In this paper, we consider the modelling of diffracted stray light in heliospheric imagers. The emphasis is on the imagers proposed by RAL Space as part of the phase A/B1 study for ESA’s Vigil (formerly called Lagrange) L5 monitoring mission. In order to handle the extreme diffraction angles, a one-dimensional version of the PROPER diffraction modelling library has been developed. This is used to compute patterns at the lens aperture, and the standard two-dimensional version is then used to continue propagation to the sensor plane. The effects of key instrument and modelling parameters are analysed with a view to optimize accuracy of the modelling and the diffraction performance of the instrument.

In two earlier articles (Tappin, Eyles and Davies in Solar Phys. 290, 2143, 2015, and Solar Phys. 292, 28, 2017), we used the stellar photometry to determine the calibration parameters and long-term trends of the Heliospheric Imagers (HI) on board the Solar Terrestrial Relations Observatory (STEREO). In this article we provide an update on these determinations for the ahead spacecraft (STEREO-A) to incorporate the interval after solar superior conjunction (when STEREO-B was non-operational). We describe the modifications needed to our photometry procedures to accommodate the reduced pointing stability following the switch-off of the spacecraft gyros shortly prior to conjunction. We find a small revision to the absolute levels is required. We also show that the very low rates of degradation (less than 0.2% per year) found in the earlier determinations have continued beyond solar conjunction.

Magnetars are strongly magnetized, isolated neutron stars1-3 with magnetic fields up to around 1015 gauss, luminosities of approximately 1031-1036 ergs per second and rotation periods of about 0.3-12.0 s. Very energetic giant flares from galactic magnetars (peak luminosities of 1044-1047 ergs per second, lasting approximately 0.1 s) have been detected in hard X-rays and soft γ-rays4, and only one has been detected from outside our galaxy5. During such giant flares, quasi-periodic oscillations (QPOs) with low (less than 150 hertz) and high (greater than 500 hertz) frequencies have been observed6-9, but their statistical significance has been questioned10. High-frequency QPOs have been seen only during the tail phase of the flare9. Here we report the observation of two broad QPOs at approximately 2,132 hertz and 4,250 hertz in the main peak of a giant γ-ray flare11 in the direction of the NGC 253 galaxy12-17, disappearing after 3.5 milliseconds. The flare was detected on 15 April 2020 by the Atmosphere-Space Interactions Monitor instrument18,19 aboard the International Space Station, which was the only instrument that recorded the main burst phase (0.8-3.2 milliseconds) in the full energy range (50 × 103 to 40 × 106 electronvolts) without suffering from saturation effects such as deadtime and pile-up. Along with sudden spectral variations, these extremely high-frequency oscillations in the burst peak are a crucial component that will aid our understanding of magnetar giant flares.

Rouillard, A. P., Davies, J. A., Forsyth, R. J., Rees, A., Scott, C. J., Harrison, R. A., Lockwood, M., Bewsher, D., Crothers, S. R., Eyles, C. J., Hapgood, M. and Perry, C. H. (2008) First imaging of corotating interaction regions using the STEREO spacecraft. Geophysical Research Letters, 35 (10). L10110. ISSN 0094­8276 doi: https://doi.org/10.1029/2008GL033767 Available at http://centaur.reading.ac.uk/7508/

The Heliospheric Imagers (HI) on the Solar TErrestrial RElations Observatory (STEREO) observe the solar wind and disturbances therein as it propagates from close to the Sun to 1 AU and beyond. In this article we use stellar photometry over much of the mission to date to make a determination of the long-term evolution of the photometric response of the inner (HI-1) cameras. We find very slow degradation rates of the order of 0.1 % per year, similar to those found for HI-2 by Tappin, Eyles and Davies (Solar Phys. 290, 2143, 2015) and significantly slower than rates found for other comparable instruments. We also find that it is necessary to make a small (\({\approx}\,1~\%\)) revision to the photometric calibration parameters used to convert instrument units into physical units. Finally, we briefly discuss the effects of pointing instabilities on the measurement of stellar count rates.

