Instrumental oscillations in RHESSI count rates during solar flares

¹ Solar Physics LaboratoryHeliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt MD, 20771 USA
e-mail: a.r.inglis@gmail.com
² Space Research Institute, Russian Academy of Sciences, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
³ School of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK
⁴ Centre for Fusion, Space and Astrophysics, University of Warwick, Coventry, CV4 7AL, UK
⁵ School of Mathematics and Statistics, University of St Andrews, St Andrews, KY16 9SS, UK
⁶ Central Astronomical Observatory at Pulkovo of the Russian Academy of Sciences, 196140 St Petersburg, Russia
⁷ Wyle Information Systems Inc., McLean, VA 22102, USA

Received: 14 December 2010
Accepted: 6 April 2011

Abstract

Aims. We seek to illustrate the analysis problems posed by RHESSI spacecraft motion by studying persistent instrumental oscillations found in the lightcurves measured by RHESSI’s X-ray detectors in the 6–12 keV and 12–25 keV energy range during the decay 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 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.