Remote Sensing of the Heliosphere with the Murchison Widefield Array

The Murchison Widefield Array (MWA) is one of the new technology low frequency radio interferometers currently under construction at an extremely radio-quiet location in Western Australia. The MWA design brings to bear the recent availability of powerful high-speed computational and digital signal processing capabilities on the problem of low frequency high-fidelity imaging with a rapid cadence and high spectral resolution. Solar and heliosphere science are among the key science objectives of the MWA and have guided the array design from its very conception. We present here a brief overview of the design and capabilities of the MWA with emphasis on its suitability for solar physics and remote-sensing of the heliosphere. We discuss the solar imaging and interplanetary scintillation (IPS) science capabilities of the MWA and also describe a new software framework. This software, referred to as Haystack InterPlanetary Software System (HIPSS), aims to provide a common data repository, interface, and analysis tools for IPS data from all observatories across the world.     

Smooth models of overshooting at the base of the solar convective zone

A set of smoothed temperature gradient profiles around overshooting layers at the solar convective zone bottom is considered. In classical local theories of convection the one point defined according to the Schwarzschild criterion is enough to describe a convective boundary. To get a sophisticated picture of the overshooting we use four points to compute the transition overshooting functions. Analyzing the transition gradient profiles we found that the overshooting convective flux may be either positive or negative. A negative overshooting flux appears in nonlocal convective theories and causes a steep temperature gradient profile. But we propose an evenly smoothed gradient which corresponds to a convective flux positive everywhere. To outline the effect of the temperature gradient on the solar oscillations the squared Brunt–Väisälä frequency N ² is calculated. In local convective theories the N ² profile shows the discontinuity of the first derivative at the convective boundary, while all smoothed profiles eliminate the break.     

Oscillatory Response of the 3D Solar Atmosphere to the Leakage of Photospheric Motion

The direct propagation of acoustic waves, driven harmonically at the solar photosphere, into the three-dimensional solar atmosphere is examined numerically in the framework of ideal magnetohydrodynamics. It is of particular interest to study the leakage of 5-minute global solar acoustic oscillations into the upper, gravitationally stratified and magnetised atmosphere, where the modelled solar atmosphere possesses realistic temperature and density stratification. This work aims to complement and bring further into the 3D domain our previous efforts (by Erdélyi et al., 2007, Astron. Astrophys. 467, 1299) on the leakage of photospheric motions and running magnetic-field-aligned waves excited by these global oscillations. The constructed model atmosphere, most suitable perhaps for quiet Sun regions, is a VAL IIIC derivative in which a uniform magnetic field is embedded. The response of the atmosphere to a range of periodic velocity drivers is numerically investigated in the hydrodynamic and magnetohydrodynamic approximations. Among others the following results are discussed in detail: i) High-frequency waves are shown to propagate from the lower atmosphere across the transition region, experiencing relatively low reflection, and transmitting most of their energy into the corona; ii) the thin transition region becomes a wave guide for horizontally propagating surface waves for a wide range of driver periods, and particularly at those periods that support chromospheric standing waves; iii) the magnetic field acts as a waveguide for both high- and low-frequency waves originating from the photosphere and propagating through the transition region into the solar corona. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

The Recovery of CME-Related Dimmings and the ICME’s Enduring Magnetic Connection to the Sun

It is generally accepted that transient coronal holes (TCHs, dimmings) correspond to the magnetic footpoints of CMEs that remain rooted in the Sun as the CME expands out into the interplanetary space. However, the observation that the average intensity of the 12 May 1997 dimmings recover to their pre-eruption intensity in SOHO/EIT data within 48 hours, whilst suprathermal unidirectional electron heat fluxes are observed at 1 AU in the related ICME more than 70 hours after the eruption, leads us to question why and how the dimmings disappear whilst the magnetic connectivity is maintained. We also examine two other CME-related dimming events: 13 May 2005 and 6 July 2006. We study the morphology of the dimmings and how they recover. We find that, far from exhibiting a uniform intensity, dimmings observed in SOHO/EIT data have a deep central core and a more shallow extended dimming area. The dimmings recover not only by shrinking of their outer boundaries but also by internal brightenings. We quantitatively demonstrate that the model developed by Fisk and Schwadron (Astrophys. J. 560, 425, 2001) of interchange reconnections between “open” magnetic field and small coronal loops is a strong candidate for the mechanism facilitating the recovery of the dimmings. This process disperses the concentration of  “open” magnetic field (forming the dimming) out into the surrounding quiet Sun, thus recovering the intensity of the dimmings whilst still maintaining the magnetic connectivity to the Sun. Electronic Supplementary Material  The online version of this article () contains supplementary material, which is available to authorized users.     

