In-situ LF electric and magnetic field measurements of the solar wind expansion: Solar Orbiter capabilities Thomas Chust1, Fouad Sahraoui1, Alessandro Retinò1, Matthieu Berthomier1, Paul Leroy1, Alexis Jeandet1, Ondrej Santolik2,3, Andris Vaivads4, Volodya Krasnoselskikh5, Milan Maksimovic6 and Stuart D. Bale7 (1) LPP/CNRS-Ecole Polytechnique-UPMC-P11, Palaiseau–St Maur, France (2) Institute of Atmospheric Physics, Czech Academy of Sciences, Prague, Czech Republic (3) Charles University, Prague, Czech Republic (4) Swedish Institute of Space Physics, Uppsala, Sweden (5) LPC2E/CNRS, Orléans, France (6) LESIA, Observatoire de Paris, Meudon, France (7) Department of Physics and Space Sciences Laboratory, University of California Berkeley, USA

Outlines Electron kinetics Ions kinetics Turbulence Solar Orbiter mission and Radio & Plasma Waves (RPW) instrument RPW Low Frequency Receiver (LFR) Science objectives Electron kinetics Ions kinetics Turbulence Science requirements Overall description and working modes Engineering model Conclusion / Summary

Solar Orbiter Solar Orbiter Exploring the Sun-Heliosphere Connection First medium-class mission of ESA’s Cosmic Vision 2015-2025 programme, implemented jointly with NASA Dedicated and combined payload of 10 remote-sensing and in-situ instruments measuring from the photosphere into the solar wind Nominal launch 2017

Solar Orbiter Exploring the Sun-Heliosphere Connection Mission Summary Launch Date: 2017 Cruise Phase: 3 years Nominal Mission: 3.5 years Extended Mission: 2.5 years Orbit: 0.28–0.91 AU (P=150-168 days) Out-of-Ecliptic View: Multiple gravity assists with Venus to increase inclination out of the ecliptic to >24° (nominal mission), >34° (extended mission) Reduced relative rotation: Observations of evolving structures on solar surface & in heliosphere for almost a complete solar rotation

RPW Instrument Overview Will allow the characterization of the electric and magnetic fields associated to the dynamics of the near-Sun heliosphere from near DC up to 20 MHz Main Electronic Box (MEB) Electric Antennas (ANT) 3xV V LF Bias Unit Sp W 1 V 1HF Floating volt age dr iver V 5xV BIAS 2HF V 3HF 3xV HF TNR-HFR V Auto & cr oss-spectr a Sp W 1LF (4kHz-20MHz) V 1xB HF 2LF V V V 2 3LF 3 3xV Nom. SpW HF TDS to/from S/C 1xB HF Wavefor m @ 500kS/s Sp W 3xV BIAS + LFR Redundancy RPW-DPU Search Coil Magnetometer 3xB LF (SCM) Red. SpW to/from S/C B 2LF 5xV LFR BIAS Waveform up to 25kS/s Sp W B 3LF + Auto & cross- spectr a 3xB LF + k- vector (~DC-10kHz) B 1LF 3.3V 2. 5V 5V +/ -1 2V B 1HF 28 V LVPS-PDU from S/C Low Frequency Receiver

LFR Science Objectives Study the electromagnetic/electrostatic activity in the extended corona and the near-Sun solar wind, from near DC to 10 kHz. Will cover the electron gyrofrequency and most of the Doppler- shifted frequencies of the low frequency/small scale plasma waves, structures & turbulence. Kinetic or inertial Alfven waves Ion cyclotron waves Ion acoustic waves Magnetosonic or whistler mode waves ... Their characterization and the determination of their respective role in heating and accelerating the solar wind, during its expansion, is the main scientific issue addressed by LFR. Role of the low frequency/small scale plasma waves & structures associated to solar wind disturbances, e.g. interplanetary shocks, current sheets...

Doppler-Shifted Spatial Scales whistler mode waves → a fraction of fce ion acoustic waves → possibly up to 10 kHz and more electron Larmor radius → 100-1500 Hz electron inertial length → 80-600 Hz proton Larmor radius → 1-50 Hz Δ𝑓= 𝑉 𝑆𝑊 λ (k ρ = 1)

Kinetics of the Solar Wind Electrons Fundamental role of the field/particle interactions. Role of electron-scale (k┴ ρe ≥ ~1), low-frequency (~fp ≤ f ≤ ~fe) plasma processes (waves, structures, turbulence, …) ? Core (95 %) bi-Maxwellian Halo Isotropic Strahl Magnetically field-aligned Stverak et al., JGR, 2009

