Hard X-ray Bremsstrahlung Emission Research Articles

The STIX Aspect System (SAS): The Optical Aspect System of the Spectrometer/Telescope for Imaging X-Rays (STIX) on Solar Orbiter

The Spectrometer/Telescope for Imaging X-rays (STIX) is a remote sensing instrument on Solar Orbiter that observes the hard X-ray bremsstrahlung emission of solar flares. This paper describes the STIX Aspect System (SAS), a subunit that measures the pointing of STIX relative to the Sun with a precision of pm 4'', which is required to accurately localize the reconstructed X-ray images on the Sun. The operating principle of the SAS is based on an optical lens that images the Sun onto a plate that is perforated by small apertures arranged in a cross-shaped configuration of four radial arms. The light passing through the apertures of each arm is detected by a photodiode. Variations of spacecraft pointing and of distance from the Sun cause the solar image to move over different apertures, leading to a modulation of the measured lightcurves. These signals are used by ground analysis to calculate the locations of the solar limb, and hence the pointing of the telescope.

Surface-plasmon-enhanced MeV ions from femtosecond laser irradiated, periodically modulated surfaces

Enhanced emission of high energy ions is measured from sub-wavelength metallic grating targets under irradiation by intense (1015−1016 W cm−2), p-polarized, 50 fs, 800 nm laser pulses. The maximum ion energy is 55% higher and the ion flux is 60% higher for the modulated surface in comparison with polished surfaces of the same atomic composition. The ion emission, a result of enhanced light coupling and hot electron production in the grating targets, is correlated with enhancement in hard x-ray bremsstrahlung emission. The results are well reproduced by particle-in-cell simulations. The study reveals that the enhanced laser coupling by surface plasmon excitation on metallic gratings is directly responsible for the enhancement of ion energies.

X-RAY EMISSION FROM THE BASE OF A CURRENT SHEET IN THE WAKE OF A CORONAL MASS EJECTION

Following a CME which started on 2002 November 26, RHESSI, the Ramaty High Energy Solar Spectroscopic Imager, observed for 12 hours an X-ray source above the solar limb, at altitudes between 0.1 and 0.3 RS above the photosphere. The GOES baseline was remarkably high throughout this event. The X-ray source's temperature peaked around 10-11 MK, and its emission measure increased throughout this time interval. Higher up, at 0.7 RS, hot (initially >8 MK) plasma has been observed by UVCS on SoHO for 2.3 days. This hot plasma was interpreted as the signature of a current sheet trailing the CME (Bemporad et al. 2006). The thermal energy content of the X-ray source is more than an order of magnitude larger than in the current sheet. Hence, it could be the source of the hot plasma in the current sheet, although current sheet heating by magnetic reconnection within it cannot be discounted. To better characterize the X-ray spectrum, we have used novel techniques (back-projection based and visibility-based) for long integration (several hours) imaging spectroscopy. There is no observed non-thermal hard X-ray bremsstrahlung emission, leading to the conclusion that there is either very little particle acceleration occurring in the vicinity of this post- are X-ray source, or that either the photon spectral index would have had to be uncharacteristically (in flare parlance) high (gamma >= 8) and/or the low-energy cutoff? very low (Ec <= 6 keV).

Experimental and theoretical investigation of local synergy between ion Bernstein and lower hybrid waves in the Princeton Beta Experiment—Modified

The local synergistic behavior of lower hybrid and ion Bernstein waves on the Princeton Beta Experiment-Modified tokamak [Phys. Fluids B 2, 1271 (1990)] is experimentally studied using a two-dimensional hard x-ray camera. The hard x-ray bremsstrahlung emission from suprathermal electrons, generated with lower hybrid current drive, is enhanced during ion Bernstein wave power injection. This enhancement is observed in limited regions of space, suggesting the formation of localized current channels. The effects on plasma electrons during combined application of these two types of waves are theoretically investigated using a quasilinear model. The numerical code simultaneously solves the three-dimensional (R,Z,Φ) torbidal wave equation for the electric field, in the Wentzel–Kramers–Brillouin–Jeffrey approximation, and the Fokker–Planck equation for the distribution function in two dimensions (v∥,v⊥) with an added quasilinear diffusion coefficient. The radial profile of the noninductively generated current density, the transmitted power traces, and the total power damping curve are calculated. The beneficial effects of a combined utilization of ion Bernstein and lower hybrid waves on the current drive are emphasized. The numerical results are compared with the experimental observations.

Hard X-ray bremsstrahlung emission and polarization in a flaring loop affected by electron beams

Using a kinetic approach the hard X-ray bremsstrahlung emission and polarization produced by an electron beam injected, with an power law energy distribution from the corona into the chromosphere are investigated. The time-dependent kinetic equation was solved numerically in a flaring loop taking into account the energy losses and anisotropy of electron scattering for the following channels: Coulomb collisions and inelastic ones with neutral atoms, Ohmic dissipation and adiabatic scattering in a converging magnetic field. The temporal hard X-ray bremsstrahlung emission profiles are symmetric ones which resemble the observations for events with the timescale around a few seconds. The intensity of the X-ray bremsstrahlung emission increases and its directivity decreases with the time of a beam injection. The polarization is found to vary noticeably with the emergent photon energies below 40 keV being up to 30% and down to −10% for different angles of view. Harder X-ray polarization is almost constant with increase of photon energy, within a fixed angle of view.

