Parker Solar Probe: In Situ Solar Exploration

Updated 4 December 2025

Parker Solar Probe is a NASA mission that performs direct in situ studies of the solar corona, capturing plasma, magnetic fields, and energetic particles near the Sun.
Its advanced instrumentation—including SWEAP, FIELDS, ISOIS, and WISPR—provides high-cadence measurements to analyze solar wind acceleration, turbulence, and magnetic topology.
PSP data refines global MHD models by mapping the Alfvén surface, turbulence, shock acceleration, and dust impacts, offering practical insights for heliospheric research.

The Parker Solar Probe (PSP) is NASA's flagship solar mission designed for in situ exploration of the solar corona and the near-Sun heliosphere, with a perihelion as small as 9.86 R⊙ (0.046 AU). PSP enables direct measurements of plasma conditions, magnetic fields, energetic particles, and dust in regions previously accessible only to remote sensing, transforming the understanding of solar wind acceleration, heliospheric structure, turbulence and particle acceleration, the Alfvén critical surface, and coronal magnetic topology.

1. Mission Profile, Instrumentation, and Observational Capabilities

PSP executes a heliocentric orbit with repeated gravity assists at Venus, reducing perihelion with each encounter and ultimately sampling the inner corona and sub-Alfvénic wind regime. Its principal science payload consists of:

SWEAP (Solar Wind Electrons, Alphas and Protons):
- Solar Probe Cup (SPC): Sun-pointing Faraday cup. Measures ions (100–6000 V E/q; $v_p$ from 139–1072 km s⁻¹), electrons (100–1500 V). Full VDFs at up to 4.6 Hz cadence; robust to temperatures >1400 °C at closest perihelion. Enables determination of moments $n, u, w_\mathrm{th}$ and electron/proton/alpha compositions (Case et al., 2019).
- SPAN-Ion and SPAN-Electron: Top-hat ESAs providing 3D VDFs for ions and electrons (2 eV–30 keV, full-sky merging), capturing core, halo, and strahl subpopulations at 0.2 s (electrons) to 0.8 s (ions) cadence, resolving anisotropies, heat fluxes, and kinetic instabilities (2002.04080, Halekas et al., 2019).
FIELDS: Measures DC/AC magnetic and electric fields (fluxgate and search-coil), DC B up to ≈100 Hz, E and B up to ≈10 kHz, plus quasi-thermal noise for $n_e$ . Also includes radio antennas (10.5 kHz–19.2 MHz) for in situ radio burst detection (Stanislavsky et al., 2021).
ISOIS (Integrated Science Investigation of the Sun): EPI-Lo and EPI-Hi telescopes for energetic ions (20 keV–200 MeV), omnidirectional composition, anisotropy, time profiles; sectoring allows field-aligned anisotropy measurement (Leske et al., 2019).
WISPR (Wide-field Imager for Solar Probe): White-light coronagraph with two telescopes (13.5°–53°, 50°–108° elongation), 0.1° pixels, imaging coronal structure inside 0.25 AU down to ≲10 R⊙ at sub-degree resolution (Poirier et al., 2019, Liewer et al., 2022).
Onboard data acquisition is continuous during perihelion encounters. For co-rotational phases where PSP's angular rate matches the Sun's, constant longitude sampling enables true temporal evolution analysis rather than spatial scanning (Roberts et al., 2017).

2. Solar Wind Structure, Alfvén Surface, and Coronal Connectivity

PSP's prime mission objective is the direct measurement and reconstruction of the nascent solar wind from its roots in the coronal magnetic field through the critical Alfvén surface to the interplanetary medium. Recent results (Finley, 8 Sep 2025) have established:

Alfvén surface position ( $r_A$ ): The transition where the wind speed exceeds Alfvén speed, $v(r) = v_A(r)$ , is mapped using FIELDS for $B$ and SWEAP for $n, v$ . The average $r_A$ increased from 11 R⊙ (solar minimum) to 16 R⊙ (solar maximum, Cycle 25), calculated using

