David Alexander
Rice University

Thierry Appourchaux
Ins9tut d’Astrophysique Spa9ale

The Team
T. Appourchaux1, P. Liewer2, M. WaB3, D. Alexander4, V. AndreBa5, F. Auchère1, P. D’Arrigo3,J.Ayon2, T. Corbard6, S. Fineschi5, W. Finsterle7, L. Floyd8, G. Garbe9, L. Gizon10, D. Hassler11,L. Harra12, A. Kosovichev13, J. Leibacher1,14, M. Leipold15, N. Murphy2, M. Maksimovic16, V.
Mar9nez‐Pillet17, S.A. MaBhews12, R. Mewaldt18, D. Moses19, J. Newmark19, S. Régnier20, W.
Schmutz8, D. Socker19, D. Spadaro21, M. StuBard3, C. Trosseille1, R.Ulrich22, M.Velli2,A.Vourlidas19, R. Wimmer‐Schweingruber23, T. Zurbuchen24
1) Ins9tut d’Astrophysique Spa9ale, Orsay, France, 2) Jet Propulsion Laboratory / California
Ins9tute of Technology, 3) EADS Astrium, Stevenage, UK, 4) Rice University, 5) INAF/
Osservatorio Astronomico di Capodimonte, Napoli, Italy, 6) Observatoire de la Côte d’Azur,
Nice, France, 7) PMOD/WRC, Davos Dorf, Switzerland, 8) Interferometrics Inc., Virginia, 9)Na9onal Space Science and Technology Center, 10) Max‐Planck‐Ins9tut fürSonnensystemforschung, Katlenburg‐Lindau, Germany, 11) Southwest Research Ins9tute,Boulder, Colorado, 12) Mullard Space Science Laboratory, UK, 13) Stanford University, 14)
Na9onal Solar Observatory, 15) Kayser‐Threde GmbH, München, Germany, 16) Observatoirede Paris‐Meudon, France, 17) Ins9tuto de Astrohsica de Canarias, La Laguna, Spain, 18)
California Ins9tute of Technology, 19) Naval Research Laboratory, 20) University of St
Andrews, UK, 21) INAF / Osservatorio Astroﬁsico di Catania, Italy, 22) University ofCalifornia, Los Angeles, California 23) Chris9an‐Albrechts‐Universität, Kiel, Germany, 24)
University of Michigan

Solar Polar Imager: Observing Solar Ac<vity from aNew Perspec<ve
D. Alexander1, PauleU Liewer2, J. Ayon2, L. Floyd3, G. Garbe4, B. Goldstein2, D. Hassler5, A. Kosovichev6,R. Mewaldt7, D. Moses3, N. Murphy2, M. Neugebauer8, D. Socker3, S. Suess4, R. Ulrich9, M. Velli2, A.
Vourlidas3, T. Zurbuchen10, A. Sandman1
Our current understanding of the Sun and its atmosphere is severely limited by a lack of observa<ons of the polar regions. The Solar
Polar Imager mission uses solar sail propulsion to place a spacecraE in a 0.48 AU circular orbit around the Sun with an inclina<on of 75°.
This ﬁrst direct view of the polar regions of the Sun enables crucial observa<ons not possible from the usual eclip<c viewpoint and will
revolu<onize our understanding of the internal structure and dynamics of the Sun. The rapid 4 month polar orbit and the combined insitu and remote sensing instrument suite allows unprecedented studies of the link between the Sun and the solar wind and solar
energe<c par<cles. Moreover, SPI can serve as a pathﬁnder for a permanent solar polar sen<nel for space weather predic<on in supportof the Explora<on Ini<a<ve.
The Solar Polar Imager will provide the capability to:•••••••••Monitor Earth‐directed CMEs from high la<tudes
Greatly improve models of the global heliosphere using beUer magnetogram coverage in longitude and la<tude
Observe ac<ve regions for much longer than 13 days
Monitor ARs before they “appear” around the east limb
Provide beUer coverage of AR sources of CMEs and SEPs
Yield more complete informa<on on subsurface ﬂows and evolving ARs.
Provide in‐situ measurements of magne<c ﬁelds, solar wind and SEPs out of the eclip<c plane
Determine the la<tudinal dependence of the total solar irradiance variability
Monitor Mars‐directed CMEs from high la<tude

In this paper we report on the results of a recent Vision Mission study reﬁning the science objec<ves, instrument suite, orbit andtrajectory analysis, solar sail characteris<cs and measurement strategy. The SPI provides an important step in improving ourunderstanding of the physics governing solar variability on long and short <me‐scales and on local and global scales.