Electron Time-of-Flight Differences in Solar Flares

view Abstract Citations (94) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Electron Time-of-Flight Differences in Solar Flares Aschwanden, Markus J. ; Schwartz, Richard A. ; Alt, Daniel M. Abstract In the thick-target model for hard X-ray (HXR) emission in solar flares, electron acceleration is assumed to occur in flaring loops at coronal heights, while HXR bremsstrahlung emission is produced in the chromosphere. Under this assumption, the velocity spectrum of the accelerated electrons causes time-of-flight differences that are expected to result in the lower energy HXRs to be delayed with respect to the higher energies. Here we report on the first observational evidence for such a delay. The electron time-of-flight differences between electrons that produce 25-50 keV and 50-100 keV HXR emission are found to have a distribution with a mean of τ = 16.7±1.9 ms and a standard deviation of στ = 16.8 ms. This result is based on the statistics of 5430 HXR pulses detected during 640 solar flares, recorded in the Discriminator Science Data (DISCSC) burst trigger mode with a time resolution of 64 ms by the Burst and Transient Source Experiment (BATSE) onboard the Compton Gamma Ray Observatory (CGRO). From the time-of-flight differences we infer a mean altitude of the acceleration site of H = 7300±800 km (with a standard deviation of σH = 7300 km) above the level at which the electrons lose their energy. This derived mean loop height should be considered as a lower limit because it is based on the predominance of time-of-flight effects over opposite delay effects caused by pitch-angle scattering or trapping. For the electron density in the flare loops we find an upper limit of ne ≤ 4 × 1012 cm-3, based on the requirement that the electron travel time has to be shorter than the collision time. The relatively small time-of-flight differences correspond typically to only ≍3% of the HXR pulse duration, and, therefore, no rapid variation in the spectral slope of the observed HXR spectrum is expected. Publication: The Astrophysical Journal Pub Date: July 1995 DOI: 10.1086/175930 Bibcode: 1995ApJ...447..923A Keywords: RADIATION MECHANISMS: NONTHERMAL; SUN: CORONA; SUN: FLARES; SUN: PARTICLE EMISSION; SUN: X-RAYS; GAMMA RAYS full text sources ADS |

Experimental studies of the radial diffusion of energetic electrons in lower-hybrid wave heated plasmas in the Alcator-C tokamak

An upper limit for the cross-field spatial diffusion coefficient of energetic electrons in moderate density tokamak discharges has been measured. The energetic electrons were generated by square wave-modulated lower-hybrid (LH) wave injection. The decay of the fast electron population for electron energies εd≥100 keV following LH radio-frequency (RF) power shutoff was inferred from the time dependence of the radial profiles of the hard x-ray bremsstrahlung emission. The experimental results were fitted to a diffusive model that includes collisional losses and electric field acceleration. The fast electron diffusion coefficient consistent with the data is Df≤0.2 m2/s, at least in the inner half of the plasma. The corresponding global confinement time of the energetic electrons is estimated to be at least a factor of 2 greater than the energy confinement time of the bulk plasma in discharges of density n̄e=8×1019 m −3.

Return Current Instability in Flares

AbstractWe consider the problem of ion-acoustic wave generation, and resultant anomalous Joule heating, by a return current driven unstable by a small-area thick-target electron beam in solar flares. We find that, contrary to the usual assumption, the hard X-ray bremsstrahlung emission may actually be enhanced in comparison to conventional thick-target models. This present paper is a summary of the work of Cromwell, McQuillan and Brown (1988).

Pulsed soft x-ray source for laser-plasma diagnostic calibrations

Improvement in laser-plasma analysis makes it necessary to perform accurate calibration in the soft x-ray range. For this purpose we have modified a plasma focus device (27 kJ, 40 kV, and 400 kA) in order to enhance its soft x-ray emission: A hollow anode is used to prevent hard x-ray bremsstrahlung emission, and a lot of filling gases are chosen for their characteristic lines in the keV and sub-keV range (Ne, Ar, O, N, and CH4). These emissions have been absolutely measured with an x-ray bolometer and can be used directly or indirectly (in the fluorescent mode: Mg Kα at 1.254 keV) for x-ray calibration of various diagnostics. Some detector calibrations in pulsed regime (10 ns) are presented [films, solid state Si doped p-intrinsic-n (PIN) diodes, and charge-coupled devices (CCD) arrays.]

The evolution of the spatial structure of thick-target hard X-ray emission in solar flares

We examine the spatial distribution of hard X-ray bremsstrahlung emission from an electron-heated target, using a self-consistent calculation of the hydrodynamic response of the atmosphere to heating by the electrons to compute the density-height structure of the target atmosphere at various times. In this way we derive the predicted temporal evolution of the hard X-ray spatial structure at various photon energies. We compare our results with existing observations from the Solar Maximum Mission Hard X-Ray Imaging Spectrometer and give a prognosis for the type of structure to be expected at the sub-arc second resolution planned for future instrumentation.