STEPS: Suprathermal & Energetic Ion Spectrometer

• STEPS is a suprathermal and energetic ion spectrometer that quantifies non-Maxwellian particle populations, crucial for understanding shock acceleration and energy partitioning in space plasmas.
• It employs six Si-PIN detector units with distinct GSE viewing directions, ensuring precise, simultaneous directional sampling and stable calibration over a broad energy range.
• Validated against ACE-EPAM, STEPS offers reliable insights on solar energetic particles, anisotropy in ion distributions, and magnetospheric coupling under varying plasma conditions.

Searching arXiv for STEPS / ASPEX / Aditya-L1 papers to ground the article in current literature. arXiv search query: all:"ASPEX STEPS Aditya-L1" Searching arXiv for papers on ASPEX-STEPS and related suprathermal/energetic particle measurements. Supra-Thermal and Energetic Particle Spectrometer (STEPS) is the suprathermal and energetic ion spectrometer within the Aditya Solar wind Particle EXperiment (ASPEX) aboard Aditya-L1. It is designed to measure suprathermal and energetic ions in the interplanetary medium from the Sun–Earth L1 point with directional resolution from six look directions, and it has also been used during Aditya-L1’s Earth-bound phase to sample the interplanetary medium, magnetosheath, and magnetosphere. In the recent literature, STEPS is presented as an instrument for investigating suprathermal seed populations, solar energetic particles (SEPs), SIR/CIR-associated energetic particles, directional anisotropy, and the non-Maxwellian particle tail relevant to shock acceleration and space-weather forcing (Sebastian et al., 24 Jul 2025).

1. Mission placement and scientific remit

STEPS is one of the two spectrometers in ASPEX. The other subsystem is SWIS, the Solar Wind Particle Spectrometer, aimed at bulk solar-wind measurements, whereas STEPS is aimed at energetic ions. In the L1 configuration, STEPS is used to study suprathermal and energetic ions in the interplanetary medium; during the Earth-bound commissioning phase, it provided measurements in the magnetosphere, magnetosheath, and interplanetary medium. The cited studies emphasize its relevance to the origin of suprathermal seed particles, SEPs, SIR/CIR-associated energetic particles, and upstream particle populations in the near-Earth and heliospheric environment (Chakrabarty et al., 27 Jun 2025).

The scientific role of STEPS is closely aligned with energy-budget studies of interplanetary shocks. Independent in-situ analysis of 18 shocks at 1 AU with ACE and Wind found that the downstream non-Maxwellian particle population carried 2.14% to 16.3% of the total downstream energy flux, with a mean of 8.15% and a median of 8.25%; that work did not use STEPS data, but it analyzed the same suprathermal and energetic tail that STEPS is designed to measure (David et al., 2021). This identifies STEPS as an instrument for quantifying particle populations that are not captured by standard thermal plasma moments.

2. Instrument architecture and viewing geometry

STEPS comprises six detector units with distinct nominal viewing directions in Geocentric Solar Ecliptic (GSE) coordinates. SR, IM, PS, and EP lie in the ecliptic plane, NP points toward ecliptic north, and SP points toward ecliptic south. This geometry is central to the instrument concept because it permits simultaneous directional sampling rather than inference from temporal modulation (Sebastian et al., 24 Jul 2025).

Unit	Nominal direction	Operational note in cited studies
SR	Sun Radial	Saturation issues; not used for nominal science analysis
PS	Parker Spiral	Stable; used extensively
IM	Intermediate between SR and PS	Stable; used extensively
EP	Earth Pointed	Stable; used extensively
NP	North Pointed	Stable; used extensively
SP	South Pointed	Saturation issues; not used for nominal science analysis

Each unit uses a fully depleted Si-PIN detector with pulse height proportional to deposited particle energy. SR, PS, and EP employ 300 μm detectors in a dual-window configuration, with an inner region dead layer of about 0.1 μm over 7 mm diameter and an outer region dead layer of about 0.8 μm over 7–18 mm diameter. IM, NP, and SP use 250 μm detectors in a single-window configuration with a dead layer of about 0.2 μm. In the Earth-bound analysis, the PS detector is additionally described as a stacked custom dual-window Si-PIN + scintillator/SiPM assembly, while NP is a single-window detector with a 0.2 μm dead layer (Sebastian et al., 24 Jul 2025).