In this article we describe the methods used to determine the photometric calibration parameters for the outer Heliospheric Imagers (HI-2) onboard the Solar Terrestrial Relations Observatory (STEREO) spacecraft from measurements of background stars, and we present those values that represent small corrections to the values predicted from pre-launch calibrations. Conversion factors to physical units are also derived. We determine the degradation of these instruments over the course of the mission to date; this is found to be around an order of magnitude slower than for white-light instruments on other spacecraft. We compute a correction to the large-scale flatfield for HI-2A, allowing for vignetting in the outer parts of the images. In addition, we consider the effects of pixel saturation and the implications for the use of the HI-2 instruments for stellar photometry. We also discuss the limitations of the currently employed geometrical projection assumptions.

The Solar Mass Ejection Imager (SMEI) was the first of a new class of heliospheric and astronomical white-light imager. A heliospheric imager operates in a fashion similar to coronagraphs, in that it observes solar photospheric white light that has been Thomson scattered by free electrons in the solar wind plasma. Compared with traditional coronagraphs, this imager differs in that it observes at much larger angles from the Sun. This in turn requires a much higher sensitivity and wider dynamic range for the measured intensity. SMEI was launched on the Coriolis spacecraft in January 2003 and was deactivated in September 2011, thus operating almost continuously for nearly nine years. Its primary objective was the observation of interplanetary transients, typically coronal mass ejections (CMEs), and tracking them continuously throughout the inner heliosphere. Towards this goal it was immediately effective, observing and tracking several CMEs in the first month of mission operations, with some 400 detections to follow. Along with this primary science objective, SMEI also contributed to many and varied scientific fields, including studies of corotating interaction regions (CIRs), the high-altitude aurora, zodiacal light, Gegenschein, comet tail disconnections and motions, and variable stars. It was also able to detect and track Earth-orbiting satellites and space debris. Along with its scientific advancements, SMEI also demonstrated a significantly improved accuracy of space weather prediction, thereby establishing the feasibility and usefulness of operational heliospheric imagers. In this paper we review the scientific and operational achievements of SMEI, discuss lessons learned, and present our view of potential next steps in future heliospheric imaging.

Gamma-Ray Bursts (GRBs) are the most energetic explosions in the universe, their optical photon flux rise very quickly, typically within one minute, then fall off gradually. Hundreds of GRBs optical light curves have been measured since the first discovery of GRB in 1967. However, only a handful of measurements have been made within a minute after the gamma ray signal. Because of this drawback, the short-hard type GRBs and rapid-rising GRBs, which may account for 30% of all GRBs, remain practically unexplored. To reach sub-minute timescales, the Ultra-Fast Flash Observatory (UFFO) uses a rapidly moving mirror to redirect the optical beam instead of slewing the entire spacecraft. The first realization of this concept is UFFO-pathfinder, which is equipped with fast-response Slewing Mirror Telescope (SMT) and a UFFO Burst Alert and Trigger Telescope (UBAT). SMT has a slewing mirror to redirect optical photons into a telescope and then record them by an intensified CCD. UBAT uses coded mask to provide X-ray trigger from a GRB and provides the GRB location for SMT. UFFOs sub-minute measurements of the optical emission of dozens of GRBs each year will result in a more rigorous test of current internal shock models, probe the extremes of bulk Lorentz factors, provide the first early and detailed measurements of fast-rise GRB optical light curves, and help verify the prospect of GRB as a new standard candle. The UFFO-pathfinder is fully integrated with the Lomonosov satellite and is scheduled to be launched in late 2013 or early 2014. We will present the latest progress in this conference.

We present the lessons learned about the degradation observed in several space solar missions, based on contributions at the Workshop about On-Orbit Degradation of Solar and Space Weather Instruments that took place at the Solar Terrestrial Centre of Excellence (Royal Observatory of Belgium) in Brussels on 3 May 2012. The aim of this workshop was to open discussions related to the degradation observed in Sun-observing instruments exposed to the effects of the space environment. This article summarizes the various lessons learned and offers recommendations to reduce or correct expected degradation with the goal of increasing the useful lifespan of future and ongoing space missions.

