Solar Wind Acceleration and Waves in the Corona

Solar Wind Acceleration and Waves in the Corona Perspectives for a spectrometer on Solar C/Plan A L. Teriaca Max-Planck-Institut für Sonnensystemforschung 09 – 12/03/10: Second Solar-C Science Definition Meeting

Coronal Holes & Fast Wind • It is well established that the fast wind emanates from the super-radially expanding polar CHs (Krieger et al. 1973; Woch et al. 1997; Mc. Comas et al. 2000). However, many important questions are still open. • Which structures, within polar CHs are the source regions of the wind? • What fraction of the wind is coming from open field structures where it is powered by waves induced by photospheric motions? • What fraction is coming from episodes of reconnection of closed loops with the open field?

Coronal Holes • Long lasting large-scale areas of reduced coronal emission characterised by mostly open magnetic field. EIT λ 17. 1 – T= 0. 9 MK Plumes • Denser and cooler (e. g. , Wilhelm 2006) than the surrounding corona, plumes are the prominent feature of the Coronal Holes characterizing the solar poles at activity minimum. • Plumes have been observed and studied since long time in white light during total eclipses (e. g. , Abbot 1900, Saito 1965, Koutchmy 1977) as linear structures rooted in coronal holes and extending towards the interplanetary space. Pasachoff et al. 2009 Composite 17. 1 and VL eclipse images (Wang et al. 2006).

Wind from Plumes or Interplumes? From Doppler dimming analysis: • At the base of the corona < 30 km/s in interplumes. No significant speed in plumes (Teriaca et al. 2003, Ap. J 588, 566). • Velocities in plumes exceed 60 km/s below 1. 5 R/RSun, larger than in interplumes (Gabriel et al. 2003, 2005). • It is generally agreed that interplumes are faster above 1. 5 R/RSun (Gordano et al 2000; Teriaca et al. 2003; Gabriel et al. 2005; Raouafi et al. 2007). From Doppler shift measurements: • On disk: Outflows from the darker regions in Ne VIII (0. 63 MK) (Wilhelm et al. 2000). Similar results from EIS data (Tian et al. 2010). • Off-disk: Small outflows in plumes (SUMER: Wilhelm et al. 1998; Feng et al. 2009; Teriaca et al. 2010).

Requirements for Doppler Shift measurements • Leaving the ecliptic plane, will allow direct measurements of the flows at the base of PCHs. With a LOS component of the flow speed of about 50% (for quasi-radial flows), it will be possible measuring flows of few km/s, e. g. , in Ne VIII 770 Å, δv < 2 km/s → Δλ ~30 mÅ/pixel. • High λ/Δλ → information on sub-resolution flows. • Spatial scale 1″ (Comparable to EIS and SUMER). • Improve time resolution by reaching exposure times of about 2 s on disk (at 1″ scale). • Large FOV to encompass the whole polar cap (1000″× 600″) at 40 degrees orbit inclination.

Waves • Waves (usually Alfvén waves) are a very promising mechanism for transporting the energy from the solar surface (where the waves are driven by the convective shuffling of the magnetic field lines) into the corona, where they are dissipated by turbulent processes (e. g. , Cranmer et al. 2007). • In general, waves with periods from 7 to 20 min have been detected in both plumes and interplumes above the limb (De. Forest & Gurman 1998; Ofman et al. 2000; Banerjee et al. 2000, 2001, 2009 a) and interpreted as slow magnetoacoustic waves. • Alfvénic fluctuations have been observed in the chromosphere (Hinode/SOT: De Pontieu et al. 2007) and in AR coronal loops (Co. MP: Tomczyk et al. 2007). • In CHs some evidence for Alfvén waves has been found by Banerjee et al. (1998, 2009 b), Dolla & Solomon (2008) from the measurement of line broadenings and by Gupta et al. (2010).

Requirements for waves detection Interplume – EIS Fe XII • A substantial improvement in the throughput (with respect to current spectrometers) could allow searching for higher frequencies and for wave signatures in width and velocity. • To put constraints on the wave nature high throughput and spectral resolution are required. Propagation speeds from ~ 130 km/s 20″ above the limb up to 330 km/s 160″ above limb. Periods from 15 to 30 min. Gupta et al. 2010 (submitted).

Off-limb studies: Requirements • Record the O VI 1032 and 1037 profiles. • Simultaneous detection of line pairs sensitive to densities between 106 and 108 cm-3. • Simultaneous detection of line pairs sensitive to temperatures around 106 K (e. g. , Mg IX 706/749). • Acquire all needed observables over the required FOV (600″× 600″) in a much shorter time by increasing throughput and by using multiple detectors (like CDS or EIS). • Very good stray-light performance.

