MAVEN & Tianwen-1: Martian Plasma Observations

• MAVEN and Tianwen-1 observations are coordinated multi‐spacecraft measurements that map Mars' induced magnetosphere and plasma boundaries with high temporal resolution.
• They employ triaxial fluxgate magnetometers and plasma instruments to quantify solar wind dynamic pressure, IMF orientation, and induced field strength.
• Integrated data analysis reveals how solar wind drivers modulate boundary positions, magnetic draping, and atmospheric escape processes.

Mars’ upper atmosphere is shaped by the interaction with the solar wind, in the absence of a global intrinsic dipole field. Coordinated multi-spacecraft observations from NASA’s MAVEN and China’s Tianwen-1 missions have enabled a detailed, quantitatively robust characterization of this induced magnetospheric system, with simultaneous access to the undisturbed upstream solar wind and downstream plasma boundary responses. The synergistic use of magnetic and ion measurements—leveraging MAVEN MAG and SWIA, Tianwen-1’s MOMAG and MINPA—provides cross-validated, high-resolution mapping of Martian plasma boundaries, solar-wind drivers (dynamic pressure, IMF orientation/magnitude), transient solar-wind structures, and their downstream atmospheric and ionospheric consequences.

1. Instrumentation, Calibration, and Data Coverage

Both MAVEN and Tianwen-1 are equipped with high-sensitivity triaxial fluxgate magnetometers (MAVEN/MAG, Tianwen-1/MOMAG) and solar wind/plasma instrumentation (MAVEN/SWIA, Tianwen-1/MINPA).

• MAVEN: Dual-sensor fluxgates on a 3.6 m boom, intrinsic noise floor ≲0.03 nT, sampling at up to 32 Hz. SWIA measures upstream solar wind ion density ($\rho$) and velocity ($\mathbf{v}$), with frequent crossings into both undisturbed solar wind and Mars’ magnetosheath/induced magnetosphere.
• Tianwen-1: MOMAG features two fluxgate sensors on a 3.19 m boom (outer sensor), with dynamic range ±1000 nT and noise floor ≲0.05 nT. The orbiter’s highly inclined apoapsis (to $\sim$4.17 $R_\mathrm{Mars}$) ensures prolonged sampling (~50–75% of each orbit) of the undisturbed interplanetary field. MINPA provides ion and plasma moments.

Calibration of MOMAG relies on a dual-sensor (gradiometric) approach. Artificial “jumps” (instrumental/spacecraft-induced) are removed by cross-correlation. Static offset is determined via the Wang–Pan method, exploiting the near-constant magnitude of Alfvénic fluctuations in the solar wind. These procedures, reinforced by time-domain smoothing (72 hr window), achieve cross-instrument agreement with MAVEN at the sub-0.1 nT level in the solar wind, even in the challenging uncleaned spacecraft environment (Zou et al., 2023).

2. Simultaneous Upstream–Downstream Observational Methodology

Crucially, the coordinated orbit geometries allow one spacecraft (usually Tianwen-1) to be positioned in the true upstream solar wind, while the other (MAVEN) samples the magnetosheath or induced magnetosphere. Real-time in situ upstream measurements of $\rho$, $\mathbf{v}$, and IMF vector $\mathbf{B}$—from SWIA (MAVEN) or MINPA+MOMAG (Tianwen-1)—are relayed to the downstream observer. Bow-shock positions are systematically determined from abrupt magnetic-field jumps ($>|B|$ increases of $>5$ nT, $\sim$1-min $\sigma_B$ doubling), wave signatures, and ion heating.

The resulting dataset comprises $\sim$1.7 million simultaneous upstream–downstream pairs (Nov 2021–Aug 2022), with upstream conditions unambiguously classified by real-time bow-shock identification (Cheng et al., 21 Dec 2025).

3. Plasma and Magnetic Field Parameters: Definitions and Statistical Distributions

Key physical quantities mapped include:

• Solar-wind dynamic pressure: $P_{\rm dyn} = \rho v^2$, median $P_{\rm dyn} \approx 0.7$ nPa.
• IMF magnitude: $B_{\rm IMF} = |\mathbf{B}|$, median $B_{\rm IMF} \approx 3.02$ nT.
• Alfvén Mach number: $M_A = v/v_A$, $v_A = B / \sqrt{\mu_0 \rho}$, with $M_A^2 = P_{\rm dyn} / P_B \approx 152$ ($M_A \approx 12.3$).
• Induced field strength: $B_{\rm ind} = |\mathbf{B}_{\rm down}|$ downstream of the shock.
• Compression ratio: $C_R = B_{\rm ind} / B_{\rm IMF}$, typically ranging from $\sim 2$ (near subsolar MPB) to $\sim 1.2$ (flanks).
• Clock-angle departure: For vector $\mathbf{B}$ in the $(Y_{\rm MSO}, Z_{\rm MSO})$ plane, the clock angle is $\theta = \arctan2(B_{Y}, B_{Z})$; $$\Delta\theta = \theta_{\rm down} - \theta_{\rm IMF}$$ quantifies rotation between upstream IMF and downstream field.

Statistical analysis—binning by distance from the bow shock in MSE coordinates, IMF orientation, and upstream drivers—produces full 2D spatial maps of $|B_{\rm ind}|$, $C_R$, and $|\Delta\theta|$ (Cheng et al., 21 Dec 2025).

