E1 Plasma Detectors

The E1 Helios plasma data is extremely valuable data. It was the only publicly available dataset from the inner heliosphere, between 0.3 and 1 AU, up until Parker Solar Probe (PSP) and Solar Orbiter (SO). This dataset is particularly important for inter-calibration and inter-comparisons with plasma datasets from the recent PSP and SO missions.

Experiment E1 investigated the characteristics of the solar wind plasma. Four instruments measured different plasma properties; three detectors (I1a, I1b, and I3) measured the properties of protons and heavier ions and the fourth detector (I2) measured electrons. The measurements were taken at different energy ranges, azimuth ranges, and elevation ranges.

Instrumentation Summary

  • I1a instrument provides 3D ion distribution functions. It measures count rates of positive ions and reports a distribution function as a function of Ep (energy assuming all particles are protons).
    • Corrections to the distribution bins are explained in Stansby’s paper [2]
    • Measurement range: 0.155 — 15.32kV (32 channels)
    • Azimuth range: -54.5° — 32.7° (16 channels)
    • Elevation range -20° — 20° (9 channels)
  • I1b instrument provides 1D (reduced) ion distribution functions. It measures the ion current and, like I1a, creates a distribution report out of energy-per-charge (E/q) bins.
    • Measurement range: 0.145 — 14.32kV (32 channels)
    • Azimuth range: -56.25° — 118° (1 channels)
    • Elevation range -40° — 40° (1 channels)
  • I2 instrument provides 2D electron distribution functions. It measures electrons in two different modes (A & B — manually toggle). The only difference is the energy channel spacing. "There is only 1 elevation bin, so effectively a 2D cut through the electron distribution is measured in the ecliptic.” [2]
    • E/q range (A): 0.5 — 15.5V (16 channels)
    • E/q range (B): 10.7 — 1660V (16 channels)
    • Azimuth range: 360° (8 channels)
    • Elevation range -9° — 9° (1 channels)
  • I3 instrument measures the velocity and mass-per-charge (m/q) of positive ions.
    • Velocity range: 199 — 767 km/s (16 channels)
    • Azimuth range: -53.2° — 30.8° (16 channels)
    • Elevation range: -20° — 20° (9 channels)
    • m/q range: 1 — 5.33 (15 channels)

Original Data Description

  • In summary the existing E1 data consists of ion distribution functions (3D & 1D measurements), electron distribution functions (only 2D measurements) and corresponding proton & alpha particle moments (density, velocity, temperature, etc). [1]
  • The best available and most original E1 dataset, was processed in the 1990’s, by Schwenn's team in Germany, then saved in ASCII files on a set of DVDs. These are highly processed datasets, the original (raw) E1 data is gone forever. These files are organized by individual (time tag) measurement. Each file contains a header with various parameters (such as time, spacecraft position, heliocentric distance, magnetic field averages, etc) and old 1D moments of the ion distribution function, followed by an ion and electron distribution function. [1]
  • David Stansby’s Helios Plasma Instruments Overview contains detailed and updated information of the onboard E1 experiment instruments
    • Measurement ranges, azimuth ranges, and elevation ranges for all 4 instruments can be found at in Stansby’s paper [2]
    • Details of data processing/transmission are quantified in Stansby’s paper [2]
  • How were the old E1 ion moments (in the ascii files) calculated? To the best of our knowledge:
    • The 1D distribution functions, from I1a, has a dip that corresponded to a separation between proton and alpha populations
    • Each distribution is then integrated to produce its associated moments (density, speed, and temperature)
    • By integrating the 3D distribution functions from I1a, a velocity vector is obtained
    • The original code that provided these moments in Fortran is included in the document provided by R. Schwenn (see part1 and part2). Also, various codes in Fortran and IDL provided by E. Marsch are archived for reference (see below, last bullet point in this section)
  • Issues with this plasma data and old plasma moments
    • Low energy bins contain instrument noise floor for the ion distributions
    • The units of the 1D reduced distributions (either for the I1a distributions or I1b current distributions) are unclear
    • Ascii data files omitted zero counts in the distribution functions, so all zero counts are missing
    • The energies of the electron distribution functions were corrected by a very simplified form of the spacecraft potential  φ = const. log(Np) :
      • Φs/c, Schween-Marsch = 3.85 * [2.13 - loge(Ne * dAU2)]
        • Ne is electron density in volts/cm3
        • dAU is the spacecraft distance to the sun in AU 1.5*106 km
      • This makes it easy to undo and then apply a more sophisticated correction
      • Only the higher energy mode (mode-B) channels were transmitted back to earth, so we can’t use the low energy mode-A data to calculate a more accurate potential