1Rice University, 2Jet Propulsion Lab., 3Naval Research Lab., 4Marshall Space Flight Center, 5Southwest Research Ins9tute,
6Stanford University, 7California Ins9tute of Technology, 8University of Arizona, 9University of California Los Angeles,
10University of Michigan

MissionSpacecraft in highly
inclined ~75°
heliocentric orbit
0.48AU
Uses solar sail to reachhigh inclination
Science?

Dynamo:Helioeismolgy & magnetic
fields of polar regions
Polar view ofcorona,CMEs, solar
irradiance

Artificial polar view -- magnetogram + LASCO

Link high latitude
solar wind &energetic particles tocoronal sources

POLARIS: High Level Science Goals• What is the rela9onship between the magne9sm anddynamics of the Sun’s polar regions and the solardynamo?
• What advantages does the polar perspec9ve provide forspace weather predic9on?
• What is the azimuthal structure and dynamics of thecorona and CMEs?
• How are varia9ons in the solar wind linked to the Sun at
all la9tudes?
• How are solar energe9c par9cles accelerated andtransported in radius and la9tude?
• How does the solar irradiance vary with la9tude?

POLARIS is designed to observe thepolar regions for extended periods of9me.
Concept heritage from
Solar Polar Imager
POLARIS: ESA Cosmic Vision concept
SPI: NASA Vision Mission concept

POLARIS ‐ Mission Concept
Uses Solar Sail to attain 75° inclination
orbit at 0.5 AU in ~5-7 years• Spiral in to 0.48 AU (2-3 years), then crank up
to 75° final orbit• In situ data collected during cruise• Imaging data collected after final orbit
attained and sail is jettisoned• 29% of time in final orbit is above 60°
Instruments• Helioseismograph (velocities,magnetic field)• Coronagraph (CMEs, coronal structures, blobs)• EUV (2 channels; structure & flares for context)• UV Spectrometer (coronal base outflows)• Solar wind protons & electrons (SW variations)• Solar Energetic Particles (impulsive & gradual)• Magnetometer (in situ magnetic field)• Tolar solar irradiance variability

Payload Accommodation• Total Instrument Mass ~97.5 kg• Average data rate >60 kbps
• Store &dump data, ~2 DSN passes/week• Gimbaled antenna for uninterrupted
helioseismology observations

RD1 and RD2 are two separate sail studies which consider diﬀerent performance sails.

POLARIS Goal 1:

What is the rela<on between the magne<sm and dynamics of theSun’s polar regions and the solar cycle?
Major Goal: Knowledge of the high‐la9tude meridional circula9on and diﬀeren9al rota9on is
crucial for understanding magne9c ﬂux transport, polar ﬁeld reversals, solar cycle models
and predic9ons. These are the main ingredients of the solar dynamo.

( rotation averages)

Magnetic Field Evolution:
Magnetic fields first appear
associated with ARs. Meridional
flow moves fields poleward.
Trailing spot is poleward of theleading spot. Causes reversal of
the global dipole field.
Torsional Oscillations: Solardifferential rotation undergoes acyclic pattern of increased anddecreased rotation rate called“torsional oscillations”. Global
magnetic field evolution is closely
related to the torsional oscillations.

Courtesy: Roger Ulrich

POLARIS Goal 1 (con<nued):

What is the rela<on between the magne<sm and dynamics of theSun’s polar regions and the solar cycle?
Current State of Knowledge
Solar Rotation Rate from SOHO/MDI• Internal structure and diﬀeren9al
rota9on measured with great precision
except in the polar regions (75‐90ºla9tude), the energy‐genera9ng core
and the thin subsurface“superadiaba9c” layer. (Globalhelioseismology)• Large‐scale dynamics inves9gated indetails in the mid‐la9tude zone from‐60 to 60 degrees in the upper
convec9on zone, 30 Mm deep. (Localhelioseismology)
RPOLARIS can ﬁll in polar regions and, Solar rota9on rate (Ω/2π nHz) with uncertain
in conjunc9on with SDO or GONG,polar and deep regions shown in white (basedon Schou 1998).
measure ﬂows in deep interior?

POLARIS Goal 1 (con<nued):

What is the rela<on between the magne<sm and dynamics of theSun’s polar regions and the solar cycle?
•••

What are the surface & subsurface ﬂow paBerns in the polar regions?
What are the internal ﬂows deep in the convec9on zone that drive the solar
dynamo? (POLARIS + SDO or GONG)
How do the solar magne9c ﬁelds evolve?