Continuous monitoring of comet Holmes from before the 2007 Outburst

The outburst and subsequent brightness evolution of comet 17P/Holmes has been observed using the MMT Observatory’s All-Sky Camera (Pickering 2006) on Mt. Hopkins near Tucson, Arizona, USA. The comet was picked up at the limiting visual magnitude of 5.5 on October 24.38 and tracked by the camera continuously until sunrise four hours later. During this time the comet brightened to visual magnitude 3.5. Comet Holmes was next observed just after sunset on October 25.23 at visual magnitude 2.5, where it remained approximately constant over the next three days. The comet then began to dim slowly and was followed into the early months of 2008 with periods of dense time coverage.     

High-Frequency Evolving Emission Lines for the 25 August 1999 Solar Flare

We analyze a special kind of temporal fine structure in microwave radio emission for the 25 August 1999 solar flare observed by the PMO spectrometer over the range of 4.5 – 7.5 GHz. This flare displays continuum emission after a group of reverse-slope type III bursts around 6 GHz. High-resolution dynamic spectra reveal three evolving emission lines (EELs) following the type III group. They are characterized by isolated, narrow, and continuous emission strips, which display frequency fluctuations with time. Their frequency-drift rates are between −2 and 3 GHz s⁻¹. Distinct from the EELs at lower frequencies, three EELs have a very short duration of a few seconds. They show an average bandwidth of Δf≈330 MHz and a relative bandwidth of Δf/f≈0.057. This is the first time that this kind of fine structure has been observed around 6 GHz.     

Atmospheric parameters of the B-supergiant HD 198478 from the UV spectra

We determined atmospheric parameters of the Galactic early B-supergiant HD 198478 (55 Cyg) from the UV silicon lines taken from the high-resolution 1150–1980 Å IUE spectra. TLUSTY numerical code was used to model the stellar atmosphere and to determine the temperature and surface gravity assuming a non-LTE plane parallel hydrostatic stellar atmosphere with microturbulence. The synthesized spectra were broadened by the IUE instrumental profile, rotational and macroturbulent velocity with ROTIN numerical code. The silicon 1264 Å, 1309 Å, 1312 Å, 1417 Å and 1294–1303 Å multiplet lines of different stages of ionization (Si II and Si III) and Balmer Hδ 4101 Å line were modeled, leading to the temperature, surface gravity, rotational and macroturbulent velocity values. Our results have shown that the line broadening cannot be explained by rotational velocity only, but additional macroturbulent velocity component should be taken into account. HD 198478 shows a significant degeneracy in velocity, which means that the individual contributions of the macroturbulence and rotation in the total velocity broadening cannot be distinguished. Adequate fit of TLUSTY models to the observed non-resonant silicon lines suggests that the non-LTE plane-parallel hydrostatic stellar model without wind contribution can be used to explain such lines. We have obtained similar results using the HST STIS spectra in the same procedure, showing that the IUE spectra, despite their lacking quality compared to the STIS spectra, are reliable enough in determination of the B supergiants’ photospheric parameters.     

Grand Minima of Solar Activity and the Mean-Field Dynamo

We demonstrate that a simple solar dynamo model, in the form of a Parker migratory dynamo with random fluctuations of the dynamo governing parameters and algebraic saturation of dynamo action, can at least qualitatively reproduce all the basic features of solar Grand Minima as they are known from direct and indirect data. In particular, the model successfully reproduces such features as an abrupt transition into a Grand Minimum and the subsequent gradual recovery of solar activity, as well as mixed-parity butterfly diagrams during the epoch of the Grand Minimum. The model predicts that the cycle survives in some form during a Grand Minimum, as well as the relative stability of the cycle inside and outside of a Grand Minimum. The long-term statistics of simulated Grand Minima appears compatible with the phenomenology of the Grand Minima inferred from the cosmogenic isotope data. We demonstrate that such ability to reproduce the Grand Minima phenomenology is not a general feature of the dynamo models but requires some specific assumption, such as random fluctuations in dynamo governing parameters. In general, we conclude that a relatively simple and straightforward model is able to reproduce the Grand Minima phenomenology remarkably well, in principle providing us with a possibility of studying the physical nature of Grand Minima.     

