Time distributions in satellite constellation design

The aim of the time distribution methodology presented in this paper is to generate constellations whose satellites share a set of relative trajectories in a given time, and maintain that property over time without orbit corrections. The model takes into account a series of orbital perturbations such as the gravitational potential of the Earth, the atmospheric drag, the Sun and the Moon as disturbing third bodies and the solar radiation pressure. These perturbations are included in the design process of the constellation. Moreover, the whole methodology allows to design constellations with multiple relative trajectories that can be distributed in a minimum number of inertial orbits.     

Improvements on a unified dark matter model

We study, by means of a spherical collapse model, the effect of shear, rotation, and baryons on a generalized Chaplygin gas (gCg) dominated universes. We show that shear, rotation, and the baryon presence slow down the collapse with respect to the simple spherical collapse model. The slowing down in the growth of density perturbation is able to solve the instability of the unified dark matter (UDM) models described in previous papers (e.g., Sandvik et al. 2004) at the linear perturbation level, as also shown by a direct comparison of our model with previous results.     

Calibration of the ART-XC mirror modules at MSFC

The Astronomical Röntgen Telescope X-ray Concentrator (ART-XC) is a hard X-ray telescope with energy response up to 30 keV, to be launched on board the Spectrum Röntgen Gamma (SRG) spacecraft in 2018. ART-XC consists of seven identical co-aligned mirror modules. Each mirror assembly is coupled with a CdTe double-sided strip (DSS) focal-plane detector. Eight X-ray mirror modules (seven flight and one spare units) for ART-XC were developed and fabricated at the Marshall Space Flight Center (MSFC), NASA, USA. We present results of testing procedures performed with an X-ray beam facility at MSFC to calibrate the point spread function (PSF) of the mirror modules. The shape of the PSF was measured with a high-resolution CCD camera installed in the focal plane with defocusing of 7 mm, as required by the ART-XC design. For each module, we performed a parametrization of the PSF at various angular distances Θ. We used a King function to approximate the radial profile of the near on-axis PSF (Θ < 9 arcmin) and an ellipse fitting procedure to describe the morphology of the far off-axis angular response (9 < Θ < 24 arcmin). We found a good agreement between the seven ART-XC flight mirror modules at the level of 10%. The on-axis angular resolution of the ART-XC optics varies between 27 and 33 arcsec (half-power diameter), except for the spare module.     

Efficient design of direct low-energy transfers in multi-moon systems

In this contribution, an efficient technique to design direct (i.e., without intermediate flybys) low-energy trajectories in multi-moon systems is presented. The method relies on analytical two-body approximations of trajectories originating from the stable and unstable invariant manifolds of two coupled circular restricted three-body problems. We provide a means to perform very fast and accurate computations of the minimum-cost trajectories between two moons. Eventually, we validate the methodology by comparison with numerical integrations in the three-body problem. Motivated by the growing interest in the robotic exploration of the Jovian system, which has given rise to numerous studies and mission proposals, we apply the method to the design of minimum-cost low-energy direct trajectories between Galilean moons, and the case study is that of Ganymede and Europa.     

Bayesian Methods for Reconstructing Sunspot Numbers Before and During the Maunder Minimum

The Maunder Minimum (MM) was an extended period of reduced solar activity in terms of yearly sunspot numbers (SSN) during 1610?–?1715. The reality of this “grand minimum” is generally accepted in the scientific community, but the statistics of the SSN record suggest a need for data reconstruction. The MM data show a nonstandard distribution compared with the entire SSN signal (1610?–?2014). The pattern does not satisfy the weakly stationary solar dynamo approximation, which characterizes many natural events spanning centuries or even millennia, including the Sun and the stars. Over the entire observation period (1610?–?2014), the reported SSN exhibits statistically significant regime switches, departures from autoregressive stationarity, and growing trends. Reconstruction of the SSN during the pre-MM and MM periods is performed using five novel statistical procedures in support of signal analysis. A Bayesian–Monte Carlo backcast technique is found to be most reliable and produces an SSN signal that meets the weak-stationarity requirement. The computed MM signal for this reconstruction does not show a “grand” minimum or even a “semi-grand” minimum.     

Changing Relationships Between Sunspot Number,Total Sunspot Area and $F_{10.7}$ in Cycles 23 and 24

This article is an update of a study (Tapping and Valdès in Solar Phys. 272, 337, 2011) made in the early part of Cycle 24 using an intercomparison of various solar activity indices (including sunspot number and the 10.7 cm solar radio flux), in which it was concluded that a change in the relationship between photospheric and chromospheric/coronal activity took place just after the maximum of Cycle 23 and continued into Cycle 24. Precursors (short-term variations) were detected in Cycles 21 and 22. Since then the sunspot number index data have been substantially revised. This study is intended to be an update of the earlier study and to assess the impact of the revision of the sunspot number data upon those conclusions. This study compares original and revised sunspot number, total sunspot area, and 10.7 cm solar radio flux. The conclusion is that the transient changes in Cycles 21 and 22, and the more substantial change in Cycle 23, remain evident. Cycle 24 shows indications that the deviation was probably another short-term one.     

Observations of a Radio-Quiet Solar Preflare

The preflare phase of the flare SOL2011-08-09T03:52 is unique in its long duration, in that it was covered by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Nobeyama Radioheliograph, and because it showed three well-developed soft X-ray (SXR) peaks. No hard X-rays (HXR) are observed in the preflare phase. Here we report that no associated radio emission at 17 GHz was found either, despite the higher sensitivity of the radio instrument. The ratio between the SXR peaks and the upper limit of the radio peaks is higher by more than one order of magnitude than the ratio in regular flares. The result suggests that the ratio between acceleration and heating in the preflare phase was different than in regular flares. Acceleration to relativistic energies, if any, occurred with lower efficiency.     

Comparative Study of the Three-Dimensional Thermodynamical Structure of the Inner Corona of Solar Minimum Carrington Rotations 1915 and 2081

Using differential emission measure tomography (DEMT) based on time series of EUV images, we carry out a quantitative comparative analysis of the three-dimensional (3D) structure of the electron density and temperature of the inner corona (\(r<1.25\,\mathrm{R}_{\odot}\)) between two specific rotations selected from the last two solar minima, namely Carrington Rotations (CR)1915 and CR-2081. The analysis places error bars on the results because of the systematic uncertainty of the sources. While the results for CR-2081 are characterized by a remarkable north–south symmetry, the southern hemisphere for CR-1915 exhibits higher densities and temperatures than the northern hemisphere. The core region of the streamer belt in both rotations is found to be populated by structures whose temperature decreases with height (called “down loops” in our previous articles). They are characterized by plasma \(\beta\gtrsim1\), and may be the result of the efficient dissipation of Alfvén waves at low coronal heights. The comparative analysis reveals that the low latitudes of the equatorial streamer belt of CR-1915 exhibit higher densities than for CR-2081. This cannot be explained by the systematic uncertainties. In addition, the southern hemisphere of the streamer belt of CR-1915 is characterized by higher temperatures and density scale heights than for CR-2081. On the other hand, the coronal hole region of CR-1915 shows lower temperatures than for CR-2081. The reported differences are in the range \({\approx}\,10\,\mbox{--}\,25\%\), depending on the specific physical quantity and region that is compared, as fully detailed in the analysis. For other regions and/or physical quantities, the uncertainties do not allow assessing the thermodynamical differences between the two rotations. Future investigation will involve a DEMT analysis of other Carrington rotations selected from both epochs, and also a comparison of their tomographic reconstructions with magnetohydrodynamical simulations of the inner corona.