– What are the magne9c ﬁelds in the polar regions?
– How is the ﬂow and dissipa9on of magne9c energy in the corona aﬀected by theinterior and surface ﬂows? (POLARIS Uses EUV + coronagraph to observe coronalresponse to ﬂows)

POLARIS will measure (using both global and local helioseismology)••••

Diﬀeren9al and torsional oscilla9ons at high la9tudes, any polar jets or vor9ces
Meridional circula9on, la9tudinal and longitudinal structures, secondary cells, rela9onship to ac9ve
longitudes, magne9c ﬂux transport
Supergranula9on and large‐scale convec9on paBerns in polar regions (super‐rota9on, wave‐likebehavior, network, ﬂux transport, rela9onship to coronal holes)
Tomography of the deep interior (POLARIS + SDO or GONG)

11/18/08

12

POLARIS Goal 1 (con<nued):

What is the rela<on between the magne<sm and dynamics of theSun’s polar regions and the solar cycle?
What is the large-scale circulation pattern in polar
regions and its role the magnetic field dynamics?

What is the high-latitudestructure of the meridional
circulation?

What is the shape ofthe tachocline, prolate?
Converging flows around
active regions play important rolein 11/18/08
their evolution.

How deep is the meridional flow?
13POLARIS Goal 2:

Coronagraph/EUV Science Goals•Primary– Observe GLOBAL eﬀect of CMEs•

– Follow EUV Waves/Shocks over thecomplete solar surface– Determine true longitudinal extent ofCMEs/Streamers•– Locate SEP injection sites

Accurate mass//density/energy determina9on– 3D reconstruc9on of CMEs•

– Direct observations of geoeffectiveflares

BeBer determina9on of CME substructures– 360˚ Evolu9on of Streamer Belt

– 3D structure of erupting prominenceor loop arcades– Plume/Coronal Hole structure••

3D plume structure (orbit)
Coronal sources of plumes– Follow active region evolution overfull rotation (360˚?)– Current Sheet/Streamer structure••

Filamentary? Uniform density?

Secondary– Momentum loss in Solar Wind
– Ini9a9on & Evolu9on of Earth‐directedCMEs.

CoronagraphPrimary•

Secondary– Determine spatial relation between
CMEs and their coronal sources– Image the network/corona at poles

EUV Imager

POLARIS Goal 2:

What is the azimuthal structure and dynamics of the coronaand CMEs?

EUV Imager: EUV Simulations
Earth View

South Pole

North Pole

Alexander & Sandman

EUV Imager: EUV Simula<ons

Eclip<c View

POLARIS View

EUV Imager: X‐ray Simula<ons

POLARIS View

POLARIS Goal 2:

Synergy with Magnetograph andHelioseismology Goals
EUV IMAGER
connectivity of magnetic field through coronal emission, subject to optical-thin l-o-s
integration, assuming 171Å or 195Å option.
coronal response to field dynamics (emergence, magnetoconvective motions)coronal hole boundary locations for refining global field topology.
polar crown arcade observations to determine large-scale neutral lines near poles

CORONAGRAPH
large-scale (‘global’) magnetic topology
coronal response to “field-reversal” or poleward drift of field

POLARIS Goal 3:

How are varia9ons in the solar wind linked to the Sun at all
la9tudes?
• Can we identify coronal sources of slow wind?
• What is the underlying cause of the variabilityof the slow wind?
• What are the processes that create the slowwind?
• Can a direct connection be made between insitu and remote observations of plumes orstreamers?
POLARIS can:

POLARIS will address these ques9ons
beDer than exis9ng & planned missions.

 sample slow wind closer to Sun and polar
field lines less wound provide rapid (4 months) latitude scans atfixed radius yield more accurate mapping using
magnetograms with increased coverage inlongitude and latitude including pole!
 combine in situ, coronagraph, EUV and UV
spectrometer information

POLARIS Goal 4:

POLARIS will allow much better
determination of SEP event onset timeand solar source to address thesequestions because path length tosource is shorter (closer, less field linewinding) and dispersion & scattering
less.
• Requires SW plasma, energetic particle
instruments, magnetometer, EUV (flareonset) & coronagraph (CME height vs. time)

Time (hours)

How are solar energe9c par9cles accelerated and transported
in radius and la9tude?

Inverse velocity (c/v)

Velocity dispersion of observed energeCc parCcles,
extrapolated to determine SEP event injecCon Cme.
Shows injecCon >1 hour aLer ﬂare. Suggests SEPS due toCME shock in low corona, not ﬂare. (Mewaldt et al)

POLARIS Goal 5:

What advantages does the polar perspec9ve provide for space
weather predic9on?
POLARIS can serve as proving ground for a permanent polar orbiter for
space weather prediction in support of the Exploration Initiative• Monitor Earth-directed CMEs from high latitudes (get height-time

profiles for “halo” CMEs)• Greatly improve models of the global heliosphere using better
magnetogram coverage in longitude and latitude (better BCs)
• See some active regions much longer than 13 days
• See some ARs before “appear” on east limb.
• More frequent observations of AR sources of CMEs, SEPs
• Much more complete information on subsurface flows and evolving
ARs.
• Monitor Mars-directed CMEs from high latitude

POLARIS Goal 6:

How does the solar irradiance vary with la9tude?
All current measurements of Total Solar Irradiance (TSI) from ecliptic
plane- Measure TSI variations of ~0.1%.
- Why is the variation ~1/3 of variationin other Sun-like stars? Are we seeing themfrom the polar view point?
- Is missing ecliptic radiation funneled out throughthe polar regions or does the Sun store thisenergy?
- Define a irradiance/dynamo relationship forapplication to stars

Measurements of TSI variation withlatitude will answer a basic question insolar physics/astrophysics and constrainmodels of TSI
Note: Ac9ve cavity TSI instruments are notoriously diﬃcult to calibrate, but measure varia9ons in TSI very
accurately. Goal of SPI solar irradiance instrument is ONLY to measure VARIATIONS in irradiance. Eliminates need forcomplex calibra9ons and simpliﬁes instrument

Instrument Payload
The remote-sensing instruments are:
• Dopplergraph and magnetograph imager [MDI, HMI, SoLO heritage]
The Doppler and Stokes imager will provide solar radial velocity every 45 seconds (in orderto resolve the helioseismology oscillations), with a pixel resolution of 4”, and vector
magnetic field maps of the whole Sun every five minutes with a pixel resolution of 2”.

• White-light coronagraph [COR2 heritage]
The optical design has been modified to incorporate a 5-element external occulter and toachieve good stray light performance over a 16° field of view with a 60-cm long instrument.
The coronagraph will observe the corona from 1.5 to 15 solar radii with 28” pixels.

• EUV imager [SoLO heritage, new Fresnel zone- based instrument]
Two telescopes with baselined lines centered at 195 and 304 Å respectively. full-disk
observations of the corona out to 1.4 RSun with 2.6” pixels• UV spectrograph [RAISE heritage]

The spectrograph includes a single-element off-axis parabola mirror and a toroidal variable
line space grating with an instantaneous field of view of the entrance slit of 10” x 85’ (fullsolar disk at 0.48 AU), with a spatial resolution of 10”, corresponding to 3700 km on theSun. A single raster scan can produce a full-Sun spectroheliogram in 17 minutes. The
baseline design we have chosen covers the 1166Å – 1266Å wavelength range.

• Total Solar Irradiance monitor
The POLARIS TSI Monitor is a new generation, room-temperature absolute radiometerbased on the electrical substitution principle (ESR). It employs 3 cavities that are all pointed
towards the Sun but can be shaded individually.

All imaging instruments will have a typical pixel resolution of 2 to 4 arcseconds.

Instrument Payload
The in‐situ instruments are:
• Magnetometer• Solar‐wind ion composi9on and electron spectrometer• Energe9c par9cle package• Radio and plasma wave package
Table 1 summarizes the scien9ﬁc objec9ves that are addressed by each instrument,
while Table 2 sums up the required resources. All remote instruments shall be nolonger than one meter.

Coronagraph/EUV Imager
Mini‐COR

CCD
Assembly

Radiator

A0 Aperture

Tube Diameter = 132 mm

Total Length = 605 mm
Total Mass = 5.25 Kg

SecondaryMirror

Primary Mirror(130mm)Al Filter& Shutter

FPA2kx2k CCD

see Vourlidas

One-shot Door

Length 700 mm
Mass 5.6 Kg

EUV Imager

Implica<ons/Lessons for Solar‐C Plan A
When considering polar orbits, mission
becomes more like a planetary mission:
Time to science orbit much longer thansolar physicists are used to.

Implica<ons/Lessons for Solar‐C Plan A~30% of orbit > 60° la9tude : ~50% of orbit > 30° la9tude

Implica<ons/Lessons for Solar‐C Plan A
Ra9onale for as high an inclina9on orbit as possible:
If you are going to look at the pole do it properly
Increase observa9on 9me above la9tude of interest (say 30°)
360° view of interes9ng la9tudes for ﬂux emergence andcycle studies
Increased viewing 9me of features of interestImplica<ons/Lessons for Solar‐C Plan A

Good situa9on for stereoscopic helioseismology (see Gizon)

Implica<ons/Lessons for Solar‐C Plan A
Circular vs. ellip9cal orbit (e.g. e=0.3 SEP op9on)

PROSCONSCIRCULARELLIPTICAL• Fixed solar distance• Uniform orbital velocity• “ﬁxed” radial velocity• thermal issues cleaner• Longer dwell 9mes
at highest
inclina9ons (but onlyin one hemisphere)• Diﬀerent viewing
angles than Earth• Higher res. atperihelion• Limited 9me at highla9tude• Poten9ally not ideal forstereoscopy• Telemetry• Thermal issues atperihelionImplica<ons/Lessons for Solar‐C Plan A
If helioseismology/dynamo/polar magne9c ﬁeld isprimary science then this should drive the orbit
design and instrument choices:‐ single instrument mission?