Radial Evolution of the Electron Core Temperature Continuous heating of the SW electrons ? Helios + Ulysses Stepan Stverak, Thesis Double-adiabatic CGL laws : 𝑇= 𝑇 ∥ 𝑇 ⊥ 2 1 3 ∝ 𝑅 −4 3 BUT : 𝑣 𝑡ℎ𝑒 ≥ 𝑉 𝑆𝑊 Thus, it is not possible to conclude ... 0.3 4

Evidence for Instabilities Shaping the Electron VDFs Helios, Cluster, Ulysses Data set : ~ 125,000 samples from 0.3 to 4 AU Stverak et al., JGR, 2008 β ∥𝑒 = 𝑛 𝑒 𝑘 𝑏 𝑇 ∥𝑒 𝐵 0 2 2 μ 0 Similar for protons (Kasper et al., SW Ten, 2003; Hellinger et al., GRL, 2006; Marsch et al., AG, 2006) Strong departures from isotropy are constrained by e.m. instabilities?

Radial Evolution of the Electron VDF Components Helios, Cluster, Ulysses Data set : ~ 240,000 samples from 0.3 to 4 AU Stverak et al., JGR, 2009 During expansion, strahl is transformed into halo by wave-particle interactions ? 0.3 4 Similar to fast solar wind (Maksimovic et al., JGR, 2005)

Continuous Heating of the Solar Wind Protons Fundamental role of field/particle interactions. Proton magnetic moment with distance to Sun 𝑣 𝑡ℎ𝑝 ≪ 𝑉 𝑆𝑊 Role of proton-scale (k┴ ρp ≥ ~1), low-frequency (~fp ≤ f ≤ ~fe) plasma processes (waves, structures, turbulence, …) ? Thus, for a stationary SW expansion, CGL laws should apply ... (E. Marsch 1991)

Evidence for Instabilities Shaping the Proton VDFs Wind Slow solar wind (< 600 km/s) core protons Data set: 1995–2001 Proton cyclotron Mirror Hellinger et al., GRL, 2006 Ωp Ωp % ∥𝑝 = 𝑛 𝑝 𝑘 𝑏 𝑇 ∥𝑝 𝐵 0 2 2 μ 0 Similar for electrons (Stverak et al., JGR, 2008) Parallel fire hose Oblique fire hose Strong departures from isotropy are constrained by e.m. instabilities?

Radial Evolution of the SW Proton Temperature Anisotropy Helios, Ulysses Proton cyclotron instability Mirror instability Fast solar wind (> 600 km/s) protons (all) Matteini et al., GRL, 2007 CGL adiabatic prediction Strong departures from isotropy are constrained by e.m. instabilities? Extended heating needed for the protons ? Parallel fire hose β ∥𝑝 = 𝑛 𝑝 𝑘 𝑏 𝑇 ∥𝑝 𝐵 0 2 2 μ 0 Oblique fire hose

Ion Kinetics and Micro-turbulence Wind Solar wind expansion and compression drive the proton distributions towards pressure anisotropy instability thresholds: 1. Alfven/Ion-cyclotron 2. Mirror mode 3. Firehose instabilities Wind measurements (> 1M) show δB fluctuations associated with these instability thresholds, suggest mirror and oblique firehose. These instabilities inject fluctuation power directly at kρi~1 (in contrast to the turbulent cascade). mirror oblique fire hose β ∥𝑝 = 𝑛 𝑝 𝑘 𝑏 𝑇 ∥𝑝 𝐵 0 2 2 μ 0 Bale et al., PRL, 2009

Enhanced proton temperatures near the instability thresholds Parallel heating near firehose threshold Perpendicular heating near Ion Cyclotron/Mirror threshold Wind Kasper et al., ArXiv, 2009

Turbulent Heating by KAW / Whistler Waves? Turbulence: “energy” flows from large to small scales This is manifested by power-law spectra connecting these scales LFR range of observation ? ? ? 𝑘 ⊥ ρ 𝑒 ∼1 In SW at 0.3-1 AU : (Δf ~ 1-50 Hz) (Δf ~ 100-1500 Hz)

Evidence for a ┴ Cascade up to the Proton Scale Cluster wavelet Phase speed is ~Alfven speed Dispersion at short wavelength is like Kinetic Alfvén Wave Strong evidence/consistency with KAW turbulence FFT E B (FGM data) (22 samples / sec) Evidence for a turbulent dissipation at the proton gyroscale? What above kρp ~ 1 ? Cascade up to the electron scale? ~ phase speed (dispersion like KAW) Bale et al., PRL, 2005 (also Sahraoui et al., PRL, 2009)

Evidences for a Cascade well above the Proton Scale Cluster A first case study Another similar study (several cases) SCM spectral data SCM waveform data FGM SCM noise floor Alexandrova et al., PRL, 2009 Sahraoui et al., PRL, 2009 Evidences of a cascade up to the electron gyroscale? What after ? Exponential spectral feature or power law? Observational limitations : noise floor both for δE and δB …