Off-limb spectroscopy To observe the coronal emission with un-occulted spectrometers above 1. 2 R/RSun, we need a very good control of the stray-light (micro-roughness ~2 Ǻ rms). The introduction of a moveable occulter would allow observations up to 2 or 3 R/RSun opening up the large field of investigation we know from UVCS (see e. g. Teriaca et al. 2003 Reviews such as Kohl 2006; Cranmer 2009). However, it should be checked whether a substantial improvement with respect to UVCS can be achieved.

How do plumes form and evolve? • Plumes are sometimes associated to Coronal Bright Points (CBP) (Feng et al. 2009). • CBPs are mostly red-shifted (Madjarska et al. 2003), blue-shifted (Popescu et al. 2004), with no net shift (Wilhelm et al. 2000), with both (EIS: Tian et al. 2010). • Plumes seem to be associated to jets (Lites et al. 1999; Raouafi et al. 2008), but also Young et al. (1999)? • The time domain needs to be far better explored. • The magnetic field and atmospheric structure at the footprints need to be investigated.

Polar Coronal Jets • Polar jets are frequent (between 60 and 200/day) in XRT images (e. g. , Cirtain et al. 2007). • Are they relevant to the wind (but also Wang et al. 1998 for EUV/WL jets)? • Much faster raster scans to fully understand the dynamics of these events and measure their physical parameters under reduced line of sight effects. • Multi-thermal nature: simultaneous observations in lines at temperatures from chromosphere to corona. • It would benefit from an X-ray or EUV imager.

Spectroscopy for Plan-A Polar view Ecliptic view Ideal for studies of the on-disk coronal holes: • Doppler shifts at the base of the corona. • Relation with the magnetic field. • Increased spatial resolution (a factor ~2. 5 for feature at 70°). Ideal for studies of the off-limb corona: • Doppler Dimming: radial outflow speeds. • Off-limb profiles: waves and turbulence studies.

Spectroscopy for Plan-A Polar view • Combined studies with probes at different ecliptic latitudes (Earth orbit, Solar Orbiter) will Ecliptic view provide insight on the 3 -D of coronal structures. With Plan-A instrumentation, • a plume seen on the plane of sky from Earth will have its foot point region probed by measuring the photospheric magnetic field and the plasma dynamics. Changes in the latter parameters will be related to changes in the off-limb plume (waves, flows, jets). • science encompassing STEREO-like observations would become feasible at all angles between 0° and 40°.

Relation with Solar Orbiter • Its payload includes an imaging spectrograph with a movable occulter to observe the solar corona both on the solar disk and off limb out to 3 R/RSun. • It will undoubtedly address many of the open questions related to the creation and acceleration of the solar wind. • However, Solar Orbiter will be an encounter mission with limited observing time. • The challenging mission profiles imposes severe mass, size and telemetry constraints.

Advantages of the plan-A mission profile. • Plan-A foresees a final stable orbit of 1 AU radius and 40° inclination that provides stable thermal conditions. • Orbiting in synchrony with Earth provides a larger and more uniform telemetry rate (= more science). • Reasonable space and mass for a spectrometer. • All latitudes beween − 40° and +40° will be explored during each orbit.

Science and Synergies of plan-A • It will provide a more extended spatial and temporal coverage of the different structures that will be observed in rarer occasions by the Solar Orbiter spectrometer with higher spatial resolution. • Simultaneous observations with the Solar Orbiter spectrometer of the poles from likely different angles could provide insight on the 3 -D structure of the observed features. • It would provide a second point of view for combined studies with Solar Probe.

Conclusions • The polar view allows a much better study of the coronal hole regions. This is important not only for VUV spectroscopy (dynamics of the TR and low corona) but also for measurements of the polar magnetic field. A polar mission is, thus, scientifically very important. • The plan-A mission would offer systematic studies of the polar regions from different viewing angles throughout the solar cycle.

Conclusions • The above ecliptic point of view coupled with much higher time resolution, and good spectral resolution, would allow: – identifying the structures from where the fast wind originates. – Study their temporal evolution. – Detect wave signatures in width and velocity, posing constrains on the wave nature. • 13× 13 aperture, 130 cm focal, two elements spectrograph (such as EIS) would already provide 1″ over 600″× 600″ FOV.

Conclusions • Working at longer wavelengths would have several vantages: lines covering from chromosphere to corona, higher spectral resolution, a relatively large effective area (10 times that of SUMER at 1000 Å). • At least 2 wavelenght bands including the O VI lines (Doppler Dimming) and the Ne VIII 770 line (velocity fields at the base of the corona). • An additional band could be centered around the H I Lyman α line (including lines of Si III, C III, N V, Fe XII in the first order and He I, O V, Mg X, FE XIX in the second order). • Synergies with Solar Probe+ and Solar Orbiter.

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