Since the advent of wide-angle imaging of the inner heliosphere, a plethora of techniques have been developed to investigate the three-dimensional structure and kinematics of solar wind transients, such as coronal mass ejections, from their signatures in single- and multi-spacecraft imaging observations. These techniques, which range from the highly complex and computationally intensive to methods based on simple curve fitting, all have their inherent advantages and limitations. In the analysis of single-spacecraft imaging observations, much use has been made of the fixed ϕ fitting (FPF) and harmonic mean fitting (HMF) techniques, in which the solar wind transient is considered to be a radially propagating point source (fixed ϕ, FP, model) and a radially expanding circle anchored at Sun centre (harmonic mean, HM, model), respectively. Initially, we compare the radial speeds and propagation directions derived from application of the FPF and HMF techniques to a large set of STEREO/Heliospheric Imager (HI) observations. As the geometries on which these two techniques are founded constitute extreme descriptions of solar wind transients in terms of their extent along the line of sight, we describe a single-spacecraft fitting technique based on a more generalized model for which the FP and HM geometries form the limiting cases. In addition to providing estimates of a transient's speed and propagation direction, the self-similar expansion fitting (SSEF) technique provides, in theory, the capability to estimate the transient's angular extent in the plane orthogonal to the field of view. Using the HI observations, and also by performing a Monte Carlo simulation, we assess the potential of the SSEF technique.

The SECCHI Heliospheric Imager (HI) instruments on-board the STEREO spacecraft have been collecting images of solar wind transients, including coronal mass ejections, as they propagate through the inner heliosphere since the beginning of 2007.The scientific use of the images depends critically on the performance of the instruments and its evolution over time. One of the most important factors affecting the performance of the instrument is the rejection of straylight from the Sun and from other bright objects located both within and outside the HI fields of view.This paper presents an analysis of the evolution of the straylight-rejection performance of the HI instrument on each of the two STEREO spacecraft over the three first years of the mission. The straylight level has been evaluated and expressed in mean solar brightness units, in which such scientific observations are usually quoted, using photometric conversion factors.

The aim of this paper is to determine whether the photometric response of the STEREO HI-1A and HI-1B Heliospheric Imagers has evolved or degraded during the mission to date. This is done using the methodology of Bewsher et al. (Solar Phys. 264, 433, 2010) to calculate the photometric calibration factor, μ, over the four complete STEREO heliocentric orbits between the start of the science mission operations in April 2007 and June 2011, and to evaluate whether this parameter has undergone any significant variation. It is shown that the photometric response of the instruments has remained stable to within ∼ 1%. The value of μ across different parts of the field of view is also determined to evaluate the validity of the calibrated large-scale flatfield determined by Bewsher et al. (Solar Phys. 264, 433, 2010). Based on the results, new revised values for the photometric calibration factor and conversion factors for diffuse sources are presented.

The Heliospheric Imager (HI) instruments on board the STEREO spacecraft are used to analyze the solar wind during August and September 2007. We show how HI can be used to image the streamer belt and, in particular, the variability of the slow solar wind which originates inside and in the vicinity of the streamer belt. Intermittent mass flows are observed in HI difference images, streaming out along the extension of helmet streamers. These flows can appear very differently in images: plasma distributed on twisted flux ropes, V‐shaped structures, or “blobs.” The variety of these transient features may highlight the richness of phenomena that could occur near helmet streamers: emergence of flux ropes, reconnection of magnetic field lines at the tip of helmet streamers, or disconnection of open magnetic field lines. The plasma released with these transient events forms part of the solar wind in the higher corona; HI observations show that these transients are frequently entrained by corotating interaction regions (CIRs), leading to the formation of larger, brighter plasma structures in HI images. This entrainment is used to estimate the trajectory of these plasma ejecta. In doing so, we demonstrate that successive transients can be entrained by the same CIR in the high corona if they emanate from the same corotating source. Some parts of the streamers are more effective sources of transients than others. Surprisingly, evidence is given for the outflow of a recurring twisted magnetic structure, suggesting that the emergence of flux ropes can be recurrent.