The readout electronics provide two gain chains per detector, high gain (HG) and low gain (LG), digitized into 256 ADC channels. Channels 0–127 correspond to HG and 128–255 to LG. In nominal operation, HG typically covers energies up to about 1.3 MeV for IM and NP, and up to about 2 MeV for PS and EP. The quiet-time directional study used PS, IM, EP, and NP, with SR and SP excluded because of light-saturation issues (Gupta et al., 16 Jul 2025).

3. Operations, calibration, and in-flight performance

A one-year operational assessment covered 08 January 2024 to 28 February 2025 after insertion into the final halo orbit around L1. Four of the six units—PS, EP, IM, and NP—exhibited stable detector response throughout the observation period. Their housekeeping HVM values stayed near nominal levels, which was interpreted as evidence of operational robustness. Pre-launch nominal HVM values were about 0.95 V for SR, PS, and EP, and about 0.52 V for IM, NP, and SP; during flight, IM, EP, PS, and NP remained close to nominal, while SR dropped to about 0.83 V and SP to about 0.48 V, sometimes lower (Sebastian et al., 24 Jul 2025).

The underperforming units had distinct failure modes. SR was intended to view along the Sun-radial direction with a 1.5° offset from the +yaw axis and collimators to limit direct sunlight, yet scattered sunlight still caused saturation when the spacecraft’s +yaw-to-Sun angle was below about 16.5°. When the spacecraft rotated and sunlight no longer reached the detector, SR recovered to nominal voltage. SP also showed persistent saturation, with HVM near 0.48 V and sometimes about 0.4 V, but unlike SR its loading was not strongly affected by spacecraft rotation. Possible causes suggested in the paper include sunlight reflection from the engine cone and multiple reflections from MLI. Because SR shares front-end electronics with IM, and SP shares front-end electronics with EP, the saturated units were left operational to preserve the healthy paired units. Their data in the nominal configuration were therefore treated as not usable for scientific analysis (Sebastian et al., 24 Jul 2025).

Calibration stability was monitored through calibration pulse analysis. The centroid of Gaussian fits was used to track gain stability, the maximum observed channel deviation was about 0.2 ADC channels, and the slope of the linear calibration curve,

$\text{ADC channel} = m \times E + c,$

remained unchanged over time. In the cited interpretation, this indicates stable electronics gain rather than secular in-flight drift (Sebastian et al., 24 Jul 2025).

4. Spectral coverage, response, and analysis constraints

The instrument literature describes STEPS as covering approximately $20~\mathrm{keV/n}$ to $6~\mathrm{MeV/n}$, but specific analyses restrict attention to the ranges where the detector response is linear and the relevant units are healthy. In the quiet-time suprathermal-ion study, the fitted energy intervals were 0.36–1.32 MeV for PS, 0.14–1.22 MeV for IM, 0.39–1.33 MeV for EP, and 0.12–1.23 MeV for NP. The spectra were treated as power laws,

$J = A E^{-m},$

with $J$ the differential directional flux and $m$ the spectral index, and the fits were performed only over the linear portions of the log-log spectra because the low-energy end is affected by the low-level discriminator (LLD) (Gupta et al., 16 Jul 2025).

The Earth-bound phase analysis imposed similar constraints. For that interval, STEPS measured ions roughly in the 0.1–2 MeV range, with PS data extending to about 1.89 MeV and NP data to about 1.23 MeV. The LLD suppressed background noise and produced a non-linear low-energy response below threshold, so spectral fits were started only above about 0.25 MeV for PS and about 0.16 MeV for NP. That study computed spectral indices both from flux versus ADC channel over the linear portion and from flux versus six grouped energy bins, finding nearly the same results and using the energy-bin spectral index for interpretation (Chakrabarty et al., 27 Jun 2025).

