arXiv subjects

D. Bisikalo

Publications and source records attributed to D. Bisikalo.

6 recordsLinked to original sources

arXiv2026

The impact of electron precipitation on Earth's thermospheric NO production and the drag of LEO satellites

We investigate the response of space weather events on Earth's upper atmosphere over the polar regions by studying their effect on the drag of the CHAMP and GRACE satellites. Increasing solar activity that results in heating and the expansion of the upper atmosphere threatens low Earth orbit (LEO) satellites. Auroral events are closely related to the stellar energy deposition of solar EUV radiation and precipitating energetic electrons, which influence photochemical processes such as the production of nitric oxide (NO) in the upper atmosphere. To study the production of NO molecules and their influence on the thermospheric structure and satellite drag, we first model Earth's background thermosphere with the 1D upper atmosphere model Kompot by considering the incident X-ray, EUV, and IR radiation during selected space weather events. To investigate the effect of electron precipitation in the production of NO molecules in the polar thermosphere, we apply a Monte Carlo model accounting for the stochastic nature of collisional scattering of auroral electrons in collisions with the surrounding N$_2$-O$_2$ atmosphere, including the production of suprathermal N atoms. The observed effect of the atmospheric drag on CHAMP and GRACE during the two studied events indicates that a sporadic enhancement of NO molecule production in the polar thermosphere and its IR-cooling capability, which counteracts thermospheric expansion and can lead to an ``overcooling'' with decreased density after the space weather event, can have a protective effect on LEO satellites. Their production efficiency, however, is highly dependent on the energy flux of the precipitating electrons. Our results have direct implications for empirical satellite orbit prediction models, as our simulations highlight the need to consider precipitation-induced NO production to improve the predictive power of these models.

astro-ph.EP↗

arXiv2015

Hubble Space Telescope observations of the NUV transit of WASP-12b

We present new observations of four closely-spaced NUV transits of the hot Jupiter-like exoplanet WASP-12b using HST/COS, significantly increasing the phase resolution of the observed NUV light curve relative to previous observations, while minimising the temporal variation of the system. We observe significant excess NUV absorption during the transit, with mean normalised in-transit fluxes of $F_\mathrm{norm}\simeq0.97$, i.e. $\simeq$2-5 $σ$ deeper than the optical transit level of $\simeq0.986$ for a uniform stellar disk (the exact confidence level depending on the normalisation method used). We further observe an asymmetric transit shape, such that the post-conjunction fluxes are overall $\simeq$2-3 $σ$ higher than pre-conjunction values, and characterised by rapid variations in count rate between the pre-conjunction and out of transit levels. We do not find evidence for an early ingress to the NUV transit as suggested by earlier HST observations. However, we show that the NUV count rate observed prior to the optical transit is highly variable, but overall $\simeq$2.2-3.0 $σ$ below the post-transit values and comparable in depth to the optical transit, possibly forming a variable region of NUV absorption from at least phase $ϕ\simeq$0.83, limited by the data coverage.

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arXiv2015

Colliding Winds in Low-Mass Binary Star Systems: wind interactions and implications for habitable planets

Context. In binary star systems, the winds from the two components impact each other, leading to strong shocks and regions of enhanced density and temperature. Potentially habitable circumbinary planets must continually be exposed to these interactions regions. Aims. We study, for the first time, the interactions between winds from low-mass stars in a binary system, to show the wind conditions seen by potentially habitable circumbinary planets. Methods. We use the advanced 3D numerical hydrodynamic code Nurgush to model the wind interactions of two identical winds from two solar mass stars with circular orbits and a binary separation of 0.5 AU. As input into this model, we use a 1D hydrodynamic simulation of the solar wind, run using the Versatile Advection Code. We derive the locations of stable and habitable orbits in this system to explore what wind conditions potentially habitable planets will be exposed to during their orbits. Results. Our wind interaction simulations result in the formation of two strong shock waves separated by a region of enhanced density and temperature. The wind-wind interaction region has a spiral shape due to Coriolis forces generated by the orbital motions of the two stars. The stable and habitable zone in this system extends from approximately 1.4 AU to 2.4 AU. (TRUNCATED)

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arXiv2013

XMM-Newton monitoring of the close pre-main-sequence binary AK Sco. Evidence of tide driven filling of the inner gap in the circumbinary disk

AK~Sco stands out among pre-main sequence binaries because of its prominent ultraviolet excess, the high eccentricity of its orbit and the strong tides driven by it. AK Sco consists of two F5 type stars that get as close as 11R$_*$ at periastron passage. The presence of a dense ($n_e \sim 10^{11}$~cm$^{-3}$) extended envelope has been unveiled recently. In this article, we report the results from a XMM-Newton based monitoring of the system. We show that at periastron, X-ray and UV fluxes are enhanced by a factor of $\sim 3$ with respect to the apastron values. The X-ray radiation is produced in an optically thin plasma with T$\sim 6.4\times 10^{6}$ K and it is found that the N$_H$ column density rises from 0.35$\times 10^{21}$~cm$^{-2}$ at periastron to 1.11$\times 10^{21}$~cm$^{-2}$ at apastron, in good agreement with previous polarimetric observations. The UV emission detected in the Optical Monitor band seems to be caused by the reprocessing of the high energy magnetospheric radiation on the circumstellar material. Further evidence of the strong magnetospheric disturbances is provided by the detection of line broadening of 278.7~km~s$^{-1}$ in the N~V line with HST/STIS. Numerical simulations of the mass flow from the circumbinary disk to the components have been carried out. They provide a consistent scenario with which to interpret AK~Sco observations. We show that the eccentric orbit acts like a gravitational piston. At apastron, matter is dragged efficiently from the inner disk border, filling the inner gap and producing accretion streams that end as ring-like structures around each component of the system. At periastron, the ring-like structures come into contact, leading to angular momentum loss, and thus producing an accretion outburst.

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arXiv2012

3D Gas Dynamic Simulation of the Interaction Between the Exoplanet WASP-12b and Its Host Star

HST transit observations in the near-UV performed in 2009 made WASP-12b one of the most "mysterious" exoplanets; the system presents an early-ingress, which can be explained by the presence of optically thick matter located ahead of the planet at a distance of 4-5 planet radii. This work follows previous attempts to explain this asymmetry with an exospheric outflow or a bow shock, induced by a planetary magnetic field, and provides a numerical solution of the early-ingress, though we did not perform any radiative transfer calculation. We performed pure 3D gas dynamic simulations of the plasma interaction between WASP-12b and its host star, and describe the flow pattern in the system. In particular, we show that the overfilling of the planet's Roche lobe leads to a noticeable outflow from the upper atmosphere in the direction of the L1 and L2 points. Due to the conservation of the angular momentum, the flow to the L1 point is deflected in the direction of the planet's orbital motion, while the flow towards L2 is deflected in the opposite direction, resulting in a non-axisymmetric envelope, surrounding the planet. The supersonic motion of the planet inside the stellar wind leads to the formation of a bow shock with a complex shape. The existence of the bow shock slows down the outflow through the L1 and L2 points, allowing us to consider a long-living flow structure which is in the steady-state.

astro-ph.EP↗

arXiv2001

Bending Instability of an Accretion Disc Around a Black Hole

We demonstrate that generically an accretion disk around a compact object could have a new type of instability in that the accretion flow need not be symmetric with respect to the equatorial plane even when matter is supplied symmetrically farther out. We find that this behaviour is mainly due to interaction of outgoing matter bounced off the centrifugal barrier and the incoming accretion. We believe that X-ray variability could be developed by this instability.

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