Martin D. Fivian

A large excess in apparent solar oblateness due to surface magnetism.

The shape of the Sun subtly reflects its rotation and internal flows. The surface rotation rate, ∼2 kilometers per second at the equator, predicts an oblateness (equator-pole radius difference) of 7.8 milli–arc seconds, or ∼0.001%. Observations from the Reuven Ramaty High-Energy Solar Spectroscopic Imager satellite show unexpectedly large flattening, relative to the expectation from surface rotation. This excess is dominated by the quadrupole term and gives a total oblateness of 10.77 ± 0.44 milli–arc seconds. The position of the limb correlates with a sensitive extreme ultraviolet proxy, the 284 angstrom limb brightness. We relate the larger radius values to magnetic elements in the enhanced network and use the correlation to correct for it as a systematic error term in the oblateness measurement. The corrected oblateness of the nonmagnetic Sun is 8.01 ± 0.14 milli–arc seconds, which is near the value expected from rotation.

CO-SPATIAL WHITE LIGHT AND HARD X-RAY FLARE FOOTPOINTS SEEN ABOVE THE SOLAR LIMB

We report analysis of three solar flares that occur within 1? of limb passage, with the goal to investigate the source height of chromospheric footpoints in white light (WL) and hard X-rays (HXR). We find the WL and HXR (?30 keV) centroids to be largely co-spatial and from similar heights for all events, with altitudes around 800 km above the photosphere or 300?450 km above the limb height. Because of the extreme limb location of the events we study, emissions from such low altitudes are influenced by the opacity of the atmosphere and projection effects. STEREO images reveal that for SOL2012-11-20T12:36 the projection effects are smallest, giving upper limits of the absolute source height above the nominal photosphere for both wavelengths of ?1000 km. To be compatible with the standard thick target model, these rather low altitudes require very low ambient densities within the flare footpoints, in particular if the HXR-producing electrons are only weakly beamed. That the WL and HXR emissions are co-spatial suggests that the observed WL emission mechanism is directly linked to the energy deposition by flare accelerated electrons. If the WL emission is from low-temperature ( K) plasma as currently thought, the energy deposition by HXR-producing electrons above ?30 keV seems only to heat chromospheric plasma to such low temperatures. This implies that the energy in flare-accelerated electrons above ?30 keV is not responsible for chromospheric evaporation of hot ( K) plasma, but that their energy is lost through radiation in the optical range.

RHESSI imager and aspect systems

RHESSI uses nine Rotating Modulation Collimators (RMCs) for imaging, each consisting of a pair of grids mounted on the rotating spacecraft. The angular resolutions range from 2.3 arcsec to 3arcmin. The relative twist between the two grids of each pair is the most critical parameter. It must be less than 20 arcsec for the finest grid. After precision alignment, it is monitored by the Twist Monitoring System (TMS) to a few arcsec. The Sun-pointing must be known better than 0.4 arcsec for the image reconstruction. This is achieved by the Solar Aspect System (SAS), which consists of a set of three Sun sensors. Each sensor is focusing the filtered Sun light onto a linear CCD. The onboard Aspect Data Processor (ADP) selects the 6 limb positions, which over-define the pointing offset of the Sun center in respect to the imaging axis of the imager. The Roll Angle System (RAS) continuously measures the roll angle of RHESSI within arcmin accuracy. The RAS is a continuously operating CCD star scanner. The time of the passage of a star image over the CCD is recorded and defines the roll angle, comparing its pixel position and amplitude with a star map.

Energy Deposition and Hard X-Ray Source Motions in the 2002 July 23 γ-Ray Flare

Reconnection models of solar flares predict a systematic motion of hard X-ray (HXR) footpoints as magnetic energy is released and electrons are accelerated. While the correlation of the HXR flux with the apparent motion of the footpoints has previously been investigated, we derive and investigate for the first time the correlation between cumulative deposited energy at the footpoints and their separation. Providing excellent statistics, data from the Reuven Ramaty High Energy Solar Spectroscopic Imager of the 2002 July 23 flare are re-analyzed. The data show an excellent correlation for most of the time intervals. However, despite the good correlation, for some time ranges the derived amount of released magnetic energy is far too small to account for the energy in HXR-producing electrons.

Solar Aspect System (SAS) for the High-Energy Solar Spectroscopic Imager (HESSI)

The HESSI SAS is a set of three Sun sensors, which shall provide high bandwidth information on the solar pointing of the rotating spacecraft. The precision of <EQ 0.4 arcsec relative is necessary in order to obtain the HESSI imaging resolution of 2 arcsec; the absolute accuracy of 1 arcsec is required for comparison with other measurements. Each SAS is based on focusing the Sun through a narrow bandwidth filter on to a 2048-element x (13(mu) )2 linear CCD. A digital threshold algorithm is used to select N pixels that span each solar limb for inclusion in the telemetry. Determination of the 6 limb crossing locations provided by the 3 subsystems defines the position offset of the Sun in the rotating frame. In this paper we describe the mechanical and electronic configuration of the SAS FM and the results of the first test measurements.

RHESSI Aspect System and In-flight Calibration

Precise knowledge of the pointing and the roll angle of the rotating spacecraft is needed in order to reconstruct images with 2 arcsec resolution using the modulation patterns seen on each of the detectors of the bi-grid rotating collimators. Therefore, the aspect system consists of two subsystems of sensors, the Solar Aspect System (SAS) and Roll Angle System (RAS). The transmitted data consists of Solar limb data from the SAS (CCD pixels around the intersection of a Solar image with three linear CCDs) and Star event data from the RAS (CCD pixels induced by passages of Star images over a linear CCD). In order to meet the RHESSI requirements, the reconstructed pointing needs to be ≤ 0.4 arcsec (rms) relatively (≤ 1 arcsec absolutely) and the determination of the roll angle needs to be better than 1 arcmin (rms). Beside of understanding and calibrating each sensor, the error budget on the aspect system requires an alignment of the relevant features of the 1.55 m extended telescope on a micron level. This could be achieved by a combination of on-ground and in-flight calibration.

Roll Angle System (RAS) for the High-Energy Solar Spectroscopic Imager HESSI

The purpose of the HESSI RAS is to provide information on the roll angle of the rotation spacecraft. Precise knowledge of the roll angle is a necessary ingredient for image reconstruction. The RAS is a continuously operating star scanner that points out radially and observes stars at 75 degrees from the Sun direction using a commercial lens and a fast CCD. The passage of a star image over the CCD charges one or several pixels above threshold and the timing of this signal defines the roll angle, once the star has been identified by comparing its pixel position and amplitude with a star map. Roll angles at intermediate times are inferred by assuming uniform rotation. With a limiting star magnitude of mv equals 3 we expect to observe at least 1 star per revolution over 1 year; on the average we will detect about 10 stars/revolution.