ASPEX: In-Situ Particle Measurements on Aditya-L1

• ASPEX is an in-situ particle payload on Aditya-L1 that combines two subsystems, SWIS and STEPS, to measure solar wind ions and suprathermal particles.
• Its dual instruments—SWIS with top-hat electrostatic analyzers and STEPS with fixed silicon detectors—offer 3D velocity distribution functions and directional flux measurements across a broad energy range.
• Data from ASPEX facilitate advanced studies of heliospheric plasma dynamics, shock acceleration, and space weather, with calibration validated through cross-comparisons with ACE and GOES.

Searching arXiv for papers on ASPEX and Aditya-L1 to ground the article. arxiv_search(query="ASPEX Aditya L1 solar wind particle experiment") Aditya Solar Wind Particle EXperiment (ASPEX) is the in-situ particle payload on board Aditya-L1, the Sun–Earth L1 mission of India, and comprises two subsystems: the Solar Wind Ion Spectrometer (SWIS) and the Supra-Thermal and Energetic Particle Spectrometer (STEPS). From the L1 halo orbit reached on 6 January 2024 after launch on 2 September 2023, ASPEX provides continuous measurements of the upstream solar wind, suprathermal ions, and energetic particles in a vantage geometry suited to heliospheric transport studies and space-weather monitoring (Gupta et al., 16 Jul 2025, Kumar et al., 23 Jul 2025). The published record to date presents ASPEX both as an instrument suite and as a coordinated measurement system: SWIS resolves low-energy ion velocity distribution functions, while STEPS samples directional ion fluxes from suprathermal to MeV energies, enabling joint analyses of background solar-wind structure, shock-driven acceleration, and quiet-time seed populations (Sebastian et al., 24 Jul 2025, Parashar et al., 15 Dec 2025).

1. Mission placement and payload scope

Aditya-L1 has been described as India’s first observatory-class solar and heliospheric mission and, in parallel, as India’s first dedicated solar observatory. Its placement around the Sun–Earth L1 point enables continuous in-situ monitoring of the undisturbed solar wind, which is central to both heliospheric plasma diagnostics and upstream warning of geoeffective disturbances (Gupta et al., 16 Jul 2025, Kumar et al., 23 Jul 2025).

ASPEX is the mission’s in-situ plasma suite. One line of work treats it as one of three in-situ payloads dedicated to solar-wind ions and suprathermal or energetic particles; another describes it as the in-situ plasma suite itself, emphasizing its division into SWIS and STEPS (Gupta et al., 16 Jul 2025, Kumar et al., 23 Jul 2025). These descriptions are complementary rather than contradictory: both identify ASPEX as the mission’s primary particle instrument package for the ion population from solar-wind energies to MeV-class suprathermals.

During the earth-bound phase, STEPS was operated whenever the spacecraft altitude exceeded $52{,}000$ km during 11–19 September 2023, providing measurements in the magnetosphere, magnetosheath, and interplanetary medium before routine L1 operations (Chakrabarty et al., 27 Jun 2025). That early phase established that ASPEX science is not restricted to nominal halo-orbit operations.

Subsystem	Primary measurement domain	Configuration
SWIS	$0.1$–$20$ keV ion population; three-dimensional velocity distribution functions of protons and alpha particles	Two orthogonally mounted top-hat electrostatic analyzers, THA-1 and THA-2
STEPS	Suprathermal and energetic ions from $\sim 20$ keV/n up to a few MeV/n; operation reports also describe $20$ keV–$6$ MeV/nucleon ions	Six fixed silicon detector units: SR, IM, PS, NP, SP, EP

2. Instrument architecture and directional sampling

SWIS uses two orthogonally mounted top-hat electrostatic analyzers on a three-axis stabilized platform. THA-1 samples the ecliptic plane and THA-2 samples the perpendicular meridional plane; both provide $360^\circ$ azimuthal coverage with a narrow out-of-plane acceptance of $\pm 1.5^\circ$. THA-1 employs a 16-sector ring anode with $22.5^\circ$ sector resolution, whereas THA-2 uses a 32-sector anode with $11.25^\circ$ resolution. The reported energy range is $0.1$0–$0.1$1 keV with $0.1$2, a nominal full-sweep cadence of $0.1$3 s, and chevron-pair microchannel plates with position-sensitive anodes (Kumar et al., 23 Jul 2025).