4. Joint Magnetospheric Findings: Solar Wind Forcing and Magnetosheath Structure

4.1 Influence of Upstream Dynamic and Magnetic Pressure

• Both $B_{\rm ind}$ and $C_R$ increase with solar-wind dynamic pressure $P_{\rm dyn}$, but $C_R$ decreases with increasing $B_{\rm IMF}$. High magnetic pressure “stiffens” IMF draping around the planetary obstacle, reducing compression.
• Quasi-perpendicular IMF configurations ($\mathbf{B} \perp \mathbf{v}$, $\theta_{Bn} > 45^\circ$) yield $B_{\rm ind}$ values $15-20\%$ larger than quasi-parallel IMF, reflecting enhanced mass-loading efficiency. Mass loading and associated field amplification are suppressed in quasi-parallel orientation ($\mathbf{B} \parallel \mathbf{v}$).

4.2 Spatial Evolution: Clock-angle Departure

• $|\Delta\theta|$ immediately downstream of the bow shock is low ($\lesssim 30^\circ$), rising across the magnetosheath to $\sim 45^\circ$ at the magnetic pile-up boundary, and exceeding $60^\circ$ within the induced magnetosphere. This identifies a transition from well-organized IMF draping to increasingly turbulent or locally structured magnetospheric fields.
• The magnitude and evolution of $|\Delta\theta|$ depend on upstream drivers: low $P_{\rm dyn}$, high $B_{\rm IMF}$, and low $M_A$ minimize clock-angle departure, and under quasi-parallel IMF, departures exceed $50^\circ$, challenging use of local clock angle as an IMF proxy. For quasi-perpendicular IMF, $|\Delta\theta| \lesssim 40^\circ$, validating single-spacecraft proxies (Cheng et al., 21 Dec 2025).

5. Validation, Inter-Calibration, and Event Studies

Extensive cross-comparison between processed MOMAG and MAVEN/MAG confirms high statistical and event-level consistency:

Parameter	Tianwen-1 (MOMAG/MINPA)	MAVEN (MAG/SWIA)	Agreement Metrics
$|B|$ mean (solar wind)	3.02 nT	3.07 nT	Median diff: 0.05 nT, $r > 0.9$
$v$ (solar wind)	--	--	$|\Delta v|/v < 3\%$, $r = 0.92-0.94$
$T$ (solar wind)	--	--	Underestimate: $\sim$21\%, $r = 0.74$

Event studies (e.g. simultaneous bow shock crossings, magnetic holes, SIR/ICME passages) show near one-to-one correspondence in $|B|$, vector orientation, and fluctuation morphology. Especially, joint analysis of 158 MOMAG bow shock crossings (Nov 13–Dec 31, 2021) reveals excellent spatial match to modeled average boundaries, with real-time dual-point crossing pairs confirming south–north asymmetry and rapid (up to $20$ km/s) flank motion (Wang et al., 2023, Zou et al., 2023, Chi et al., 2023).

MINPA plasma velocity matches SWIA within a few percent post background correction. Density and pressure, affected by MINPA's $2\pi$ FOV, can be corrected either by geometric scaling ($k=2$) or Maxwellian extrapolation, provided the velocity distribution is close to isotropic (Wang et al., 20 Mar 2024).

6. Solar Transients, Atmospheric Coupling, and Multi-Point Science

Coordinated observations have produced the first reliable multi-point ICME and SIR catalogue at Mars. For example:

• ICME-1 (2021-12-10): $⟨|B|⟩ = 7.29$ nT (MOMAG), $6.74$ nT (MAVEN), $r=0.97$; $\sigma(|B|) \approx 1.2$ nT.
• SIRs: Mean $|B|$ between $4.95$ and $9.5$ nT, MOMAG/MAVEN agreement $<0.7$ nT.

ICMEs produce up to fourfold increases in dynamic pressure, compressing bow shock and magnetic pile-up boundaries, transiently enabling solar-wind field penetration below $400$ km altitude. SIRs also drive periodic compressions, reconnection enhancement, and variable ion outflow rates—linking heliospheric drivers directly to atmospheric escape (Chi et al., 2023).

Multi-instrument coverage (MAG, SWIA, STATIC on MAVEN; MOMAG, MINPA, MEPA on Tianwen-1) enables resolution of spatial versus temporal variations, shock-normal determinations, and 4D dissection of solar transient evolution as they interact with Martian boundaries and atmosphere.

7. Implications for Magnetospheric Physics and Future Research

The coordinated MAVEN–Tianwen-1 dataset constitutes a transformative advance for Martian space plasma science:

• It removes the need for steady-state assumptions in boundary-proxy modeling, for the first time quantitatively attributing downstream responses to contemporaneous solar-wind drivers.
• It reveals how $P_{\rm dyn}$, $B_{\rm IMF}$, $M_A$, and IMF geometry separately control induced field strength, draping, magnetosheath structure, and proxy reliability.
• Inter-instrument calibration experiments provide a robust blueprint for future magnetometry on uncleaned orbiters and enable decadal-scale continuity for Mars’ upstream space-weather monitoring.
• Broader implications include coupling of Martian atmospheric loss processes to heliospheric forcing, the mapping of global and hemispheric asymmetries in the bow shock, and direct comparative planetology with other unmagnetized bodies.

Planned synergies with terrestrial and heliospheric missions will, during future solar conjunctions, allow full-Sun-to-Mars CME and SIR tracking, advancing predictive Mars space weather forecasting and atmospheric evolution studies (Cheng et al., 21 Dec 2025, Chi et al., 2023, Zou et al., 2023, Wang et al., 2023, Wang et al., 20 Mar 2024).