the nuclear surface and form a well defined tangential discontinuity surface (or ionopause) only when d reaches the value 2.65 AU. When d 2.65 AU an inner shock could, in principle, also form within the cometary ionosphere, although its existence would depend on the detailed thermodynamics of the cometary ionosphere. Resolution of this question is beyond the scope of the present analysis.The conclusions of the present study would be qualitatively valid for other comets having sizes, surface optical properties and chemical compositions, different from those adopted here. The helio-centric distances at which the various transitions take place from the one mode of solar wind interaction to another, would, of course, be different, with all these distances being smaller for less active comets.     

Multiwavelength focusing with the Sun as gravitational lens

The light deviation caused by the gravitational potential in the vicinity of the sun could be used as a means of focussing radiation that cannot be focussed easily otherwise. The gravitational lens formed by the sun is not stigmatic, but does have the advantage of being achromatic and acts identically on all types of mass-less radiations. For a source at infinity, its geometrical characteristics present a “caustic” line starting at 550 astronomical units (UA) downstream from the sun. In a plane perpendicular to that caustic line, images of distant objects are formed.The perturbations by the solar corona plasma will significantly blur electromagnetic radiation for wavelengths longer than those of the IR domain. At shorter wavelengths, for example the γ domain, the focussing process could lead to 10⁸ amplification factors. In order to reach the regions where images are formed, long distance space missions are necessary. Once launched, missions of this type would be dedicated to a single field. Some possible targets are considered, such as Sagitarius A observed in X and γ rays.In this paper we study the point spread function (PSF) of the sun as a gravitational lens. Taking into account perturbations by the planets, the non sphericity of the sun and coronal plasma index, we derive limits within which such observations could be possible.     

The colours of the Sun

We compile a sample of Sun-like stars with accurate effective temperatures, metallicities and colours (from the ultraviolet to the near-infrared). A crucial improvement is that the effective temperature scale of the stars has recently been established as both accurate and precise through direct measurement of angular diameters obtained with stellar interferometers. We fit the colours as a function of effective temperature and metallicity, and derive colour estimates for the Sun in the Johnson–Cousins, Tycho, Strömgren, 2MASS and SDSS photometric systems. For  ( B − V )_⊙  , we favour the 'red' colour 0.64 versus the 'blue' colour 0.62 of other recent papers, but both values are consistent within the errors; we ascribe the difference to the selection of Sun-like stars versus interpolation of wider colour– T _eff–metallicity relations.     

The abundance of chlorine in the Sun

The Cli line 8375.943 (4s ⁴P_5/2 – 4p ⁴D7/2) is identified in the solar spectrum. This is the first identification of a chlorine line in solar spectrum. The measured equivalent width (W = 0.8 mÅ) corresponds to an abundance log N(Cl) = 5.65 on the scale log N(H) = 12.00.     

Chromospheric downflow velocities as a diagnostic in solar flares

We explore the dynamics of chromospheric condensations driven by evaporation during the impulsive phase of solar flares. Specifically, we find that the maximum chromospheric downflow speed obeys the approximate relation υ_d= 0.4 (F/ϱ_ch)^1/3, where F is that part of the flare energy flux driving chromospheric evaporation, and ϱ_ch is the mass density in the preflare chromosphere just below the preflare transition region. This implies that chromospheric downflows as measured by Hα asymmetries may be a powerful probe of flare energetics.

     

Chromospheric downflow velocities as a diagnostic in solar flares

We explore the dynamics of chromospheric condensations driven by evaporation during the impulsive phase of solar flares. Specifically, we find that the maximum chromospheric downflow speed obeys the approximate relation _d = 0.4 (F/ _ch )^1/3, where F is that part of the flare energy flux driving chromospheric evaporation, and _ch is the mass density in the preflare chromosphere just below the preflare transition region. This implies that chromospheric downflows as measured by H asymmetries may be a powerful probe of flare energetics.     

The chemical composition of the Sun

We present a redetermination of the solar abundances of all available elements. The new results have very recently been published by Asplund et al. (Annu. Rev. Astron. Astrophys. 47:481, 2009). The basic ingredients of this work, the main results and some of their implications are summarized hereafter.     

The early despinning of the Sun

It is shown that if the Sun passed through a T Tauri stage, then a mass loss of only 15% would be sufficient to despin the Sun to an angular velocity of 0 (10^–5 rad/sec) at 10⁷ years without the additional braking effect of an enhanced magnetic field. Thus the present Sun could have a core rotating at most ten times faster than its surface.     

The EUV chromospheric network in the quiet Sun

Investigations on the structure and intensity of the chromospheric network from quiet solar regions have been carried out with EUV data obtained from the Harvard spectroheliometer on the Apollo Telescope Mount of Skylab. The distribution of intensities within supergranulation cell interiors follows a near normal function, where the standard deviation exceeds the value expected from the counting rate, which indicates fine-scale structure below the 5 arc sec resolution of the data. The intensities from the centers of supergranulation cells appear to be the same in both quiet regions and coronal holes, although the network is significantly different in the two types of regions. The average halfwidth of the network elements was measured as 10 arc sec, and was independent of the temperature of formation of the observing line for 3.8 < log T _e < 5.8. The contrast between the network and the centers of cells is greatest for lines with log T _e 5.2, where the network contributes approximately 75% of the intensity of quiet solar regions. The contrast and fractional intensity contributions decrease to higher and lower temperatures characteristic of the corona and chromosphere.