A notable instrumental development was the introduction of a toggling mode to address the HG/LG transition gap near ADC channel 128. In the default mode, the analog multiplexer selected HG or LG dynamically, which limited continuous spectral coverage. In toggling mode, HG and LG alternate every 5 minutes, with 1-second cadence retained within each 5-minute block, yielding an effective 10-minute combined spectral cadence. Continuous toggling began on 11 May 2024 and enabled reconstruction of continuous spectra beyond about 1.3 MeV for IM and NP and beyond about 2 MeV for PS and EP (Sebastian et al., 24 Jul 2025).

The detector response also imposes physically important upper-energy transitions. Because of finite silicon thickness, STEPS fully absorbs protons below about 5.5 MeV for IM and NP and below about 6.0 MeV for PS and EP. Above those energies, the detectors increasingly sample heavier ions instead of protons. In some high-energy events, another spectral jump was reported around about 24 MeV for PS and EP and about 22 MeV for IM and NP, attributed to removal of $\mathrm{He}^{++}$ from the ion composition (Sebastian et al., 24 Jul 2025).

5. L1 observations: quiet-time suprathermal ions and transient events

The quiet-time L1 analysis used the four unsaturated directions PS, IM, EP, and NP to test whether suprathermal ions are anisotropic or isotropic on timescales of a few days. Across quiet intervals in January–November 2024, the fitted spectral indices were consistently close to $m \approx 2.0$ in all directions. PS, IM, and EP sample directions nearly aligned with the ecliptic plane, whereas NP samples a mutually orthogonal direction roughly along +Z in GSE. The near-equality of the spectral indices was interpreted as evidence that the quiet-time suprathermal ions are approximately isotropically distributed over timescales of a few days (Gupta et al., 16 Jul 2025).

The same study evaluated whether this apparent isotropy could be a viewing-frame artifact through the Compton–Getting effect. The correction depends on the relative speed between solar wind and spacecraft, the particle speed, and the viewing angle. The reported viewing angles were $52^\circ$ for PS, $30^\circ$ for IM, $20~\mathrm{keV/n}$0 for EP, and $20~\mathrm{keV/n}$1 for NP. Because $20~\mathrm{keV/n}$2, the correction was found to be small; for NP, where $20~\mathrm{keV/n}$3, it vanishes. The isotropy inference was therefore taken as physical rather than kinematic (Gupta et al., 16 Jul 2025).

The quiet-time study also addressed origin. Because STEPS does not separate species cleanly and is dominated by protons and alpha particles, ACE/ULEIS was used to examine $20~\mathrm{keV/n}$4, $20~\mathrm{keV/n}$5, and $20~\mathrm{keV/n}$6 during the same intervals. The interpretation was that the quiet-time suprathermal pool is not purely quiet-background solar-wind material, but includes leftover ions from previous impulsive SEP events and gradual SEP events. The paper estimated that about 25% of quiet intervals show impulsive SEP-like $20~\mathrm{keV/n}$7 signatures, about 45% of the population is consistent with impulsive SEP-like influence via Fe/O, and about 50% shows C/O values characteristic of gradual SEP populations (Gupta et al., 16 Jul 2025).

The one-year operations paper extends the observational picture beyond quiet-time conditions. STEPS recorded multiple transient energetic-particle enhancements above quiet background, often associated with ICME-driven interplanetary shocks and SIR/CIRs. During quiet times, the initial ADC channels are affected by LLD nonlinearity, but above that region the spectra become linear; for PS Out, IM, NP, and EP Out, spectra were reported as linear up to beyond 10 MeV. In quiet time, the inner and outer detectors of PS and EP do not match below 2 MeV, with the discrepancy stronger for EP and attributed to the ultra-thin dead layer on the inner detectors. During an ion enhancement event associated with an ICME shock, that inner/outer discrepancy largely vanished and the spectra matched well across the energy range. Event spectra derived from binned energies did not follow a single power law up to about 6 MeV; instead they showed a clear transition from suprathermal to SEP energies, similar to spectral structure seen in large gradual SEP events. This suggests that STEPS resolves spectral complexity produced by multiple acceleration and transport processes rather than only bulk flux enhancement (Sebastian et al., 24 Jul 2025).

