Heliospheric studies with LOFAR and EISCAT-3D Andy Breen, Mario Bisi & Richard Fallows

1. Heliospheric studies with LOFAR and EISCAT-3D Andy Breen, Mario Bisi & Richard Fallows Aberystwyth University

2. Solar wind • Continuous supersonic expansion of the solar atmosphere • Fills interplanetary space • Driver for all solar-terrestrial disturbances ESA/NASA SOHO|LASCO McComus et al., 2008

3. Solar wind is primarily bimodal • Fast (~600-800 km s-1), low-density streams above large regions of open field on Sun • Relatively uniform • Slow (~250-400 km s-1), higher-density streams above bright coronal streamers • Highly structured • Many transient structures • Large transient structures released by solar eruptions or destabilisation of coronal loops • “Coronal mass ejections” ESA/NASA SOHO|LASCO NASA STEREO|SECCHI (RAL)

4. Large solar eruption • Release of stored magnetic energy • Burst of radiation • Ejection of matter/magnetic field (Coronal Mass Ejection – “CME”) Potentially highly geoeffective… … depending on whether the mass ejection passes over Earth and what the orientation of its magnetic field is. ESA/NASA SOHO EIT & LASCO

5. Significant evolution in solar wind structure between Sun and 1 AU • Interaction between streams with different speeds • Forms regions of compression and rarefaction • “Stream interaction regions” or “Co-rotating interaction regions” • Shocks form on stream boundaries by 1 AU – can be geoeffective • Interaction between solar wind transients and background wind • Transients (including CMEs) slowed or accelerated by interaction with solar wind • Changes path of transient • Smaller transients entrained by stream interaction regions • Larger ones (esp. CMEs) disrupt stream interaction regions • Structure of large transients changes through interaction with background wind • Over-expansion into faster wind • Magnetic field orientation can change

6. Smaller transients conspicuous in interplanetary observations • Appear to be associated with short-period magnetic field rotations (Dorrian et al., 2009) • May be capable of perturbing comet tails(?)

7. Science questions Evolution of large-scale solar wind structure • Interaction between CMEs and background wind • How rapidly are events accelerated/decelerated? • Interaction between CMEs and stream interaction regions All influence CME trajectory – will it hit the Earth? • Rate of over-expansion of CMEs into fast wind • Distortion of CME structure by interaction with background wind, stream interaction regions, other CMEs, smaller transients All influence CME structure and magnetic field orientation – will it be goeeffective if it hits the Earth? • Smaller transients – are they ubiquitous in the slow wind? • Origin/evolution? • Interaction with Earth’s magnetic field?

8. Requirements • Comparison of data and modelling needed to get at the underlying physics • Better data covering fine 3D structure of inner heliosphere needed to constrain models Potential data sources In-situ measurements (e.g. Messenger, Venus Express, ACE.. Measure primary physical parameters (e.g. velocity, density, magnetic field) Very limited spatial coverage Optical remote sensing (e.g.STEREO imagers, Solar Mass Ejection Imager) Radio remote sensing (radio bursts, radio scintillation)

9. Remote sensing of the solar wind White-light imaging • Visible-wavelength light from Sun scattered from electrons in solar wind • Thomson scatter • Intensity of scattered light from a volume proportional to Ne • Scattered intensity varies with viewing geometry • Distant scattering events appear fainter than nearer ones Radio scintillation observations (IPS) • Radio waves from distant astronomical sources scattered by density irregularities (few 10s to few 100s km scale) in solar wind • Phase variations in scattered waves converted to amplitude variation by interference • Variation in intensity (scintillation) roughly proportional to Ne2 • Scattering events nearer observer less clearly observed than further ones

10. White-light imaging and radio observations or, why do we need radio observations now we've got STEREO? Temporal resolution • STEREO HI cameras return images every 40 minutes (inner field, HI-1) or 2 hours (outer field, HI-2). • Radio scintillation (IPS) measurements can give density-proxy and bulk velocity estimates on < 10 minute cadences • IPS can reveal transient events better Different sensitivity to electron density • White light imagers – linear sensitivity to Ne • IPS – ~ Ne2 sensitivity • IPS better able to resolve faint structures Multi-site IPS measurements can detect other solar wind properties e.g. magnetic field rotation in CMEs/transients.... White-light imagery extremely good at giving context for events Complimentary techniques!

