X-ray spectra in the range $1.5-8.5$~keV have been analyzed for 526 large flares detected with the Solar Assembly for X-rays (SAX) on the Mercury {em MESSENGER} spacecraft between 2007 and 2013. For each flare, the temperature and emission measure of
the emitting plasma were determined from the spectrum of the continuum. In addition, with the SAX energy resolution of 0.6 keV (FWHM) at 6~keV, the intensities of the clearly resolved Fe-line complex at 6.7~keV and the Ca-line complex at 3.9~keV were determined, along with those of unresolved line complexes from S, Si, and Ar at lower energies. Comparisons of these line intensities with theoretical spectra allow the abundances of these elements relative to hydrogen to be derived, with uncertainties due to instrument calibration and the unknown temperature distribution of the emitting plasma. While significant deviations are found for the abundances of Fe and Ca from flare to flare, the abundances averaged over all flares are found to be enhanced over photospheric values by factors of $1.66 pm 0.34$ (Fe), $3.89~pm~0.76$ (Ca), $1.23~pm~0.45$ (S), $1.64~pm~0.66$ (Si), and $2.48~pm~0.90$ (Ar). These factors differ from previous reported values for Fe and Si at least. They suggest a more complex relation of abundance enhancement with the first ionization potential (FIP) of the element than previously considered, with the possibility that fractionation occurs in flares for elements with a FIP of less than $sim$7~eV rather than $sim10$~eV.
The capability of maintaining two satellites in precise relative position, stable in a celestial coordinate system, would enable major advances in a number of scientific disciplines and with a variety of types of instrumentation. The common requireme
nt is for formation flying of two spacecraft with the direction of their vector separation in inertial coordinates precisely controlled and accurately determined as a function of time. We consider here the scientific goals that could be achieved with such technology and review some of the proposals that have been made for specific missions. Types of instrumentation that will benefit from the development of this type of formation flying include 1) imaging systems, in which an optical element on one spacecraft forms a distant image recorded by a detector array on the other spacecraft, including telescopes capable of very high angular resolution; 2) systems in which the front spacecraft of a pair carries an occulting disk, allowing very high dynamic range observations of the solar corona and exoplanets; 3) interferometers, another class of instrument that aims at very high angular resolution and which, though usually requiring more than two spacecraft, demands very much the same developments.
The cause of quasi-periodic pulsations (QPP) in solar flares remains the subject of debate. Recently, Nakariakov & Zimovets (2011) proposed a new model suggesting that, in two-ribbon flares, such pulsations could be explained by propagating slow wave
s. These waves may travel obliquely to the magnetic field, reflect in the chromosphere and constructively interfere at a spatially separate site in the corona, leading to quasi-periodic reconnection events progressing along the flaring arcade. Such a slow wave regime would have certain observational characteristics. We search for evidence of this phenomenon during a selection of two-ribbon flares observed by RHESSI, SOHO and TRACE; the flares of 2002 November 9, 2005 January 19 and 2005 August 22. We were not able to observe a clear correlation between hard X-ray footpoint separations and pulse timings during these events. Also, the motion of hard X-ray footpoints is shown to be continuous within the observational error, whereas a discontinuous motion might be anticipated in the slow wave model. Finally, we find that for a preferential slow wave propagation angle of 25-28 degrees that is expected for the fastest waves, the velocities of the hard X-ray footpoints lead to estimated pulse periods and ribbon lengths significantly larger than the measured values. Hence, for the three events studied, we conclude that the observational characteristics cannot be easily explained via the Nakariakov & Zimovets (2011) propagating slow wave model when only angles of 25-28 degrees are considered. We provide suggested flare parameters to optimise future studies of this kind.