Data Reanalysis

  • The goal of this work is to reprocess the entire plasma data set, using more modern fit analysis techniques to calculate better and more accurate plasma parameters (density, velocity and temperature including anisotropies, and heat flux) for the various components forming the ion distribution function (proton core, proton beam and alpas) and the electron distribution function (core, halo and strahl). This work is ongoing and funded by NASA HDEE grant 80NSSC18K0370 to UCB.
  • This work is being done in phases:
    • Phase 1: Characterize the proton core population; work completed (see references [3], [4])
    • Phase 2: Characterize the alpha particle population; work completed (see reference [5])
    • Phase 3: Characterize the proton beam population; work in progress
    • Phase 4: Use the above ion data to estimate Helios spacecraft potential and calibrate electron measurements. Then characterize the electron — core/halo/strahl — distribution function.
  • Proton Core Fitting Process[4]: each distribution was fitted with a bi-Maxwellian distribution function
    • The distribution function was rotated to align with the magnetic field.
      • Without the magnetic field values the rotational symmetry axis of the distribution fit cannot be determined, but we can still locate the peak of the distribution
      • If the magnetic field direction varied significantly during a measurement, the distribution ended up “smeared”
    • A 3D bi-Maxwellian function was fitted to the data by obtaining a best guess of the parameters
      • There are six fit parameters: amplitude, three bulk velocity components, and two thermal speeds
      • The number density and temperatures were calculated from the thermal speeds
      • And the fitted bulk velocity gives us radial, tangential, and normal velocities (vr, vt, vn)
    • Both the python code for the Proton Core Fit and resulting dataset are included in the archive
    • David Stansby’s A New Inner Heliosphere Proton Parameter Dataset from the Helios Mission describes a reprocessing of the original Helios ion distribution functions to provide reliable and reproducible data to characterise the proton core population of the solar wind in the inner heliosphere.
  • Alpha Particle Fitting Process[5]: each distribution is also fitted by a bi-Maxwellian function:
    • The fit is done to the 3D I1A but we use the 1D I1B distribution to recognize the alpha particle population. Because alpha particles have a mass to charge ratio twice that of protons, at the same velocity they have a higher energy per charge, and therefore it is possible to separate the distribution using an energy per charge threshold, above which measurements are dominated by protons and below which they are dominated by alpha particles.
    • Similar process to the proton core fitting described above (rotation into B-frame and fit using a bi-Maxwellian model)
    • Both the python code for the Alpha Particle Fit and resulting dataset are included in the archive (In construction)
    • David Stansby’s Alpha particle thermodynamics in the inner heliosphere fast solar wind publication discusses the fit process of the alpha particles and their properties to study plasma processes and look for various radial trends. To this end, the team reprocessed the original Helios E1 data, isolated the alpha particle population, and computed the alpha particle parameters (number density, velocity, and magnetic field perpendicular and parallel temperatures).
  • Update 2020: The above proton core and alpha particle fits are being redone at UC Berkeley using E3 mag data, as they are more reliable. The E2 4Hz data above 50nT is missing, and the E2 Bz-component has a non-constant, non-zero offset that can affect parallel and perpendicular temperatures. See E2 and E3 pages for more details on the various issues affecting the mag data. (In construction)
  • The E1 ion plasma data was also reprocessed by a team from the Czech Republic (In construction)
  • Additionally the electron distribution functions were also processed by S. Stverak (In construction)