Prediction of the Maximum Amplitude and Timing of Sunspot Cycle 24

Precursor techniques, in particular those using geomagnetic indices, often are used in the prediction of the maximum amplitude for a sunspot cycle. Here, the year 2008 is taken as being the sunspot minimum year for cycle 24. Based on the average aa index value for the year of the sunspot minimum and the preceding four years, we estimate the expected annual maximum amplitude for cycle 24 to be about 92.8±19.6 (1-sigma accuracy), indicating a somewhat weaker cycle 24 as compared to cycles 21 – 23. Presuming a smoothed monthly mean sunspot number minimum in August 2008, a smoothed monthly mean sunspot number maximum is expected about October 2012±4 months (1-sigma accuracy).     

Reconstructing the 3-D Trajectories of CMEs in the Inner Heliosphere

A method for the full three-dimensional (3-D) reconstruction of the trajectories of coronal mass ejections (CMEs) using Solar TErrestrial RElations Observatory (STEREO) data is presented. Four CMEs that were simultaneously observed by the inner and outer coronagraphs (COR1 and 2) of the Ahead and Behind STEREO satellites were analysed. These observations were used to derive CME trajectories in 3-D out to ～?15?R _⊙. The reconstructions using COR1/2 data support a radial propagation model. Assuming pseudo-radial propagation at large distances from the Sun (15?–?240?R _⊙), the CME positions were extrapolated into the Heliospheric Imager (HI) field-of-view. We estimated the CME velocities in the different fields-of-view. It was found that CMEs slower than the solar wind were accelerated, while CMEs faster than the solar wind were decelerated, with both tending to the solar wind velocity.     

Automated Detection of Oscillating Regions in the Solar Atmosphere

Recently observed oscillations in the solar atmosphere have been interpreted and modeled as magnetohydrodynamic wave modes. This has allowed for the estimation of parameters that are otherwise hard to derive, such as the coronal magnetic-field strength. This work crucially relies on the initial detection of the oscillations, which is commonly done manually. The volume of Solar Dynamics Observatory (SDO) data will make manual detection inefficient for detecting all of the oscillating regions. An algorithm is presented that automates the detection of areas of the solar atmosphere that support spatially extended oscillations. The algorithm identifies areas in the solar atmosphere whose oscillation content is described by a single, dominant oscillation within a user-defined frequency range. The method is based on Bayesian spectral analysis of time series and image filtering. A Bayesian approach sidesteps the need for an a-priori noise estimate to calculate rejection criteria for the observed signal, and it also provides estimates of oscillation frequency, amplitude, and noise, and the error in all of these quantities, in a self-consistent way. The algorithm also introduces the notion of quality measures to those regions for which a positive detection is claimed, allowing for simple post-detection discrimination by the user. The algorithm is demonstrated on two Transition Region and Coronal Explorer (TRACE) datasets, and comments regarding its suitability for oscillation detection in SDO are made.     

Calibrating the Pointing and Optical Parameters of the STEREO Heliospheric Imagers

The Heliospheric Imager (HI) instruments on the Solar TErrestrial RElations Observatory (STEREO) observe solar plasma as it streams out from the Sun and into the heliosphere. The telescopes point off-limb (from about 4° to 90° elongation) and so the Sun is not in the field of view. Hence, the Sun cannot be used to confirm the instrument pointing. Until now, the pointing of the instruments have been calculated using the nominal preflight instrument offsets from the STEREO spacecraft together with the spacecraft attitude data. This paper develops a new method for deriving the instrument pointing solutions, along with other optical parameters, by comparing the locations of stars identified in each HI image with the known star positions predicted from a star catalogue. The pointing and optical parameters are varied in an autonomous manner to minimise the discrepancy between the predicted and observed positions of the stars. This method is applied to all HI observations from the beginning of the mission to the end of April 2008. For the vast majority of images a good attitude solution has been obtained with a mean-squared deviation between the observed and predicted star positions of one image pixel or less. Updated values have been obtained for the instrument offsets relative to the spacecraft, and for the optical parameters of the HI cameras. With this method the HI images can be considered as “self-calibrating,” with the actual instrument offsets calculated as a byproduct. The updated pointing results and their by-products have been implemented in SolarSoft.     

