The following list presents my publications in chronological order.

 #9S. Krucker et al.
The spectrometer/telescope for imaging X-rays on board the ESA Solar Orbiter spacecraft

Abstract
Solar Orbiter is a Sun-observing mission led by the European Space Agency, addressing the interaction between the Sun and the heliosphere. It will carry ten instruments, among them the X-ray imaging spectrometer STIX. STIX will determine the intensity, spectrum, timing, and location of thermal and accelerated electrons near the Sun through their bremsstrahlung X-ray emission. This report gives a brief overview of the STIX scientific goals and covers in more detail the instrument design and challenges.

link to this paper at ADS ]
 

   
 #8A. Białek et al.
Thermal simulations of the STIX instrument for ESA Solar Orbiter mission

Abstract
The ESA Solar Orbiter mission, planned to be launched in 2017, is going to study the Sun with ten different instruments including the Spectrometer/Telescope for Imaging X-rays – STIX. The thermal environment on the elliptical orbit around the Sun – 0.28 AU at perihelion and 0.952 AU at aphelion – is extreme, where at one point of the orbit is very hot, while on another very cold. That makes the requirements for the heat fluxes exchanged between each instrument and the spacecraft, as well as between the instrument’ subsystems, very restrictive. Here the authors discuss the thermal design with respect to the defined requirements and present the results of the thermal analyses performed with ESATAN TMS software.

link to paper at ADS ]

 
STIX Aspect System
This picture presents the early design of the STIX Aspect System. This system helps STIX to determine where it is looking at on the Sun.
   
 #7K. R. Skup et al.
Instrument data processing unit for spectrometer/telescope for imaging x-rays (STIX)

Abstract
The Spectrometer/Telescope for Imaging X-rays (STIX) is one of 10 instruments on board Solar Orbiter, an M-class mission of the European Space Agency (ESA) scheduled to be launch in 2017. STIX applies a Fourier-imaging technique using a set of tungsten grids in front of 32 pixelized CdTe detectors to provide imaging spectroscopy of solar thermal and non-thermal hard X-ray emissions from 4 to 150 keV. These detectors are source of data collected and analyzed in real-time by Instrument Data Processing Unit (IDPU). Besides the data processing the IDPU controls and manages other STIX’s subsystems: ASICs and ADCs associated with detectors, Aspect System, Attenuator, PSU and HK. The instrument reviewed in this paper is based on the design that passed the Instrument Preliminary Design Review (IPDR) in early 2012 and Software Preliminary Design Review (SW PDR) in middle of 2012. Particular emphasis is given to the IDPU and low level software called Basic SW (BSW).

link to this paper at ADS ]
 
 
The picture shows the results of the early calculations of the temperature profile of all three STIX units (X-ray windows, Imager, Detector Electronics Module) for the hot operational case.
 

 #6A. O. Benz et al.
The spectrometer telescope for imaging x-rays on board the Solar Orbiter mission

Abstract
The Spectrometer Telescope for Imaging X-rays (STIX) is one of 10 instruments on board Solar Orbiter, a confirmed M-class mission of the European Space Agency (ESA) within the Cosmic Vision program scheduled to be launched in 2017. STIX applies a Fourier-imaging technique using a set of tungsten grids (at pitches from 0.038 to 1 mm) in front of 32 pixelized CdTe detectors to provide imaging spectroscopy of solar thermal and non-thermal hard X-ray emissions from 4 to 150 keV. The status of the instrument reviewed in this paper is based on the design that passed the Preliminary Design Review (PDR) in early 2012. Particular emphasis is given to the first light of the detector system called Caliste-SO.

link to this paper at ADS ]

The status of STIX instrument reviewed in this paper is based on the design that passed the Preliminary Design Review (PDR) in early 2012.
I am mainly involved in the development of the Imager that will carry tungsten grid pairs with variable pitch. 32 cadmium telluride X-ray detectors are located behind the grids. The X-ray transmission through these grid pairs to the detectors is highly depending on the direction of incidence of the X-rays.
STIX is able to measure spectral and spatial information.
   
 #5H. Önel, G. Mann
Generation of large scale electric fields in coronal flare circuits

Abstract

A large number of energetic electrons are generated during solar flares. They carry a substantial part of the flare released energy but how these electrons are created is not fully understood yet. This paper suggests that plasma motion in an active region in the photosphere is the source of large electric currents. These currents can be described by macroscopic circuits. Under special circumstances currents can establish in the corona along magnetic field lines. The energy released by these currents when moderate assumptions for the local conditions are made, is found be comparable to the flare energy.
[ link to a blog entry on this paper (picture) by Michael Mozina ]
 

Simplified sketch of the a solar flare that closes electrical circuits through the corona by coronal magnetic field lines. This model proposes a solution to the electron number problem.
   