We have evaluated the energetics of 38 solar eruptive events observed by a variety of spacecraft instruments between February 2002 and December 2006, as accurately as the observations allow. The measured energetic components include: (1) the radiated
energy in the GOES 1 - 8 A band; (2) the total energy radiated from the soft X-ray (SXR) emitting plasma; (3) the peak energy in the SXR-emitting plasma; (4) the bolometric radiated energy over the full duration of the event; (5) the energy in flare-accelerated electrons above 20 keV and in flare-accelerated ions above 1 MeV; (6) the kinetic and potential energies of the coronal mass ejection (CME); (7) the energy in solar energetic particles (SEPs) observed in interplanetary space; and (8) the amount of free (nonpotential) magnetic energy estimated to be available in the pertinent active region. Major conclusions include: (1) the energy radiated by the SXR-emitting plasma exceeds, by about half an order of magnitude, the peak energy content of the thermal plasma that produces this radiation; (2) the energy content in flare-accelerated electrons and ions is sufficient to supply the bolometric energy radiated across all wavelengths throughout the event; (3) the energy contents of flare-accelerated electrons and ions are comparable; (4) the energy in SEPs is typically a few percent of the CME kinetic energy (measured in the rest frame of the solar wind); and (5) the available magnetic energy is sufficient to power the CME, the flare-accelerated particles, and the hot thermal plasma.
This paper describes the development of X-ray diffractive optics for imaging solar flares with better than 0.1 arcsec angular resolution. X-ray images with this resolution of the geq10 MK plasma in solar active regions and solar flares would allow th
e cross-sectional area of magnetic loops to be resolved and the coronal flare energy release region itself to be probed. The objective of this work is to obtain X-ray images in the iron-line complex at 6.7 keV observed during solar flares with an angular resolution as fine as 0.1 arcsec - over an order of magnitude finer than is now possible. This line emission is from highly ionized iron atoms, primarily Fe xxv, in the hottest flare plasma at temperatures in excess of approx10 MK. It provides information on the flare morphology, the iron abundance, and the distribution of the hot plasma. Studying how this plasma is heated to such high temperatures in such short times during solar flares is of critical importance in understanding these powerful transient events, one of the major objectives of solar physics. We describe the design, fabrication, and testing of phase zone plate X-ray lenses with focal lengths of approx100 m at these energies that would be capable of achieving these objectives. We show how such lenses could be included on a two-spacecraft formation-flying mission with the lenses on the spacecraft closest to the Sun and an X-ray imaging array on the second spacecraft in the focal plane approx100 m away. High resolution X-ray images could be obtained when the two spacecraft are aligned with the region of interest on the Sun. Requirements and constraints for the control of the two spacecraft are discussed together with the overall feasibility of such a formation-flying mission.
The abundance of iron is measured from emission line complexes at 6.65 keV (Fe line) and 8 keV (Fe/Ni line) in {em RHESSI} X-ray spectra during solar flares. Spectra during long-duration flares with steady declines were selected, with an isothermal a
ssumption and improved data analysis methods over previous work. Two spectral fitting models give comparable results, viz. an iron abundance that is lower than previous coronal values but higher than photospheric values. In the preferred method, the estimated Fe abundance is $A({rm Fe}) = 7.91 pm 0.10$ (on a logarithmic scale, with $A({rm H}) = 12$), or $2.6 pm 0.6$ times the photospheric Fe abundance. Our estimate is based on a detailed analysis of 1,898 spectra taken during 20 flares. No variation from flare to flare is indicated. This argues for a fractionation mechanism similar to quiet-Sun plasma. The new value of $A({rm Fe})$ has important implications for radiation loss curves, which are estimated.
Aims: We seek to illustrate the analysis problems posed by RHESSI spacecraft motion by studying persistent instrumental oscillations found in the lightcurves measured by RHESSIs X-ray detectors in the 6-12 keV and 12-25 keV energy range during the de
cay phase of the flares of 2004 November 4 and 6. Methods: The various motions of the RHESSI spacecraft which may contribute to the manifestation of oscillations are studied. The response of each detector in turn is also investigated. Results: We find that on 2004 November 6 the observed oscillations correspond to the nutation period of the RHESSI instrument. These oscillations are also of greatest amplitude for detector 5, while in the lightcurves of many other detectors the oscillations are small or undetectable. We also find that the variation in detector pointing is much larger during this flare than the counterexample of 2004 November 4. Conclusions: Sufficiently large nutation motions of the RHESSI spacecraft lead to clearly observable oscillations in count rates, posing a significant hazard for data analysis. This issue is particularly problematic for detector 5 due to its design characteristics. Dynamic correction of the RHESSI counts, accounting for the livetime, data gaps, and the transmission of the bi-grid collimator of each detector, is required to overcome this issue. These corrections should be applied to all future oscillation studies.