11. Some current IPS facilities – what they can do Ootacamund: single antenna, 560m x 30m, observes ~ >1000 sources/day at distances of 20-250 RSun • Can produce near-real time images of Ne proxy, bulk flow speed • Used as input to 3D tomographic reconstruction, yields 3D Ne proxy and velocity distributions with ~10° angular resolution Ootacamund radio telescope (P.K. Manoharan) Nagoya (STELab): 106m x 41m antenna, 100m x 20 m antenna, 74m x 27 m antenna, observes ~ 50 sources/day, inc. ~20-40 2-site observations at distances of 30-200 RSun • Can produce daily maps of Ne proxy, velocity • Good monthly maps of Ne proxy, velocity • Used as input to 3D tomography, ~20° angular resolution Fuji radio telescope (M. Tokumaru) EISCAT: 3 32m dishes, makes ~5 2-site observations /day at distances of ~15-100 RSun • Accurate measurements of velocity • Can detect other solar wind parameters e.g. field rotation • Even small number of long-baseline measurements greatly improve accuracy of tomographic reconstructions

12. Some limiting factors with IPS • Solar wind irregularities are “better” at producing scintillation when the radio waves are low frequency • Can get useful solar wind data from weaker radio sources at lower frequencies • Can get useful solar wind data further from Sun at lower frequencies (with same sources) • Many astronomical radio sources stronger at lower frequencies • Moving to lower frequencies can improve spatial coverage & resolution of measurements.. • .. particularly at distances from Sun where white-light imaging more difficult • Lower frequency observations harder to interpret closer to Sun • White-light imaging can help with these regions • As can IPS at higher frequencies LOFAR Hi-Band (~100-220 MHz) will give good inner heliospheric converage – as will EISCAT-3D

13. 3D velocity reconstruction from EISCAT IPS data (B.V. Jackson and M.M. Bisi) To study: • Internal structure of CMEs • CME/solar wind interaction • CME/SIR interaction • Evolution of mesoscale structure • Interaction of mesoscale structure with CMEs and SIRs • Interaction of solar wind structures with comets and planetary environments • Cometary and planetary tails Need at least as good spatial resolution (sources/day..) as Ootacamund, many more long-baseline 2-site observations/day than STELab or EISCAT

14. LOFAR LOFAR should provide all these things! Ample collecting area Plenty of combinations of 2-site observations Should be able to match Ootacamund’s number of source-observations/day, exceed 100 2-site observations/day (currently being verified!) ~5°angular resolution in tomographic reconstructions looks achievable with LOFAR data MWA will match (and probably exceed) number of source-observations/day, but won’t offer 2-site measurements Won’t be able to study physical parameters (turbulence, flow direction) that LOFAR will be able to detect

15. What’s needed IPS requires: • Rapid sampling rate (>50 Hz, ideally >100 Hz) • Wide receiver pass-band (> 10 MHz) Only total received power measurements are required • Want to observe as many sources/day as possible, on as many days as possible • Want to make many 2-site measurements • Experiment should run on “remote” (non-core) sites, ideally in background mode Need to safeguard non-core observing time for solar and heliospheric experiments IPS experiment for LOFAR needs building Initial input – data stream produced by generic solar/heliosphere mode running at each remote station? Need format for this data stream, sample data – and to start taking real data as soon as possible

16. What about EISCAT-3D? To operate on frequency close to LOFAR Hi-Band maximum (224 MHz) • Gives good potential for observing scintillation • Siting in northern Scandinavia provides excellent IPS baselines for high-resolution observations in conjunction with LOFAR sites Potential problems with ionospheric scintillation, particularly when looking at sources to the south (close to field line) Combination of LOFAR and EISCAT-3D could provide the best heliospheric observatory for the next 10+ years (certainly until SKA) Opportunities for significant science gain If interested, get in touch: azb@aber.ac.uk