STEPS consists of six independent silicon-PIN detector units mounted in fixed look directions. The named look directions are Sun-Radial (SR), Inter-Mediate (IM), Parker-Spiral (PS), North-Pointing (NP), South-Pointing (SP), and Earth-Pointing (EP). In the one-year operational analysis, four units—PS, EP, IM, and NP—are identified as the working channels delivering high-quality data, while SR and SP are affected by scattered-sunlight saturation and therefore operate with high low-level discriminator thresholds (Sebastian et al., 24 Jul 2025). In the quiet-time spectral study, SR and SP are omitted because they suffer light saturation, and the analysis is restricted to PS, IM, EP, and NP (Gupta et al., 16 Jul 2025).

The STEPS directional geometry is central to ASPEX’s scientific role. PS, IM, and EP look approximately within the ecliptic plane, whereas NP is aligned along $0.1$4, normal to the ecliptic. More specifically, PS is aligned along the nominal Parker spiral direction, IM is tilted by about $0.1$5 from the $0.1$6-axis in the ecliptic, EP points roughly sunward in the ecliptic, and NP samples the out-of-ecliptic direction (Gupta et al., 16 Jul 2025). Detector construction differs by channel: IM and NP are single-window silicon detectors with a $0.1$7 dead layer, while PS and EP are dual-window Si-PIN stacks backed by a scintillator+SiPM unit; for species-integrated spectral fits, the PS-Out and EP-Out outer detectors are used (Gupta et al., 16 Jul 2025, Sebastian et al., 24 Jul 2025).

A notable operational development is the “toggling” mode introduced on 11 May 2024, with 5 min high-gain and 5 min low-gain intervals, extending the continuous STEPS energy range from approximately $0.1$8 MeV to $0.1$9 MeV in the working units (Sebastian et al., 24 Jul 2025).

3. Measurement formalism and data reduction

For STEPS, raw count rates are converted into differential fluxes through the standard relation

$20$0

where $20$1 is the background-subtracted count rate, $20$2 is the geometric factor, $20$3 is the energy-bin width, and $20$4 is the integration time. In default mode $20$5 s, whereas in toggling mode $20$6 s (Sebastian et al., 24 Jul 2025). After transformation from spacecraft coordinates to GSE, hourly averaged directional spectra $20$7 are constructed for each look direction (Gupta et al., 16 Jul 2025).

Over quiet intervals, the STEPS spectra are modeled as power laws,

$20$8

with $20$9 obtained from a log-linear fit of $\sim 20$0 versus $\sim 20$1, equivalently

$\sim 20$2

The quiet-time study selected intervals from January to November 2024, excluding spacecraft maneuvers in April, using the Dayeh et al. (2017) mean–variance method: hourly fluxes are sorted, grouped into 24 h windows, and a threshold is identified where the variance rises sharply; fluxes below that threshold are classified as quiet (Gupta et al., 16 Jul 2025).

For SWIS, each voltage sweep yields an energy histogram of differential energy flux $\sim 20$3. These measurements are converted to velocity space using $\sim 20$4, then binned into discrete speed channels and angular sectors for partial reconstruction of the three-dimensional VDF in two planes (Kumar et al., 23 Jul 2025). Assuming an isotropic Maxwellian proton distribution in the spacecraft frame, the scalar moments are evaluated non-parametrically:

$\sim 20$5

$\sim 20$6

$\sim 20$7

In practice, bins with $\sim 20$8 are excluded to avoid low-energy noise, background and instrumental response are removed, and uncertainties from counting statistics, energy resolution, and fit errors are combined in quadrature (Kumar et al., 23 Jul 2025).

4. STEPS performance, validation, and energetic-ion observations

The first-year STEPS operations report states that four out of six units exhibit stable detector response over 08 January 2024 to 28 February 2025. Calibration-pulse monitoring showed the gain slope unchanged within $\sim 20$9 ADC channels, corresponding to $20$0 gain drift, and the high-voltage monitor remained stable within $20$1 V for PS, EP, IM, and NP. On that basis, no in-flight gain adjustments were required, with future recalibration to be triggered if gain drifts exceed $20$2 (Sebastian et al., 24 Jul 2025).

