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SECCHI/Heliospheric Imager Science Studies Sarah Matthews Mullard

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SECCHI/Heliospheric Imager Science Studies Sarah Matthews Mullard Space Science Laboratory University College London Holmbury St. Mary, Dorking Surrey RH5 6NT [email protected] Version 2, 12 December 2003 1. Introduction This document is intended to provide a focus for the scientific operation of the Heliospheric Imager camera systems that form part of the SECCHI instrument payload on STEREO. Its purpose is to provide a brief introduction to the instruments and their capabilities, within the context of the SECCHI instrumentation as a whole, and to outline a series of proposed science studies whose primary focus requires the HI1 and/or HI2, although this does not preclude the inclusion of studies whose main focus is another of the SECCHI instruments. In addition to defining the science goals for the HI these science studies also provide operational constraints that as far as possible will be fed into the on-board software requirements. In order to obtain a detailed view of the instruments, the science goals and operations, it is recommended that this document should be used together with the Heliospheric Imager Operations Document written by Richard Harrison and the Image Simulation document written by Chris Davis & Richard Harrison. 2. SECCHI and the Heliospheric Imager SECCHI is a set of remote sensing instruments designed to follow Coronal Mass Ejections (CMEs) from their origins on the Sun, out through the corona and the interplanetary medium and to possible impact with the Earth. The instrument package comprises 3 telescopes: ÿ EUV Imaging Telescope (EUVI) - a full Sun instrument which images the chromosphere and corona in 4 emission lines: He II 304 A, Fe IX/X 171 A, Fe XII 195 A and Fe XV 284 A. ÿ COR 1 and COR 2 - two white-light coronagraphs to observe the inner (1.1-3 solar radii) and outer (2-15 solar radii) corona at high temporal and spatial resolution, and provide polarization information. ÿ Heliospheric Imager (HI1 and HI2) - these are externally occulted coronagraphs that image the inner heliosphere between the Sun and the Earth between 12-215 solar radii. STEREO is the first mission dedicated to understanding the physics of CMEs and their effects on the Earth's environment. The mission's overall science objectives are as follows: ÿ Understand the causes and mechanisms of CME inititation ÿ Characterize the propagation of CMEs through the Heliosphere ÿ Discover the mechanisms and sites of energetic particle acceleration in the low corona and the interplanetary medium ÿ Improve determination of the structure of the ambient solar wind Within these objectives SECCHI has its own primary science goals to: ÿ Determine the 3D structure of coronal loops, coronal streamers and large-scale coronal structures ÿ Determine the 3D properties of CMEs ÿ Determine the timing of physical properties involved in CME initiation ÿ Determine the critical forces controlling propagation of CMEs in the corona and interplanetary medium. The HI instruments, with their unique FOV, will provide the first direct imaging observations of CMEs in the inner heliosphere. As such they will directly address the issue of the forces that control CME evolution and propagation in the corona inner heliopshere, as well as providing insights into the mechanisms and sites of particle acceleration in this region and space weather alerts. 3. Heliospheric Imager characteristics A detailed description of the design of the Heliospheric Imager can be found in the operations document and in Socker et al., 2000. The basic concept uses occultation and a baffle system to achieve the light rejection levels necessary to provide wide-angle views of the heliosphere. The two cameras are centred on the Earth-Sun line with two circular fields of view of 20o (HI-1) and 70o (HI-2), offset from the Sun at 13.65o and 53.35o, respectively. This allows us to view the whole Sun-Earth line from 12 solar radii to near Earth orbit. The detectors are CCDs of 2048x2048 13.5 micron pixels which are usually binned on board to give images of size 1024x1024 pixels, with a resolution of 70 arcsec (HI-1) and 4 arc mins (HI-2). Nominal exposure times are in the range 12-20 s for HI-1 and 60-90s for HI-2. Note that the nominal cadence for each camera is 60 min (HI-1) and 120 min (HI-2). The driver behind these numbers is the need to accumulate sufficient signal to noise and to remove cosmic rays from each exposure prior to summing to produce the image. These characteristics are summarised in Table 1 of the operations document, which we reproduce below for convenience. HI-1 HI-2 Instrument Type Externally occulted Externally occulted coronagraph coronagraph Centre of Field-of-view Along Sun-Earth line Along Sun-Earth line Direction q = 13.65 degrees q = 53.35 degrees Angular field-of-view 20 degrees 70 degrees Coronal coverage 12-84 Rsun 66-318 Rsun Overlap with COR-2 12-15 Rsun N/A Overlap with HI-1 