Imaging of the heliosphere is a burgeoning area of research. As a result, it is awash with new results, using novel applications, and is demonstrating great potential for future research in a wide range of topical areas. The STEREO (Solar TErrestrial RElations Observatory) Heliospheric Imager (HI) instruments are at the heart of this new development, building on the pioneering observations of the SMEI (Solar Mass Ejection Imager) instrument aboard the Coriolis spacecraft. Other earlier heliospheric imaging systems have STEREO Science Results at Solar Minimum Guest Editors: Eric R. Christian, Michael L. Kaiser, Therese A. Kucera. O.C. St. Cyr. R.A. Harrison (IS) • J.A. Davies A.P. Rouillard C.J. Davis • C.J. Eyles • D. Bewsher • S.R. Crothers Space Science and Technology Department. STFC Rutherford Appleton Laboratory, Chilton, Didcot. Oxfordshire, OX11 OQX UK e-mail: r.harrison@rl.ac.uk A.P. Rouillard Space Environment Physics Group, School of Physics and Astronomy. University of Southampton. Southampton, SO 17 1BJUK C.J. Eyles School of Physics and Astronomy, University of Birmingham. Birmingham, BI5 2TT UK C.J. Eyles Laboratorio de Procesado de Imagenes, Universidad de Valencia, 46071 Valencia, Spain D. Bewsher • D.S. Brown • G.D. Dorrian Institute of Mathematics and Physics. Aberystwyth University, Penglais, Aberystwyth, SY23 3BZ UK R.A. Howard • N.R. Sheeley • A. Vourlidas Space Science Division. Naval Research Laboratory, Washington DC, USA D.F. Webb Institute for Scientific Research, Boston College, Chestnut Hill, MA, USA D.F. Webb Air Force Research Laboratory. Hanscom. AFB. MA. USA 20100719182 <£} Sprii igei 220 R.A. Harrison et al. included ground-based interplanetary scintillation (IPS) facilities and the photometers on the Helios spacecraft. With the HI instruments, we now have routine wide-angle imaging of the inner heliosphere, from vantage points outside the Sun-Earth line. HI has been used to investigate the development of coronal mass ejections (CMEs) as they pass through the heliosphere to 1 AU and beyond. Synoptic mapping has also allowed us to see graphic illustrations of the nature of mass outflow as a function of distance from the Sun in particular, stressing the complexity of the near-Sun solar wind. The instruments have also been used to image co-rotating interaction regions (CIRs), to study the interaction of comets with the solar wind and CMEs, and to witness the impact of CMEs and CIRs on planets. The very nature of this area of research which brings together aspects of solar physics, space-environment physics, and solar-terrestrial physics means that the research papers are spread among a wide range of journals from different disciplines. Thus, in this special issue, it is timely and appropriate to provide a review of the results of the first two years of the HI investigations.

With the advent of the NASA STEREO mission, we are in a position to perform unique investigations of the evolution of coronal mass ejections (CMEs) as they propagate through the heliosphere, and thus can investigate the relationship between CMEs and their interplanetary counterparts, so-called interplanetary CMEs (ICMEs). ICME studies have been principally limited to single-point, in-situ observations; interpretation of the in-situ characteristics of ICMEs has been used to derive a range of ICME properties which we can now confirm or refute using the STEREO imaging data. This paper is a review of early STEREO CME observations and how they relate to our currently understanding of ICMEs based on in-situ observations. In that sense, it is a first glance at the applications of the new data-sets to this topic and provides pointers to more detailed analyses. We find good agreement with in-situ-based interpretations, but this in turn leads to an anomaly regarding the final stages of a CME event that we investigate briefly to identify directions for future study.

The aim of this paper is to calculate an accurate large-scale flatfield for the STEREO HI-1 instruments. This is done by analysing the variation in intensity of stars in the background starfield as they pass across the CCD. In order to use the background starfield, a photometric calibration is performed which defines a HI magnitude scale and a conversion between this scale and measured intensity. The photometric calibration uses stellar spectra folded through the instrument response to make initial intensity predictions. However, a secondary prediction method based on the photometric calibration, which blends the R-, V- and B-magnitudes of a star, is derived for stars with no spectral information.To perform this calibration, an initial analysis of the shape of the point spread function was required. This indicated that the PSF for the HI-1s is well approximated by a Gaussian function and does not vary substantially from the centre of the field-of-view to the corners and is essentially constant over time.In addition, a solar spectrum folded through the instrument response is used to determine conversion factors to convert from HI intensity units into mean solar brightness, S10 and SI units, for diffuse or extended sources.