STEPS: Energetic Ion Sensor on Aditya‑L1

• STEPS is a direction-resolved energetic-ion spectrometer on India's Aditya‑L1 mission that measures suprathermal and energetic ions using six detector units.
• It employs both dual- and single-window Si‑PIN detectors with high- and low-gain analog chains to cover energies from roughly 20 keV/n to over 10 MeV.
• Observations from STEPS reveal isotropic quiet-time suprathermal populations and shock-associated energization, informing studies of particle acceleration and space weather.

The SupraThermal and Energetic Particle Spectrometer (STEPS) is the high-energy ion sensor of the Aditya Solar Wind Particle EXperiment (ASPEX) aboard India’s Aditya‑L1 mission. It provides direction-resolved measurements of suprathermal and energetic ions from the Sun–Earth L1 vantage point using six detector units oriented in different directions, with nominal coverage from roughly 20 keV/n to 6 MeV/n. Across its Earth-bound phase, its first year in halo orbit, and specific investigations of quiet-time suprathermals and ICME-driven shocks, STEPS has been used to study particle acceleration, transport, anisotropy, and space-weather-relevant transients in the inner heliosphere (Sebastian et al., 24 Jul 2025).

1. Mission context and observational role

STEPS is one of two subsystems within ASPEX on Aditya‑L1, India’s first Sun–solar wind observatory at the L1 halo orbit. The spacecraft reached its final halo orbit on 06 January 2024, and the one-year operational assessment of STEPS spans 08 January 2024 through 28 February 2025. Because Aditya‑L1 is three-axis stabilized, STEPS retains directional context without the spin-modulation indirection characteristic of many legacy instruments (Sebastian et al., 24 Jul 2025).

The instrument is configured around six look directions: Sun Radial (SR), Parker Spiral (PS), InterMediate (IM), Earth‑Pointed (EP), North‑Pointed (NP), and South‑Pointed (SP). Four units, SR, PS, IM, and EP, lie in the ecliptic plane, while NP and SP view out of the ecliptic. This directional sampling is central to STEPS’ scientific role: it enables multi-axis measurements of energetic ions for studies of anisotropy, shock-associated streaming, suprathermal seed populations, and the transition between suprathermal and SEP regimes (Sebastian et al., 24 Jul 2025).

During the Earth-bound phase, before trans-L1 injection on 19 September 2023, STEPS was the first Aditya‑L1 instrument to be switched on, and it was operated whenever spacecraft altitude exceeded approximately 52,000 km during 11–19 September 2023. In that interval it sampled the interplanetary medium upstream of the bow shock, the magnetosheath, and the magnetosphere, demonstrating that the same directional ion spectrometer could be used both for heliospheric science and for near-Earth energetic-ion studies (Chakrabarty et al., 27 Jun 2025).

2. Sensor architecture, viewing geometry, and operating modes

Each STEPS unit uses a fully depleted Si‑PIN detector. PS, EP, and SR are dual-window, 300 μm thick units with an inner active region of 7 mm diameter and an ultra-thin dead layer of approximately 0.1 μm, plus an outer annulus spanning 7–18 mm diameter with a thicker dead layer of approximately 0.8 μm. IM, NP, and SP are single-window, 250 μm thick units with an approximately 0.2 μm dead layer. For PS and EP, the detector stack also includes a scintillator plus silicon photomultiplier; in the quiet-time analysis, species-integrated fluxes from PS‑Out and EP‑Out were used because deconvolution of the inner-detector species signatures was still ongoing (Sebastian et al., 24 Jul 2025).

The directional geometry used in quiet-time and event studies emphasizes four units: PS, IM, EP, and NP. In the quiet-time analysis, PS, IM, and EP lie nearly in the ecliptic plane, identified with the GSE X–Y plane, while NP points out of the plane along $+Z_{\mathrm{GSE}}$. The angles between look direction and the solar-wind velocity are reported as PS $\approx 52^\circ$, IM $\approx 30^\circ$, EP $\approx 156^\circ$, and NP $\approx 90^\circ$. In the May‑2024 ICME–ICME interaction study, PS is described as sampling particles roughly along the nominal Parker spiral direction in the ecliptic, NP as viewing out of the ecliptic toward heliographic north, EP as Earth-pointing and anti-sunward, and IM as a narrower-FOV unit sampling an additional ecliptic-plane direction. That study lists field-of-view values as IM $15^\circ$, PS $30^\circ$, NP $28^\circ$, and EP $37\%$ (Gupta et al., 16 Jul 2025).