N/A 66-84 Rsun Baseline image (2x2 1024x1024 1024x1024 binning) Image pixel scale 70 arcsec 4 arc min (binned) (binned) Spectral bandpass 630 – 730 nm 400 – 1000 nm Exposure time 12 –20 s 60 – 90 s Nominal images per 70 60 sequence Required cadence 60 min 120 min -15 -16 Brightness sensitivity 3 x 10 Bsun 3 x 10 Bsun -13 -14 Straylight rejection 3 x 10 Bsun 10 Bsun Brightness accuracy 10% 10% Table 1:Characteristics of the Heliospheric Imager 4. Science scenarios The default mode of operation of STEREO is a synoptic one , with the synoptic operation of the HI based on the values given in Table 1. However, SECCHI, and the HI in particular, can be used to answer a wide range of scientific questions, some of which will fall outside of the scope of the synoptic operational mode. In order to assess the range of operational requirements that may be needed, from an HI perspective in particular, the UK SECCHI/STEREO science team has considered a range of different scientific problems and the instrumental and operational caveats associated with them. To ensure a degree of uniformity a schema was provided for people that outlined the basic instrumental choices and the synoptic fields of view, exposure times, cadences etc. In addition people were asked to consider whether co-ordination with other instruments on STEREO was required, or with other spacecraft and ground-based facilities, since these clearly raise more complicated planning issues. The resulting 15 science questions are listed below in Table 2 along with their authors and institutes. The scientific justification and detailed observational requirements can be found in section 5 of this document. Study Author 1. Impact of CME on the Earth Richard Harrison (RAL) 2. CMEs in interplanetary space Peter Cargill (IC) 3. Understanding how observations Peter Cargill (IC) at L1 & SECCHI are related 4. 3-D structure of interplanetary CMEs Lucie Green (Cardiff) 5. CME onset Sarah Matthews (MSSL) 6. The relationship between CMEs and Sarah Matthews (MSSL) magnetic clouds 7. Particle acceleration at CME shocks Sarah Matthews (MSSL) 8. Synoptic CME programme Richard Harrison (RAL) 9. Solar wind microstructure Andy Breen (Aberystwyth) 10. Development of co-rotating interaction Andy Breen (Aberystwyth) regions 11. Differential drift velocities in the fast & Andy Breen (Aberystwyth) slow solar winds 12. Boundary regions between fast & slow Andy Breen (Aberystwyth) streams in the solar wind 13. Remote solar wind speed & direction Geraint Jones (IC) measurements from 3-D observations of cometary ion tails 14. Interplanetary acceleration of ICMEs Mathew Owens (IC) 15. Beacon mode Sarah Matthews (MSSL), Richard Harrison & Chris Davis (RAL) Table 2: list of science scenarios 5. Science scenarios Observation Title: Impact of CME on the Earth Name: Richard Harrison Institute: RAL E-mail: [email protected] Version Date: 1 May 2002 Brief Scientific Objective and Observation Overview: One of the principal aims of STEREO is to enable a better understanding of the arrival and impact of CMEs on the Earth. Thus, a major goal is the direct observation of a CME event arriving at Earth. For this, the prime instrument here is HI2, which has the Earth and CME within its field of view. However, the support of HI1, COR1 and COR2 is required to provide the most complete understanding of the CME structure and evolution as it propagates from the Sun. Sequence Details EUVI: [Bands - He II 304, Fe IX 171, Fe XII 195, Fe XV 284 Å; Resolution - 1.6 arcsec/pixel; FOV - 0.9 deg Sun-centred; Nominal cadence 30 s] ÿ Required (yes/no) - No ÿ Which bands? - n/a ÿ Image cadence? - n/a ÿ FOV (full Sun, partial Sun (define area and pointing)) - n/a ÿ Other details - n/a COR1: [Bandpass - 650-660 nm (brightness and pB); Resolution - 7.5 arcsec/pixel; FOV - 1.3-4.0 R (full revolution; Sun centred); Nominal cadence 20 s (3 images/min in different polarisation angles)] ÿ Required (yes/no) - Yes ÿ Image cadence? nominal ÿ FOV (full or partial field (define area and pointing)) - full ÿ Other details COR2: [Bandpass - 650-750 nm (brightness and pB); Resolution - 15 arcsec/pixel; FOV - 2-15 R (full revolution; Sun centred); Nominal cadence 100 s] ÿ Required (yes/no) - Yes ÿ Image cadence? - nominal ÿ FOV (full or partial field (define area and pointing)) - full ÿ Other details HI1: [Bandpass - 650-750 nm (brightness); Resolution - 35 arcsec/pixel; FOV - 20o circle centred at 13.28o to Sun centre (3.28-23.28o along ecliptic); Nominal cadence 1 hr] ÿ Required (yes/no) - Yes ÿ Image cadence? - nominal ÿ FOV (full or partial field (define area and pointing)) - full ÿ Other details HI2: [Bandpass - 400-1000 nm (brightness); Resolution - 240 arcsec/pixel; FOV - 70o circle centred at 53.36o to Sun centre (18.36-88.36o along ecliptic); Nominal cadence 2 hr] ÿ Required (yes/no) - Yes ÿ Image cadence? - 2 hr ÿ FOV (full or partial field (define area and pointing)) - see details Other details Cadence sized for fast (1000 km/s) CME travelling from 73 to 212 solar radii, i.e.
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