On the Analysis of the Complex Forbush Decreases of January 2005

In this work an analysis of a series of complex cosmic ray events that occurred between 17 January 2005 and 23 January 2005 using solar, interplanetary and ground based cosmic ray data is being performed. The investigated period was characterized both by significant galactic cosmic ray (GCR) and solar cosmic ray (SCR) variations with highlighted cases such as the noticeable series of Forbush effects (FEs) from 17 January 2005 to 20 January 2005, the Forbush decrease (FD) on 21 January 2005 and the ground level enhancement (GLE) of the cosmic ray counter measurements on 20 January 2005. The analysis is focusing on the aforementioned FE cases, with special attention drawn on the 21 January 2005, FD event, which demonstrated several exceptional features testifying its uniqueness. Data from the ACE spacecraft, together with GOES X-ray recordings and LASCO CME coronagraph images were used in conjunction to the ground based recordings of the Worldwide Neutron Monitor Network, the interplanetary data of OMNI database and the geomagnetic activity manifestations denoted by K _p and D _st indices. More than that, cosmic ray characteristics as density, anisotropy and density gradients were also calculated. The results illustrate the state of the interplanetary space that cosmic rays crossed and their corresponding modulation with respect to the multiple extreme solar events of this period. In addition, the western location of the 21 January 2005 solar source indicates a new cosmic ray feature, which connects the position of the solar source to the cosmic ray anisotropy variations. In the future, this feature could serve as an indicator of the solar source and can prove to be a valuable asset, especially when satellite data are unavailable.     

A new law emerging from the recurrences of the ‘critical-acceleration’ of MOND, suggesting a clue to unification of fundamental forces

Keeping apart the problem, whether Modified Newtonian Dynamics [MOND] can replace ‘dark matter’, this letter considers seven different theoretical recurrences of ‘critical acceleration’ of MOND noticed by Sivaram in various physical situations; adds five more observational recurrences to the list; and arrives at a set of laws which seem to be followed by all the systems bound by different fundamental forces; suggesting a clue to unification of fundamental forces. This attempt proposes a general explanation for ‘flattening of galaxies’ rotation-curves’ as well as the ‘expansion of the universe’.     

Summary of the First NUVA Conference Space Astronomy: the UV Window to the Universe

In this summary of the conference Space Astronomy: the UV Window to the Universe, held in El?Escorial, Spain, May 28 to June 1, 2007, I identify the important scientific questions posed by the speakers and the corresponding discoveries that future ultraviolet space instruments should enable. The science objectives described by the various speakers naturally fall into groups according to the needed instrumental requirements: wavelength coverage, spectral resolution, sensitivity, rapid access to targets, monitoring, and signal/noise. Although most of the science objectives presented during the conference require UV spectra in the 1,170–3,200 Å range, there are important science objectives that require spectra in the 912–1,170 Å range and at shorter wavelengths. I identify the limitations of present instruments for meeting these requirements. To avoid the upcoming UV dark age, important work must be done to properly build the World Space Observatory (WSO) and to plan future space missions.     

Role of a tachyonic field in accelerating the Universe in the presence of a perfect fluid

Recently, a tachyonic field was presented as a dark energy model to represent the present acceleration of the Universe. In this paper, we consider a mixture of tachyonic fluid with a perfect fluid. For this purpose we consider barotropic fluid and Generalized Chaplygin gas (GCG). We present a particular form of the scale factor. We solve the equations of motion to get exact solutions of the density, tachyonic potential and the tachyonic field. We introduce a coupling term to show that the interaction decays with time. We also show that the nature of the potentials vary, so the interaction term reduces the potential in both cases.     