Energy handling is implemented through two parallel analog chains, high-gain (HG) and low-gain (LG), feeding a 256-channel ADC. HG occupies channels 0–127 and typically covers up to approximately 2 MeV, while LG occupies channels 128–255 for higher energies. In default gain-switching mode, the usable range extended to approximately 1.3 MeV for IM and NP and approximately 2 MeV for PS and EP. From 11 May 2024 onward, STEPS adopted a permanent “toggling” mode in which HG and LG alternate every 5 minutes, eliminating the gap near channel 128 and extending continuous spectra beyond 2 MeV for PS/EP and beyond 1.3 MeV for IM/NP; spectra shown in the one-year operations paper extend well above 10 MeV (Sebastian et al., 24 Jul 2025).

Native accumulation is 1 s. In toggling mode, a full HG+LG spectrum requires a 10-minute cycle, while 1-second data remain available within each 5-minute segment. This produces a trade-off between broader spectral continuity and reduced effective full-spectrum temporal resolution (Sebastian et al., 24 Jul 2025).

3. Measurement formalism, products, and instrumental constraints

The STEPS studies treat the measured quantity as directional differential flux. The quiet-time analysis states the standard particle-spectrometer normalization as

$J(E) = \frac{N(E)}{G\,T\,\Delta E},$

where $\approx 52^\circ$0 is the count in the energy bin, $\approx 52^\circ$1 is the geometric factor, $\approx 52^\circ$2 is live time, and $\approx 52^\circ$3 is the energy-bin width. The May‑2024 event paper gives the corresponding form

$\approx 52^\circ$4

with $\approx 52^\circ$5 the counts in the energy bin and $\approx 52^\circ$6 the accumulation time. In the published analyses, differential fluxes are generally used as instrument-provided products rather than reconstructed from tabulated detector constants, and quantitative values of $\approx 52^\circ$7, $\approx 52^\circ$8 in physical units, efficiency curves, and formal flux units are not fully specified in the papers summarized here (Gupta et al., 16 Jul 2025).

For spectral analysis, STEPS data are commonly parameterized as a power law,

$\approx 52^\circ$9

or equivalently

$\approx 30^\circ$0

with $\approx 30^\circ$1 or $\approx 30^\circ$2 the spectral index. In quiet-time L1 studies, power-law fits are performed on the linear portion of log–log flux–energy space, with counting-statistical uncertainties included; in Earth-bound magnetospheric work, fits were performed both on the native ADC-channel spectra and on six broad energy bins constructed from the linear region above threshold (Gupta et al., 16 Jul 2025).

Low-level discriminator thresholds are a recurring constraint. They suppress electronic and background noise but also impose zero counts below threshold and a non-linear rise just above threshold. As a result, quiet-time fits exclude the low-energy non-linear region. In the L1 quiet-time study, high-energy channels above approximately 1.3 MeV for PS/EP and above approximately 1.2 MeV for IM/NP were also excluded during quiet times to avoid low statistics and SEP contamination. During the Earth-bound phase, the linear fitting region begins at approximately 0.25 MeV for PS and approximately 0.16 MeV for NP, and the analysis focuses on the high-gain channels 0–127 covering approximately 0.1–2 MeV (Chakrabarty et al., 27 Jun 2025).

Species treatment depends strongly on detector configuration and energy. In the quiet-time and May‑2024 event studies, STEPS is used in a species-integrated mode, with fluxes dominated by protons and alpha particles in the analyzed channels. The one-year operational paper also reports composition-filtering features at higher energies: protons are fully stopped below approximately 6.0 MeV in PS/EP and below approximately 5.5 MeV in IM/NP, so spectra above those energies effectively exclude H$\approx 30^\circ$3 and are dominated by heavier ions; a second discontinuity near approximately 24 MeV for PS/EP and approximately 22 MeV for IM/NP is described as consistent with the removal of He$\approx 30^\circ$4 contributions (Sebastian et al., 24 Jul 2025).

4. Operational performance, calibration, and caveats

The one-year operations assessment reports that four of the six detector units, PS, EP, IM, and NP, exhibit stable response throughout the observation period, with nominal High Voltage Monitor values maintained and no required gain-calibration updates. Onboard calibration pulses in PS show centroid deviations of approximately 0.2 ADC channels across epochs with unchanged slope, which the study interprets as stable electronics gain with small temperature-related shifts (Sebastian et al., 24 Jul 2025).

Two units, however, were not scientifically usable in nominal configuration. SR exhibits persistent loading and saturation linked to scattered sunlight entering the aperture despite its 1.5° offset from the +yaw Sun-pointing axis and collimation. Its High Voltage Monitor drops to approximately 0.83 V from a nominal approximately 0.95 V, and SR recovers only when the +yaw-to-Sun angle exceeds approximately 16.5°. SP also remains saturated, with likely causes including primary reflection from the engine cone and secondary reflections from multi-layer insulation; its High Voltage Monitor is reduced to approximately 0.48 V and can fall to approximately 0.4 V during rotations. Because SR shares front-end electronics with IM and SP shares electronics with EP, SR and SP are kept on with higher LLD thresholds to preserve IM and EP operation, but SR and SP scientific data are unsuitable for present analyses (Sebastian et al., 24 Jul 2025).

