Instrument Design STEIN Jasper Halekas and Davin Larson

Instrument Design: STEIN Jasper Halekas and Davin Larson Kyung-Hee Visit October 2009

Space Physics Instrumentation • Usually want to: – Maximise sensitivity – Maximise energy/angular resolution and coverage – Minimize weight/power – Separate different species of charged particles (and sometimes neutrals!)

Electrostatic Analyzer • Separates species very well, has very good energy/angular resolution • Low sensitivity, because of small geometric factor and need to sweep energy/angle • Can only measure one species in each detector

STEREO STE (Supra. Thermal Electrons) » Prototype detector • Very good sensitivity • Measures all energies simultaneously • Can measure ions, electrons, neutrals • But - have to have some way to separate species! • Thin window detector, very sensitive electronics, allows measurements to a few ke. V (few ke. V low energy threshold unprecedented for solid state detectors!)

er k r al a P pir ns S ctro e El STE-U w/FOV and Preamp Mounts to side of IMPACT Boom Field of view along Parker spiral – with solar wind out of FOV STE-U 70 x 70 STE-U FOV Solar Wind Ions

THEMIS SST Energy range > ~20 ke. V, electrons and ions separated

Basic design of SST instrument Al/Polyamide/Al Foil (stops ions <400 ke. V) Foil Collimator Attenuator Foil Detector Thick Detector Open Collimator Attenuator Sm-Co Magnet (sweeps away electrons <400 ke. V)

Foil Separation Energy Proton Projected Range Electron Projected Range 1 ke. V 3 x 10 -6 g/cm 2 2 ke. V 6 x 10 -6 g/cm 2 1 x 10 -5 g/cm 2 5 ke. V 1. 5 x 10 -5 g/cm 2 5. 4 x 10 -5 g/cm 2 10 ke. V 2. 7 x 10 -5 g/cm 2 1. 8 x 10 -4 g/cm 2 20 ke. V 4. 6 x 10 -5 g/cm 2 6 x 10 -4 g/cm 2 50 ke. V 9. 5 x 10 -5 g/cm 2 3 x 10 -3 g/cm 2

Toolchest 1: Interactions w/ Matter CASINO = " monte CArlo SImulation of electro. N trajectory in s. Olids "

Magnetic Deflection 1 cm B = 500 G X 2 cm pixelated (8 x 8) detector • Energy Range 2 ke. V – 50 ke. V (full angular coverage), Angular Range ± 30°. • Partial angular coverage for higher energies. • Poor angular resolution for lowest energies.

Magnetic Deflection

Magnetic Deflection

Toolchest 2: Finite Element Magnetostatics • Define a discrete grid and solve for the magnetic potential from each element • Linghua Wang used a commercially available program for this work

Toolchest 3: Tracing Particles The Lorentz Force Law provides the second order differential equation of motion for charged particles. We use the 4 th order Runge-Kutta method with an adaptive time step to solve this ordinary differential equation

Electrostatic Deflection 2 cm E-Field Region 4 2 2 cm 3 Electrons Ions W 1 L

Top View 5 cm 2 cm High Energy Ions and Electrons 3 cm Ions 4 mm Electric Field Region E = 100 -1000 V/mm Electrons Side View

Ion Pixel Electron Pixel High Energy Pixel Electrons Ions E = 100 V/mm V = 400 V

Realistic Potential Distribution

Toolchest 4: Finite Difference Electrostatics • We set the electrostatic potential of electrodes/deflectors • Then, in free space, the potential must satisfy Laplace’s equation • Davin Larson wrote code to solve on a grid using a finite difference method

Particle Trajectories in Realistic Potential Distribution

Trajectories: Collimators Electrostatic Deflectors Pixelated Detectors (Mounted Back-to-Back) 5 cm Solar Orbiter STE Low Energy Electrons Low Energy Ions Neutrals, High Energy Electrons and Ions

Energy-Angle Response for Edge Pixels

Energy-Angle Response for Middle Pixels

Center Pixel Response (Electrons) Edge Pixel Response (Electrons) Deflection Voltage Left Sweep 4 k. V 1. 5 k. V Right Sweep 600 0 600 1. 5 k. V 4 k. V • Symmetric response for ions • Neutrals measured in center pixel – cleanly separated for energies below ~15 -20 ke. V • Logarithmic voltage sweep with ten steps from 600 V to 4 k. V is optimal for covering phase space. Need positive and negative sweeps to get all angles, so 20 voltage steps desired.

Telemetry • For edge pixels need 20 voltage steps – To deconvolve, prefer linear energy bins from 2 -15 ke. V (no need to go higher for edge pixels) – Total sweep = 20 V*14 E*8 pixels*8 bits = 17920 bits • For center pixels, can sum all voltage steps (not true if you want to do neutrals). – Need 20 logarithmic energy bins to cover 3 -100 ke. V at energy resolution of 0. 2 – Total distribution = 20 E*8 pixels*8 bits = 1280 bits • Total instrumental bits per sweep = 19200 – 200 bps gives 96 s Time Resolution • For CINEMA, Time Tag all Events

Total Instrumental Count Rates at 1 AU (scale up by 25? at 0. 2 AU) Shows need for Attenuator for Solar Orbiter Similar Issue for Auroral Zone for CINEMA Quiet Time Un-Attenuated Big SEP Event Un-Attenuated Red = Edge Pixel Black = Middle Pixel Quiet Time Attenuated Big SEP Event Attenuated