$v_A(r) = \frac{B(r)}{\sqrt{\mu_0 \rho(r)}} \qquad M_A(r) = \frac{v(r)}{v_A(r)}$

Morphology: The Alfvén surface is highly structured; it is locally compressed above the heliospheric current sheet (HCS) and pseudostreamers and more radial/extended above fast, low-density streams and active-region nests. Fluctuations of 10–40% in $r_A$ were observed both longitudinally and temporally.
Angular momentum-loss: Longitudinally averaged solar wind torque increased from $1.4\times 10^{30}$ to $3\times 10^{30}\,\mathrm{erg}$ as $r_A$ grew, with torque $\tau = (2/3) \dot{M} \Omega_\odot \langle r_A\rangle^2$ .
Scaling with magnetization: The relation $\langle r_A\rangle/R_\odot = K \Upsilon_\mathrm{open}^m$ with wind magnetization $\Upsilon_\mathrm{open}$ showed empirical exponents ( $m \approx 0.36$ ) matching MHD simulations, but the observed $r_A$ was systematically $\sim$ 30% larger than predicted by axisymmetric models.
PFSS modeling: PSP coronal connectivity is reconstructed using PFSS (source surface $\sim$ 2.5 R⊙) and Parker-spiral ballistic mapping, cross-validated with FIELDS polarity and WISPR streamer structure (Finley, 8 Sep 2025, Riley et al., 2021).

3. Coronal Rays, Streamers, and Magnetic Topology

WISPR has provided the first high-resolution imaging of coronal rays and plasma sheets below 15 R⊙ (Poirier et al., 2019). These observations, in combination with large-scale MHD modeling (e.g., MULTI-VP, PFSS), reveal:

Fine-scale structure: Coronal rays corresponding to the plasma sheet above the HCS are resolved to $\Delta\theta\simeq5^\circ$ , matching in situ HPS crossings. Substructure within streamers, including lane splitting (folding in the HPS), pseudostreamers (broad, diffuse, lacking true current sheet), and $1-2^\circ$ rays at streamer edges, indicate strong flux-tube inhomogeneity.
3D mapping: The geometrically unique vantage of PSP enables stereoscopic reconstruction of the longitude and latitude of coronal rays, yielding $\pm5^\circ$ – $7^\circ$ accuracy, crucial for connecting remote imaging to in situ field/plasma measurements (Liewer et al., 2022).
Synthetic imaging and model validation: Synthetic white-light images from 1D MHD (density cubes, PFSS fields, and the Thomson integral) match observed structures. Coronal structure observed by WISPR provides constraints for validating the dynamic corona in global 3D MHD models (Poirier et al., 2019, Biondo et al., 2022).

4. Energetic Particles, Seed Populations, and Shock Acceleration

PSP's energetic particle suite (ISOIS) captures the evolution and composition of SEPs from their coronal origins:

Three-stage acceleration in SEP events: Multi-instrument analysis of a 2022 August 27 event demonstrated distinct spectral evolution: stage 1 (coronal shock at 2.85 R⊙, $\gamma_1=3.49$ ), stage 2 (CME–CME interaction at 40 R⊙ with spectral hardening, $\gamma_2=2.16$ ), stage 3 (in situ ESPs at 0.38 AU, $\gamma_3=2.59$ ) (Chen et al., 30 May 2024).
Seed population preconditioning: Solar wind suprathermal pools are strongly enhanced during ICME passage, with the spectrum hardening below 1 MeV to approach the “kappa” index limit ( $\gamma\to-1.5$ ), supporting the importance of pre-accelerated seed populations for subsequent SEP production (Schwadron et al., 2019).
Anisotropy, spectral softness, and connection mapping: Small impulsive events exhibit extreme outward anisotropy ( $A=I_\text{out}/I_\text{in}\sim30$ ), spectral softness ( $\gamma\sim-4.4$ ), and magnetic connectivity spanning $>80^\circ$ longitude, highlighting the importance of field-line expansion, HCS drift, and extended injection (Leske et al., 2019).