This operational context explains several analysis choices across the literature. The quiet-time suprathermal study uses four directionally separated sensors, PS, IM, EP, and NP, and explicitly excludes SR and SP because of light saturation. It also excludes April 2024 because frequent spacecraft rotations and thruster firings affected attitude. The Earth-bound study uses only PS and NP because those were the active units during that phase. A common misconception would be to treat STEPS as a six-direction dataset in routine science products; in practice, most quantitative analyses to date rely on the four stable units at L1, and some early-phase work used only two directions (Gupta et al., 16 Jul 2025).

Another recurring caveat concerns anisotropy quantification. Directional asymmetries are evident in several STEPS studies, but the one-year paper does not provide formal anisotropy metrics, and the quiet-time paper states explicitly that no metric such as

$\approx 30^\circ$5

is reported there. In the Earth-bound study, anisotropy is evaluated primarily through differences in fitted spectral indices between PS and NP rather than by explicit reporting of $\approx 30^\circ$6 (Chakrabarty et al., 27 Jun 2025).

5. Quiet-time suprathermals at L1

A central STEPS result is the directional investigation of quiet-time suprathermal ions at L1 during January–November 2024, with April excluded owing to rotations and thruster firings. Hourly averaged directional differential fluxes were computed for each sensor, sorted in ascending order, then re-binned with 24-hour bins so that mean and variance could be calculated. Quiet-time thresholds were identified as the mean-flux values above which variance increases monotonically; fluxes below those thresholds were labeled quiet time. Spectra were then assembled over quiet intervals lasting from a day to a few days and fit with power laws over the linear energy ranges PS 0.36–1.32 MeV, IM 0.14–1.22 MeV, EP 0.39–1.33 MeV, and NP 0.12–1.23 MeV (Gupta et al., 16 Jul 2025).

Across three analyses—common quiet intervals for all four sensors, quiet intervals selected individually per sensor, and a rotated-spacecraft configuration on 25 November 2024—the fitted spectral indices remain consistently approximately 2.0 in PS, IM, EP, and NP. Because three sensors sample distinct in-plane directions and NP samples an out-of-plane direction, this near-identity of spectral indices over multiple few-day intervals supports an isotropic quiet-time suprathermal ion distribution at L1 during 2024. The paper further examined a first-order Compton–Getting correction, using

$\approx 30^\circ$7

after neglecting the $\approx 30^\circ$8 term since suprathermal ion speeds are much larger than the solar-wind or spacecraft relative speed. With PS $\approx 30^\circ$9, IM $\approx 156^\circ$0, EP $\approx 156^\circ$1, and NP $\approx 156^\circ$2, the corrections were minimal and NP had no first-order correction because $\approx 156^\circ$3; the inferred indices remained approximately 2.0 (Gupta et al., 16 Jul 2025).

The significance of this result is methodological as well as physical. Earlier quiet-time suprathermal studies with ACE/ULEIS and Wind often used long averaging windows and spinning spacecraft, whereas STEPS provides simultaneous, fixed look directions in three ecliptic-plane directions plus one out-of-ecliptic direction. The quiet-time paper therefore presents the isotropy inference not as a consequence of temporal averaging alone, but as a direct multi-directional observational result over short, cleaner windows. It explicitly notes that the result supports the isotropy assumption used in Parker transport equation treatments (Gupta et al., 16 Jul 2025).

Source attribution in that study comes from a cross-instrument comparison with ACE/ULEIS at L1 over 0.1–1.2 MeV/n. Interval-averaged abundance ratios of $\approx 156^\circ$4He/$\approx 156^\circ$5He, Fe/O, and C/O show that approximately 25% of quiet intervals have $\approx 156^\circ$6He/$\approx 156^\circ$7He $\approx 156^\circ$8, about 45% have Fe/O near unity, roughly 20% have C/O near approximately 0.32, and approximately 50% have C/O near approximately 0.42. The study interprets these enrichments as consistent with a quiet-time suprathermal pool influenced by residual ions from previous impulsive and gradual SEP events, with potentially CIR-related contributions. It also notes that the hard spectra, $\approx 156^\circ$9, over few-day windows during solar maximum contrast with softer spectra, $\approx 90^\circ$0–4, often seen when CIRs dominate during solar minimum or declining phases (Gupta et al., 16 Jul 2025).