Evolution of the Electron Distribution Function in the Whistler Wave Turbulence of the Solar Wind

The electron distribution functions from the solar corona to the solar wind are determined in this paper by considering the effects of the external forces, of Coulomb collisions and of the wave – particle resonant interactions in the plasma wave turbulence. The electrons are assumed to be interacting with right-handed polarized waves in the whistler regime. The acceleration of electrons in the solar wind seems to be mainly due to the electrostatic potential. Wave turbulence determines the electron pitch-angle diffusion and some characteristics of the velocity distribution function (VDF) such as suprathermal tails. The role of parallel whistlers can also be extended to small altitudes in the solar wind (the acceleration region of the outer corona), where they may explain the energization and the presence of suprathermal electrons.     

Neutron star sequences and the starquake glitch model for the Crab and the Vela pulsars

We construct for the first time, the sequences of stable neutron star (NS) models capable of explaining simultaneously, the glitch healing parameters, Q, of both the pulsars, the Crab (Q≥0.7) and the Vela (Q≤0.2), on the basis of starquake mechanism of glitch generation, whereas the conventional NS models cannot give such consistent explanation. Furthermore, our models also yield an upper bound on NS masses similar to those obtained in the literature for a variety of modern equations of state (EOSs) compatible with causality and dynamical stability. If the lower limit of the observational constraint of (i) Q≥0.7 for the Crab pulsar and (ii) the recent value of the moment of inertia for the Crab pulsar (evaluated on the basis of time-dependent acceleration model of the Crab Nebula), I _Crab,45≥1.93 (where I ₄₅=I/10⁴⁵ g cm²), both are imposed together on our models, the models yield the value of matching density, E _b =9.584×10¹⁴ g cm⁻³ at the core-envelope boundary. This value of matching density yields a model-independent upper bound on neutron star masses, M _max≤2.22M _⊙, and the strong lower bounds on surface redshift z _R ≃0.6232 and mass M≃2.11M _⊙ for the Crab (Q≃0.7) and the strong upper bound on surface redshift z _R ≃0.2016, mass M≃0.982M _⊙ and the moment of inertia I _Vela,45≃0.587 for the Vela (Q≃0.2) pulsar. However, for the observational constraint of the ‘central’ weighted mean value Q≈0.72, and I _Crab,45>1.93, for the Crab pulsar, the minimum surface redshift and mass of the Crab pulsar are slightly increased to the values z _R ≃0.655 and M≃2.149M _⊙ respectively, whereas corresponding to the ‘central’ weighted mean value Q≈0.12 for the Vela pulsar, the maximum surface redshift, mass and the moment of inertia for the Vela pulsar are slightly decreased to the values z _R ≃0.1645, M≃0.828M _⊙ and I _Vela,45≃0.459 respectively. These results set an upper and lower bound on the energy of a gravitationally redshifted electron-positron annihilation line in the range of about 0.309–0.315 MeV from the Crab and in the range of about 0.425–0.439 MeV from the Vela pulsar.     

Formal solution of the asteroid motion near the 2:1 commensurability

In a previous paper (Ammar in Proc. Math. Phys. 77:99, 2002) the statement of the problem was formulated and the basic equations of motion were formed in terms of variables suitable for the applications in the problem of asteroid motion close to 2:1 commensurability. The short period terms has been eliminated up to the first order in masses O(μ), using a perturbation approach based on the Lie series, the problem is reduced to that of secular resonance one. In the present work the extended Delaunay method has been applied to develop the Hamiltonian and the generator as a power series in rather than the power of ε, where ε is a small parameter of order of the relative mass of the perturber. Hamilton–Jacobi method were used as a method of integration of the equations of the dynamical system in order to build a formal solution for the resonant problem of the type 2:1 with one degree of freedom.     

The Minimum Activity Epoch as a Precursor of the Solar Activity

This paper considers the indices characterizing the minimum activity epoch, according to the data of large-scale magnetic fields and polar activity. Such indices include: dipole–octopole index, area and average latitude of the field with dominant polarity in each hemisphere, polar activity seen in polar faculae and Ca?ii K line bright points, coronal emission line intensity (5303?Å) and others. We studied the correlation between these indices and the amplitude of the following sunspot cycle, and the relation between the duration of the cycle of large-scale magnetic fields and the duration of the sunspot cycle. The obtained relationships allow us to presume that the polar field is formed from the sources of both preceding and the current activity cycles during the decay phase and at the activity minimum. The balance in these sources would therefore determine the features of the following sunspot cycle. The prediction for the 24th activity cycle using these results leads to W=102±13.