Cross-validation against ACE-EPAM further established the L1 data quality. Comparing AL1-ASPEX-STEPS-PS outer detector with ACE-EPAM LEMS120 and LEFS over common event intervals, the study used hourly averaged fluxes, linear fits, Pearson correlation coefficients, and $20~\mathrm{keV/n}$8. The compared energy bands were 0.31–0.58 MeV and 1.89–4.75 MeV for LEMS120, and 0.55–0.76 MeV and 1.22–4.9 MeV for LEFS. All four comparisons yielded $20~\mathrm{keV/n}$9, which was interpreted as strong evidence that STEPS measurements are highly reliable and consistent with established L1 particle measurements (Sebastian et al., 24 Jul 2025).

6. Earth-bound phase, environmental diagnostics, and interpretive framework

Before routine L1 science operations, STEPS was the first Aditya-L1 instrument switched on, on 10 September 2023, and it operated during 11–19 September 2023 whenever the spacecraft was above about 52,000 km. In this Earth-bound phase, only the PS and NP units were operated. The orbit provided repeated sampling of the magnetosphere, magnetosheath, and interplanetary medium while three ICMEs hit the magnetosphere, creating an opportunity to compare external driving by ICME-related particles with internal magnetospheric acceleration and injection associated with substorms (Chakrabarty et al., 27 Jun 2025).

The analysis classified spacecraft location using model-based boundaries for the magnetopause and bow shock and examined power-law spectra under three IMF $6~\mathrm{MeV/n}$0 regimes: $6~\mathrm{MeV/n}$1, $6~\mathrm{MeV/n}$2, and $6~\mathrm{MeV/n}$3. A compact summary given in the paper is that substorm-dominated magnetospheric intervals had $6~\mathrm{MeV/n}$4, magnetosheath mixing intervals had $6~\mathrm{MeV/n}$5 to $6~\mathrm{MeV/n}$6, and hard IP/ICME-related populations had $6~\mathrm{MeV/n}$7 to $6~\mathrm{MeV/n}$8. Under northward $6~\mathrm{MeV/n}$9, the interpretation was weaker dayside reconnection and less direct penetration of hard external particles; under southward $J = A E^{-m},$0, more effective access of external ICME-driven ions into the magnetosphere hardened the spectrum (Chakrabarty et al., 27 Jun 2025).

Directional information was diagnostically important even in this reduced two-unit mode. Because the PS and NP detector units were not aligned with their nominal planned orientations during the Earth-bound phase, their look directions were reconstructed in GSE coordinates as the spacecraft moved through different orbit segments. The resulting comparison showed mild to significant anisotropy in some interplanetary and magnetospheric intervals and consistently mild anisotropy in the magnetosheath in all three cases examined. The cited interpretation was that the anisotropy reflects spatially varying mixing of particles arriving from different sources and directions rather than a single homogeneous population (Chakrabarty et al., 27 Jun 2025).

A common interpretive issue is whether the isotropy reported at L1 during quiet times conflicts with anisotropy reported during the Earth-bound phase. The published results do not support treating these as contradictory. The quiet-time study concerns short intervals of relatively undisturbed suprathermal ions near L1 and finds nearly identical slopes across four directions, whereas the Earth-bound study concerns ICME- and substorm-driven conditions in three distinct plasma regions and finds region-dependent anisotropy. This suggests that directional uniformity is regime dependent rather than universal (Gupta et al., 16 Jul 2025).

The broader significance of STEPS follows from the physical population it measures. Shock studies at 1 AU show that the suprathermal and energetic, non-Maxwellian tail is a non-negligible reservoir of shock energy, and the STEPS literature shows that this tail can be tracked directionally, over long durations, across quiet and transient conditions, and in both heliospheric and near-Earth environments. Within the cited papers, STEPS therefore functions both as a directional spectrometer for suprathermal-ion physics and as an energy-budget instrument for studying how shock acceleration, SEP production, transport, and magnetospheric coupling partition energetic particles in space plasmas (David et al., 2021).