6. Shock-associated energization, Earth-bound observations, and validation

STEPS has also been used to diagnose transient energization during an ICME–ICME interaction on 12 May 2024. In that event, time-intensity profiles from PS, IM, NP, and EP were examined across seven energy bands spanning approximately 0.12–1.9 MeV. Interval B, the downstream energization region, extends from 08:47 UTC to 21:56 UTC. The onset at 08:47 UTC is identified as a forward shock arrival, approximately 22 minutes earlier than the 09:09 UTC shock time listed in the CFA shock database. STEPS records a sudden enhancement at the shock and a subsequent gradual decay downstream, with nearly an order-of-magnitude increase in differential ion flux relative to the preceding interval. The PS, IM, and NP detectors show the shock-associated rise, while EP, pointing anti-sunward toward Earth, shows no enhancement. The paper interprets this directional selectivity as evidence for a forward shock with sunward-directed streaming and anisotropy, and as a textbook Energetic Storm Particle signature of local acceleration at the shock (Parashar et al., 15 Dec 2025).

The same study places the STEPS observations in a shock-acceleration framework. Plasma and magnetic-field context indicate a compression ratio $\approx 90^\circ$1, quasi-perpendicular geometry with $\approx 90^\circ$2 between upstream IMF and shock normal, a magnetic field jump from approximately 5 nT to approximately 11 nT, and a reduced shock speed of approximately 550.4 km/s relative to local wind of approximately 836 km/s. Using the momentum-space DSA relation

$\approx 90^\circ$3

the study obtains $\approx 90^\circ$4 for $\approx 90^\circ$5. It does not report measured STEPS spectral indices $\approx 90^\circ$6 for this event, but states that the observed multi-energy peak at the shock and downstream softening are qualitatively consistent with DSA expectations for a strong, quasi-perpendicular shock (Parashar et al., 15 Dec 2025).

The Earth-bound phase supplied a different but complementary use of STEPS. During 11–19 September 2023, PS and NP were operated at 1-minute cadence above approximately 52,000 km, covering approximately 0.1–2 MeV with the high-gain channels. By combining modeled bow-shock and magnetopause boundaries with spacecraft ephemeris, the study identified intervals in the interplanetary medium, magnetosheath, and magnetosphere, and then compared STEPS spectra with ACE/EPAM at L1 and GOES‑18/SEISS/MPS‑H at geostationary orbit. It reports that southward IMF $\approx 90^\circ$7 produces the hardest spectra across all regions, with interplanetary STEPS indices of approximately 1.07–1.39 in interval 5a and magnetosheath indices of approximately 1.86–2.09 in interval 5b, while substorm-dominated magnetospheric intervals 2 and 3 exhibit soft spectra with PS values 5.42±0.12 and 5.71±0.21 and NP values 4.84±0.05 and 5.07±0.08. In this framework, the polarity of IMF $\approx 90^\circ$8 modulates whether external ICME-associated ions or internal substorm processes dominate the 0.1–2 MeV magnetospheric environment (Chakrabarty et al., 27 Jun 2025).

That Earth-bound analysis also uses the two fixed STEPS look directions to characterize anisotropy through differences in spectral index. Significant anisotropy, with $\approx 90^\circ$9, appears inside the magnetosphere during intervals 2 and 3, when internal acceleration dominates; mild anisotropy, $15^\circ$0–0.3, persists in the magnetosheath; and anisotropy is negligible to mild in the upstream interplanetary medium. A standard two-direction anisotropy measure,

$15^\circ$1

is given for reference, but the paper primarily reports directional differences in fitted spectral indices (Chakrabarty et al., 27 Jun 2025).

Independent validation of STEPS measurements comes from cross-comparisons at L1. Over 15–18 December 2024, STEPS PS (Out) was compared with ACE/EPAM LEMS120 and LEFS in overlapping energy bands. Hourly averaged fluxes were linearly regressed in four band-pairs, and all four comparisons yielded coefficients of determination of approximately $15^\circ$2. The temporal patterns in the overlapping bands are described as similar, and the one-year paper presents this as evidence that STEPS measurements are reliable for long-term energetic-ion monitoring from L1 (Sebastian et al., 24 Jul 2025).

Taken together, the published STEPS results define the instrument as a direction-resolved energetic-ion spectrometer whose scientific distinctiveness lies in simultaneous multi-direction sampling from a three-axis-stabilized platform. The quiet-time studies support an isotropic, hard suprathermal ion population at L1 with $15^\circ$3 on few-day timescales; the May‑2024 event study shows strong directional anisotropy and local shock energization during a complex ICME interaction; the Earth-bound observations demonstrate sensitivity to regional boundaries, IMF $15^\circ$4 polarity, and external-versus-internal drivers in the near-Earth environment; and the first-year operational paper shows stable performance for four sensors and strong correlation with ACE/EPAM. A plausible implication is that STEPS occupies an observational niche between traditional single-aperture or spinning energetic-particle sensors and more composition-specific instruments: it emphasizes directional context and time-resolved spectral morphology, while species deconvolution and some absolute response parameters remain areas for further analysis (Sebastian et al., 24 Jul 2025).