 #4H. Önel [PhD thesis]
Electron Acceleration in a Flare Plasma via Coronal Circuits

Abstract [shortened]
... flares are sources for energetic particles. Hard X-ray observations (e.g., by NASA’s RHESSI spacecraft) reveal that a large fraction of the energy released during a flare is transferred into the kinetic energy of electrons. However the mechanism that accelerates a large number of electrons to high energies (beyond 20 keV) in fractions of a second is not understood yet.
The thesis at hand presents a model for the generation of energetic electrons during flares that explains the electron acceleration using real parameters obtained by real ground and space based observations.
According to this model photospheric plasma flows build up electric potentials in the active regions in the photosphere. Usually these electric potentials are associated with electric currents closed within the photosphere. However as a result of magnetic reconnection, a magnetic connection between the regions of different magnetic polarity on the photosphere can establish through the corona. Due to the significantly higher electric conductivity in the corona, the photospheric electric power supply can be closed via the corona. Subsequently a high electric current is formed, which leads to the generation of hard X-ray radiation in the dense chromosphere...

[ link to this thesis at ADS ]
 

Model parameters of the macroscopic electric circuit components (top) are obtained by observation (bottom) and microscopic plasma parameters.



 #3H. Önel, G. Mann, E. Sedlmayr
Propagation of energetic electrons through the solar corona and the interplanetary medium
Abstract
Aims: The electron transport is investigated numerically after an electron transport model is deduced.
Methods: This model for electron propagation considers global electric and magnetic fields, as well as local Coulomb collisions. A new way to handle the electron's pitch angle evolution by using a binary dice within the treatment of the Coulomb scattering is introduced. The conditions in the solar plasma are represented by average and commonly used models, such that numerical simulations can be performed easily.
Results: The model for electron propagation finally obtained makes it possible to investigate how the Coulomb collisions act on the pitch angle, while electrons are transported through the solar plasma. Some chosen numerical results for different initial conditions are presented in the paper.

[ link to this paper at ADS ]
 
This heliocentric height vs. time diagram presents 1000 electrons that are released with the same initial conditions, but they propagate differently due to Coulomb collisions.
The red line presents the best fit.
 

 #2H. Önel, G. Mann, E. Sedlmayr
Transport of energetic electrons through the solar corona and the interplanetary space
Abstract
During solar flares fast electron beams generated in the solar corona are non-thermal radio sources in terms of type III bursts. Sometimes they can enter into the interplanetary space, where they can be observed by in-situ measurements as it is done e.g. by the WIND spacecraft. On the other hand, they can be the source of non-thermal X-ray radiation as e.g. observed by RHESSI, if they precipitate toward the dense chromosphere due to bremsstrahlung. Since these energetic electrons are generated in the corona and observed at another site, the study of transport of such electrons in the corona and interplanetary space is of special interest. The transport of electrons is influenced by the global magnetic and electric field as well as local Coulomb collisions with the particles in the background plasma.

[ link to this paper at ADS ]
 
Comparison between a dynamic radio spectrum with fast electron signatures on the left to a cartoon presenting an hypothetical configuration of the magnetic field geometry at the site of electron acceleration.
   
 #1H. Önel, G. Mann, E. Sedlmayr
Propagation of Energetic Electrons in the Solar Corona and the Interplanetary Space
Abstract
Energetic electrons can be released in the solar corona (e.g. during flares) and travel along the magnetic field lines either towards the Sun or into the interplanetary space. The influences of the global magnetic and electric field as well as the Coulomb collisions on the transport of such electrons are numerically studied and a model for the electron propagation away from the acceleration site is developed. Estimating the altitude and the energy of the electron both at the acceleration site is possible with this model if one takes observations of spacecrafts as RHESSI and WIND into account.

[ link to this paper at ADS ]
 
An electron is released at a certain heliocentric distance with an initial pitch angle and a total kinetic energy. The four diagrams (heliocentric distance vs. time, electron plasma frequency vs. time, pitch angle vs. time, energy vs time) describe how the electron behaves.


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osra_19990311.fits
(1249k)
Hakan Önel,
27 Apr 2012, 07:12