External validation was carried out through cross-comparison with ACE-EPAM. During the 15–18 December 2024 event, hourly averaged STEPS-PS (outer) fluxes were compared with ACE-EPAM LEMS120 and LEFS channels in matched energy bands, yielding $20$3 values between $20$4 and $20$5. The one-year report interprets $20$6 across these comparisons as evidence of agreement in both absolute flux scale and temporal evolution (Sebastian et al., 24 Jul 2025). An earlier cross-validation during the earth-bound phase also compared STEPS with ACE-EPAM and GOES-18 SEISS-MPSH, finding $20$7 in selected ACE comparisons and $20$8 in GOES comparisons inside the magnetosphere (Chakrabarty et al., 27 Jun 2025).

Quiet-time STEPS measurements furnish one of the payload’s clearest scientific results. Across more than 20 independent intervals and across the four look directions PS, IM, EP, and NP, the spectral index is reported as $20$9, with nearly identical values in all directions on time scales of a few days. The study interprets this as demonstration that quiet-time suprathermal ions at L1 are isotropically distributed, thereby validating the isotropy assumption used in Parker transport equation models such as Fisk and Gloeckler (2008) (Gupta et al., 16 Jul 2025). A common misconception is that “quiet” intervals correspond to a compositionally pristine background. The ACE-ULEIS abundance analysis argues against that interpretation: elevated $6$0 appears in about $6$1 of quiet intervals, Fe/O near or above unity appears in about $6$2, and C/O varies between $6$3 and $6$4–$6$5, suggesting residual contributions from both impulsive and gradual SEP populations (Gupta et al., 16 Jul 2025).

Transient intervals reveal a different directional behavior. During the 17 December 2024 IP-shock-driven enhancement, the suprathermal spectrum in the $6$6–$6$7 MeV range hardened to $6$8, while above about $6$9 MeV it steepened to $360^\circ$0. Peak intensities reached $360^\circ$1 and $360^\circ$2, with stronger enhancements in PS and EP than in NP and IM, indicating directional asymmetry during the event (Sebastian et al., 24 Jul 2025).

The earth-bound STEPS campaign showed that IMF $360^\circ$3 polarity modulates the energetic-ion environment in the interplanetary medium, magnetosheath, and magnetosphere by changing the balance between external ICME-driven injections and internal substorm acceleration. In southward $360^\circ$4 intervals, hard spectra with $360^\circ$5–$360^\circ$6 were observed widely; in magnetospheric intervals associated with moderate substorm activity, soft spectra with $360^\circ$7 were measured by STEPS and GOES while ACE remained hard. The same study reported directional anisotropy through $360^\circ$8, with mild anisotropy in the sheath and larger anisotropy in selected magnetospheric intervals (Chakrabarty et al., 27 Jun 2025).

5. SWIS observations and joint ASPEX event diagnostics

A 17-month SWIS comparison against Wind-3DP, Wind-SWE-FC, and DSCOVR-PlasMag-FC found excellent agreement in bulk velocity with Wind, reporting slope values of $360^\circ$9 and $\pm 1.5^\circ$0 and $\pm 1.5^\circ$1 in both cases. Thermal speed and density showed larger scatter, which the study attributes to inter-instrument differences in energy coverage and calibration (Kumar et al., 23 Jul 2025). Across January 2024 to May 2025, SWIS reported mean bulk speed $\pm 1.5^\circ$2, mean proton density $\pm 1.5^\circ$3, mean thermal speed $\pm 1.5^\circ$4, housekeeping stability within $\pm 1.5^\circ$5 of nominal, and telemetry data gaps below $\pm 1.5^\circ$6 of total operational time (Kumar et al., 23 Jul 2025).

The 07 August 2024 ICME case study established SWIS as a solar-wind transient monitor. When the event arrived at L1 on 11 August 2024, SWIS captured the canonical ICME signatures: proton bulk speed rose from about $\pm 1.5^\circ$7 to about $\pm 1.5^\circ$8, thermal speed increased from about $\pm 1.5^\circ$9 to $22.5^\circ$0 in the sheath, proton density jumped from about $22.5^\circ$1 to about $22.5^\circ$2 at the shock, and the magnetic-field magnitude from Aditya-L1 MAG peaked near $22.5^\circ$3 nT in the cloud. Timing agreement with Wind and DSCOVR was reported to be better than 5 min for shock, sheath, and magnetic-cloud boundaries (Kumar et al., 23 Jul 2025).