5. Turbulence, Dissipation, and Solar Wind Heating

PSP in situ measurements have enabled deep tests of plasma turbulence and energy dissipation:

Spectral and cross-helicity structure: The observed inertial-range spectral slope evolves from $-3/2$ (IK) at 0.16–0.3 AU to $-5/3$ (Kolmogorov) by 1 AU (Parashar et al., 2022). Close to the Sun, normalized cross-helicity $\sigma_c\sim0.8$ –$0.9$ (outward-dominated), decaying to $\sim0.5$ at several AU. Residual energy stays negative ( $\sigma_r\approx -0.3$ – $-0.6$ ), as predicted by semi-empirical turbulence models (Cranmer, 2018).
Energy transfer rates: Turbulent cascade rates inferred from von Kármán and Yaglom scaling ( $\epsilon_\text{VK}\sim 10^5$ J kg⁻¹ s⁻¹ at 0.2 AU, scaling $\sim r^{-2}$ ) are sufficient to energize protons at the required rates for coronal heating; compressible cascade rates also fall by five orders of magnitude between 0.2 AU and 1.6 AU (Parashar et al., 2022).
Intermittency and switchbacks: Intermittent structures, especially switchbacks (magnetic field reversals caused by Alfvénic twist pulses launched by minifilament eruptions and coronal jets), are directly detected, with amplitude and duration scaling with jetlet, spicule, and jet size. Such coherent features dominate localized heating and are traced to reconnection-driven, helical pulses in the low corona (Sterling et al., 2020).
Anisotropy and PVI diagnostics: $k_\perp\gg k_\parallel$ anisotropy and enhanced local cascade in switchbacks indicate quasi-two-dimensional turbulence. Pressure-strain and local energy transfer diagnostics link intermittent structures to localized ion heating (Parashar et al., 2022).

6. Dust and the Inner Solar System Meteoroid Complex

PSP's traverse into the inner zodiacal cloud has enabled the characterization of near-Sun dust:

Impactor detection: FIELDS voltage spikes and EPI-Lo collimator anomalies identify impacts from unbound, radiation-pressure–driven “ $\beta$ -meteoroids” (hyperbolic orbits; typical size $<1\,\mu$ m, $v=100$ –250 km/s), with a radial density scaling $n(r)\propto r^{-1.97}$ (Szalay et al., 2019).
Dust ejection rate: The current mass-loss from the solar system by small grains is estimated as $1$–$14$ tons/s, in agreement with dynamical models and prior Helios measurements (Szalay et al., 2019).
Implications: As PSP reaches smaller perihelia, dust impact rates and associated plasma production are expected to increase, providing new opportunities to paper dust-plasma interactions and losses.

7. Synergy with Global MHD Models and 3D Reconstruction

PSP provides essential ground-truth for global, data-driven MHD modeling of the corona and inner heliosphere:

Data-driven modeling (RIMAP): Hybrid analytical/numerical approaches take PSP in situ data at 0.1–0.2 AU as boundary conditions (e.g., at 5 R⊙), numerically integrate the time-dependent ideal MHD equations (using PLUTO), and connect remote Metis coronagraph measurements at 3–6 R⊙ to the in situ domain, validating the structure of the Parker spiral, wind speed, and density (Biondo et al., 2022).
Validation and context: Thermodynamic MHD models (full energy equation, Spitzer–Härm conduction, radiative losses, empirical heating) most accurately capture the global topology, wind structure, and dynamic HCS alignments seen in PSP, STEREO, and ACE/Wind multi-spacecraft trackings (Riley et al., 2021).
Consequence for open flux and sub-Alfvénic domain: Even close to the Sun, a persistent “open flux deficit” is found, attributable to limitations of surface magnetograms and insufficient resolution of the global field, not to interplanetary dynamical processes.

Parker Solar Probe’s multi-instrument, multi-scale, and multi-encounter observations have redefined the empirical and theoretical basis for solar wind generation, energetic particle acceleration, coronal magnetic topology, and mass/energy loss in the heliosphere. The campaign is producing a reference dataset for calibration of MHD/stochastic wind driver models, turbulence theory, heliospheric mapping, and stellar wind spin-down laws, while also presenting new challenges such as resolving the spatiotemporal structuring of the Alfvén boundary, flux emergence and loss, and the interconnectedness of microscopic and macroscopic plasma processes (Finley, 8 Sep 2025, Poirier et al., 2019, Parashar et al., 2022, Biondo et al., 2022).