Observational Limitations : Signal to Noise Ratio Cluster (SCM waveforms) Distribution of the spectral indices (magnetic data) Solar Wind (moderate SNR) (10<SNR<20) SNR=10log 𝑆 𝑠𝑖𝑔𝑛𝑎𝑙 𝑆 𝑛𝑜𝑖𝑠𝑒 @ 30 Hz Magnetosheath (high SNR) (10<SNR<40) [Sahraoui et al., ApJ, 2013; Huang et al., arXiv, 2013]

LFR Science Requirements (1) Characterizing the LF electromagnetic waves & fields in the solar wind involves the capability of LFR : to cover turbulence and plasma instabilities to distinguish solitary waves from broadband wave activity to identify the wave modes at work (if any) Performing a multi-component analysis of the data is thus mandatory, using either a classical Fourier analysis or another treatment more appropriate to turbulence analysis. Transmission of waveforms (not only auto & cross spectra). Good frequency and time resolutions are needed. High SNR (instrument resolution) also needed !

LFR Science Requirements (2) Magnetic field fluctuations Electric field fluctuations maxima of the magnetic level values observed with Helios This is the required level for LFR strong electric field measurement (Fix the dynamics of LFR magnetic field measurements) SCM noise floor (LF band) This is the required level for LFR weak electric field measurements SCM noise floor (HF band) LFR have to be able to measure it

LFR Decimation and Processing Strategy 8 ADCs @ 98 304 Hz decimation down to ß 24 576 Hz ( f0 ) (14 bits ideally) :32 :3 (16 bits) (15 bits) (16 bits) (16 bits) 24 576 Hz 4 096 Hz 16 Hz 256 Hz shaping :3 :2 :64 :4 [20 bits ?] [18 bits ?] ( f1 ) ( f3 ) ( f2 ) :4 ( f0 ) 2  E 3  B 1  V 2  E 3  B 2  E 3  B 1  V 2  E 3  B 1  V 2  E (3  B) 2  E 3  B 1  V 2  E 3  B FFT FFT FFT (15 bits) Spectral matrices (ASM) Waveforms (WF) Spectral matrices (ASM) Waveforms (WF) Waveforms (WF) Spectral matrices (ASM) Waveforms (WF) Basic parameters (BP) Basic parameters (BP) Basic parameters (BP)

Current set of Basic Parameters “Instantaneous” 5 x 5 spectral matrix (256 FFT points) Time Averaged Spectral Matrix (ASM) 𝐀𝐒𝐌 ω 𝑗 𝑚 = 1 𝑁 𝑆𝑀 𝑚 𝑘=1 𝑁 𝑆𝑀 𝑚 𝐒𝐌 𝑘 ω 𝑗 𝑚 = 𝐒𝐌 𝑡𝑖𝑚𝑒 𝐒𝐌 ω 𝑗 𝑚 = 𝐵 1 𝐵 1 ∗ 𝐵 1 𝐵 2 ∗ 𝐵 1 𝐵 3 ∗ 𝐵 1 𝐸 1 ∗ 𝐵 1 𝐸 2 ∗ 𝑐𝑐 𝐵 2 𝐵 2 ∗ 𝐵 2 𝐵 3 ∗ 𝐵 2 𝐸 1 ∗ 𝐵 2 𝐸 2 ∗ 𝑐𝑐 𝑐𝑐 𝐵 3 𝐵 3 ∗ 𝐵 3 𝐸 1 ∗ 𝐵 3 𝐸 2 ∗ 𝑐𝑐 𝑐𝑐 𝑐𝑐 𝐸 1 𝐸 1 ∗ 𝐸 1 𝐸 2 ∗ 𝑐𝑐 𝑐𝑐 𝑐𝑐 𝑐𝑐 𝐸 2 𝐸 2 ∗ Frequency average ... 𝐒 ω 𝑗 𝑚 = 𝐀𝐒𝐌 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 ... before computations of the BPs (i.e. wave parameters) BP1 set 1: Power spectrum of the magnetic field (B) BP1 set 2: Power spectrum of the electric field (E) BP1 set 3: Wave normal vector (from B) BP1 set 4: Wave ellipticity estimator (from B) BP1 set 5: Wave planarity estimator (from B) BP1 set 6: XSO(radial)-component of the Poynting vector BP1 set 7: Phase velocity estimator BP2 set 1: Autocorrelations BP2 set 2: Normalized cross correlations Mono-k assumption : (Means, JGR, 1972) (Samson & Olson, GJRA, 1980) 𝐧×𝐄 = ω 𝑘 𝐁 𝑆 𝑖𝑗 𝑆 𝑖𝑖 𝑆 𝑗𝑗