SWIS also resolved velocity-fluctuation spectra. Power spectral densities computed via Welch’s method on 100 s-resampled data with 50% overlapping Hann windows gave an inertial-range exponent $22.5^\circ$4 over $22.5^\circ$5–$22.5^\circ$6 Hz, close to $22.5^\circ$7. The report characterizes this as a Kolmogorov-like slope and as evidence that SWIS resolves MHD-scale turbulence with high fidelity (Kumar et al., 23 Jul 2025).

The May 2024 ICME–ICME interaction study is the clearest demonstration of ASPEX as a coordinated payload rather than two separate instruments. Using SWIS THA-1 and THA-2 together with STEPS, the study reports a two-orthogonal-plane perspective of the interaction region at L1. A forward shock driven by complex ejecta impacted the trailing magnetic cloud of ICME 4 at 12 May 08:47 UTC, with reported parameters $22.5^\circ$8, upstream speed $22.5^\circ$9, $11.25^\circ$0, and compression ratio $11.25^\circ$1. The downstream interaction region persisted for about 13 h. THA-2 recorded $11.25^\circ$2–$11.25^\circ$3 times higher flux than THA-1 in the shocked interval, while STEPS observed an “Energetic Storm Particle” event in which $11.25^\circ$4–$11.25^\circ$5 MeV ion fluxes rose by about $11.25^\circ$6 at shock crossing; EP showed no increase, which the study interprets as confirming a forward shock (Parashar et al., 15 Dec 2025). The same analysis attributes the broadened VDFs and suprathermal tails to collisionless shock heating plus turbulent cascade, and emphasizes cross-plane redistribution of proton and alpha populations (Parashar et al., 15 Dec 2025).

6. Scientific interpretation, modeling relevance, and prospective use

The accumulated ASPEX literature converges on several scientific themes. First, quiet-time suprathermal ions at 1 AU appear directionally isotropic over multi-day intervals but compositionally mixed, implying continuous replenishment of the seed population by prior impulsive and gradual solar transients rather than a purely local equilibrium reservoir (Gupta et al., 16 Jul 2025). Second, strong transients break that approximate isotropy, and the directional contrast between ecliptic and out-of-ecliptic channels provides leverage on shock geometry, seed-population anisotropy, and transport timescales (Sebastian et al., 24 Jul 2025, Parashar et al., 15 Dec 2025). Third, SWIS and STEPS together connect the low-energy solar-wind phase-space structure to MeV-class suprathermal signatures within single events (Kumar et al., 23 Jul 2025, Parashar et al., 15 Dec 2025).

Pre-launch modeling work had already anticipated this multi-directional diagnostic role. The SWASTi-SW framework was developed to generate synthetic in-situ datasets tailored to SWIS, using magnetogram-driven coronal modeling, a generalized Wang–Sheeley–Arge relation, and inner-heliospheric MHD to produce directional signatures of stream interaction regions. In those synthetic observables, radial flux jumps by factors of $11.25^\circ$7–$11.25^\circ$8 over about 6 h, meridional flow $11.25^\circ$9 reverses from $0.1$00 to $0.1$01, azimuthal flow $0.1$02 oscillates through $0.1$03, and sheath heating broadens the $0.1$04–$0.1$05 keV energy response. The same framework was proposed as a means to compare observed SWIS data against model “looks” and thereby constrain thermodynamic parameters such as the effective specific-heat ratio $0.1$06 (2207.13708).

Operational papers emphasize the broader scientific utility of ASPEX at L1. For STEPS, the reported combination of multi-directional sampling, continuous energy coverage from approximately $0.1$07 MeV to $0.1$08 MeV in working channels, and stable in-flight calibration is presented as a basis for studying suprathermal seed populations, shock acceleration, corotating interaction region acceleration, and long-term space-weather monitoring (Sebastian et al., 24 Jul 2025). For SWIS, high-cadence VDF measurements, dual-plane directional coverage, and composition-sensitive proton/alpha observations are identified as enabling studies of kinetic processes, wave–particle interactions, ICMEs, shocks, and turbulence, with explicit prospects for joint analysis with MAG, PAPA/STEPS, VELC, SUIT, and ground-based radio and coronagraph observations (Kumar et al., 23 Jul 2025). Taken together, these results position ASPEX as both a measurement system for plasma kinetics and a directional observatory of heliospheric particle acceleration at the Sun–Earth L1 point.