LFR Spectral Frequencies Depending on the frequency channel, selection of 96, 104 or 88 consecutive frequency bins among 128 (NFFT = 256) of the time averaged spectral matrices. Then, the ASMs are averaged over packets of Nfreq (8 or 4) consecutive bins : Δ 𝑓 𝑚 = 𝑓 𝑚 𝑁 𝐹𝐹𝑇 × 𝑁 𝑓𝑟𝑒𝑞 𝑁 𝑓𝑟𝑒𝑞 =8 f3 = 16 Hz => waveform [DC, 8Hz] f3 / 2.5 = 6.4 Hz f2 = 256 Hz => 12 frequencies [6.5Hz, 102.5Hz] Δ f (2) = 8 Hz f2 / 2.5 = 102.4 Hz f1 = 4096 Hz => 13 frequencies [88Hz, 1752Hz] Δ f (1) = 128 Hz f1 / 2.5 = 1638.4 Hz f0 = 24576 Hz => 11 frequencies [1584Hz, 10032Hz] Δ f (0) = 768 Hz f0 / 2.5 = 9830.4 Hz 10-1 100 101 102 103 104 Hz 6.4Hz continuous waveform 6.5Hz 102.5Hz 96 bins 88Hz 1752Hz 104 bins 1584Hz 10032Hz 𝐒= 𝐀𝐒𝐌 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 88 bins

LFR Normal Mode (1) Basic Parameters sampling frequency ... BP: 1080 bps WF: 2734 bps ASM: 32 bps TM: 3846 bps Basic Parameters sampling frequency BP1 & BP2 BP1 & BP2 BP1 BP1 BP1 ASM BP1 4 SMs 64 SMs 384 SMs 4 SMs 64 SMs 384 SMs 4 SMs 64 SMs 384 SMs 4 SMs 64 SMs 384 SMs 4 SMs 64 SMs 384 SMs 4 SMs 64 SMs 384 SMs 4 SMs 64 SMs 384 SMs f0 = 24576 Hz ... TBP1_0= 4 s ... f1 = 4096 Hz ... TBP1_1= 4 s ... f2 = 256 Hz ... TBP1_2= 4 s ... ... 4 s continuous WF ... f3 = 16 Hz 20 s time

WaveForms & Averaged Spectral Matrices LFR Normal Mode (2) WaveForms & Averaged Spectral Matrices TASM= 3600 s sampling frequency TWF= 300 s WF BP1 ASM BP1 WF 384 SMs 384 SMs 384 SMs f0 = 24576 Hz 1/12 s ... ... ... 64 SMs 64 SMs 64 SMs f1 = 4096 Hz 1/2 s ... ... ... 4 SMs 4 SMs 4 SMs f2 = 256 Hz 8 s ... ... ... 2048 pts ... 4 s ... continuous WF ... f3 = 16 Hz time

LFR Selected Burst Mode 1 BP: 12672 bps WF: 393216 bps ASM: 0 bps TM: 405888 bps sampling frequency BP1 & BP2 BP1 & BP2 BP1 BP1 BP1 BP1 BP1 24 SMs 24 SMs 24 SMs 24 SMs 24 SMs 24 SMs 24 SMs f0 = 24576 Hz ... TBP1_0= 0,25 s ... 0,25 s f1 = 4096 Hz ... continuous WF ... 1 s time

LFR Selected Burst Mode 2 BP: 5760 bps WF: 24576 bps ASM: 0 bps TM: 30336 bps sampling frequency BP1 & BP2 BP1 & BP2 BP1 BP1 BP1 BP1 BP1 96 SMs 96 SMs 96 SMs 96 SMs 96 SMs 96 SMs 96 SMs f0 = 24576 Hz ... TBP1_0= 1 s ... 16 SMs 16 SMs 16 SMs 16 SMs 16 SMs 16 SMs 16 SMs f1 = 4096 Hz ... TBP1_1= 1 s ... f2 = 256 Hz ... 1 s continuous WF ... 5 s time

LFR Engineering Model FPGA ADCs (RTAX4000 emulator) November 2013

Conclusion / Summary LFR is one of the subsystems of the Solar Orbiter Radio and Plasma Waves (RPW) instrument. Will perform the on-board digital processing of the low frequency electromagnetic field & wave data (1V, 2E, 3B), measured in the extended corona and the near-Sun solar wind. Frequency range of investigation: ~0.1 Hz < f < 10 kHz. [ microphysics of the SW (heating, turbulence, VDF, ...)] Will combine different output data: from low-level (WF) to high-level (SM & BP) processed data. [ assessment of k => dedopplerization ] Will allow to increase the scientific return for a given telemetry by implementing different strategies for analyzing and transmitting the data.