# Mercury's Altered Magnetosphere During a Sub-Alfvénic ICME Event: MESSENGER Observations and Inferred Asymmetric Alfvén Wing Formation From Global MHD Simulations

Charles Bowers, Caitríona Jackman, Xianzhe Jia, L. Z. Hadid, Weijie Sun, Laura Hayes, Ryan Dewey, Brandon Burkholder, Daragh Hollman, Sebastian Cervantes, et al.

► To cite this version:

Charles Bowers, Caitríona Jackman, Xianzhe Jia, L. Z. Hadid, Weijie Sun, et al. Mercury's Altered Magnetosphere During a Sub-Alfvénic ICME Event: MESSENGER Observations and Inferred Asymmetric Alfvén Wing Formation From Global MHD Simulations. Journal of Geophysical Research Space Physics, 2025, 130 (11), pp.e2025JA034248. 10.1029/2025JA034248. hal-05438851

HAL Id: hal-05438851

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Submitted on 3 Jan 2026

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# JGR Space Physics

## RESEARCH ARTICLE

10.1029/2025JA034248

**Key Points:**

* A rare, sub-Alfvénic solar wind impacted Mercury on 1 May 2013 driven by an ICME

* During this interval, MESSENGER observed a distorted magnetosphere, including a depleted southern lobe

* Global MHD simulations predict Alfvén wings formed during the event, providing a plausible explanation for the observations

**Correspondence to:**

C. F. Bowers,
charlie.bowers33@gmail.com

**Citation:**

Bowers, C. F., Jackman, C. M., Jia, X., Hadid, L. Z., Sun, W., Hayes, L. A., et al. (2025). Mercury's altered magnetosphere during a sub-Alfvénic ICME event: MESSENGER observations and inferred asymmetric Alfvén wing formation from global MHD simulations. *Journal of Geophysical Research: Space Physics*, 130, e2025JA034248. https://doi.org/10.1029/2025JA034248

Received 5 JUN 2025
Accepted 10 NOV 2025

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## Mercury's Altered Magnetosphere During a Sub-Alfvénic ICME Event: MESSENGER Observations and Inferred Asymmetric Alfvén Wing Formation From Global MHD Simulations

Charles F. Bowers¹, Caitríona M. Jackman¹, Xianzhe Jia², Lina Z. Hadid³, Weijie Sun⁴, Laura A. Hayes¹, Ryan M. Dewey², Brandon Burkholder⁵,⁶, Daragh M. Hollman¹, Sebastian Cervantes¹, Hans L. F. Huybrighs¹, and Matthew J. Rutala¹

¹Astronomy & Astrophysics Section, School of Cosmic Physics, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin, Ireland, ²Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA, ³LPP, CNRS, Observatoire de Paris, Sorbonne Université, Université Paris Saclay, École Polytechnique, Institut Polytechnique de Paris, Palaiseau, France, ⁴Space Sciences Laboratory, University of California, Berkeley, CA, USA, ⁵Goddard Planetary Heliophysics Insitute, University of Maryland Baltimore County, Baltimore, MD, USA, ⁶Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA

**Abstract** We present observations of a rare configuration of Mercury's magnetosphere in response to sub-Alfvénic upstream conditions, driven by an interplanetary coronal mass ejection (ICME) that impacted the planet on 1 May 2013. Using data from the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft, supported by a global three-dimensional magnetohydrodynamic (MHD) simulation of the event, we demonstrate that Mercury's magnetospheric response during this interval was distinct from the typical super-Alfvénic state. During the sub-Alfvénic upstream conditions, MESSENGER measured a distorted magnetotail with a depleted southern magnetotail lobe. An MHD simulation closely reproduces these observations, providing a plausible global context for the reconfiguration of Mercury's magnetosphere under sub-Alfvénic conditions. The simulation predicts that a pair of Alfvén wings formed during this event, redirecting magnetic flux and plasma within the magnetosphere. The interplanetary magnetic field orientation during this event was primarily sunward/dawnward, generating asymmetric Alfvén wings with respect to the flow direction, in contrast to previously observed north-south wing configurations at the planet. Using Solar Orbiter observations in the inner heliosphere, we estimate that the solar wind is sub-Alfvénic approximately 2.5 times per Earth year near solar maximum, with intervals lasting between 10 s and 12 hr. Studies of these rare, sub-Alfvénic solar wind-magnetospheric interactions provide valuable insights into exoplanet-stellar wind interactions under similarly sub-Alfvénic conditions where in situ observations are not available.

**Plain Language Summary** The Sun continuously emits a stream of plasma, known as the solar wind, which travels through the solar system at hundreds of kilometers per second in all directions. Normally, the speed of the solar wind far exceeds the speeds at which waves may travel, leading to the formation of shocks on the dayside of planets and other objects in its flow, analogous to shock waves ahead of super-sonic aircraft at Earth. However, in rare cases, the wave speed of the plasma surpasses the speed of the solar wind, which has a direct impact on a planet's magnetic field environment. Here, we explore the implications for this rare regime via observations from the Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) spacecraft supported by a three-dimensional computer simulation. Under these conditions, Mercury does not develop a dayside shock, and its nightside magnetic field differs significantly relative to the typical configuration. These rare solar wind conditions offer brief, but valuable glimpses into the magnetic interactions expected in many exoplanetary systems, where close-in planets are often immersed in a similar stellar wind environment.

## 1. Introduction

The Alfvénic Mach number ($M_A$), the ratio of solar wind speed to Alfvén speed, is a key parameter governing the interaction between the solar wind and planetary magnetospheres. Beyond ~ 0.08 AU, the solar wind speed $v_{sw}$ almost always exceeds both the Alfvén speed ($v_A$) and the fast-mode wave speed ($v_f$), which is the fastest

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magnetohydrodynamic (MHD) wave mode in a plasma (Cravens et al., 1997). As a result, magnetospheric in-
teractions with the solar wind in the solar system are nominally super-Alfvénic ($M_A \gg 1$) In this regime, in-
teractions with a planet's magnetosphere give rise to structures such as a bow shock upstream of the planet and a
windsock-shaped magnetopause and magnetotail, where magnetic field lines are stretched downstream by the
solar wind.

However, during extreme solar wind conditions, often imposed by interplanetary coronal mass ejections (ICMEs), $M_A$ can drop significantly and become sub-Alfvénic ($M_A < 1$) out to at least 1 AU (Hajra & Tsurutani, 2022; Lavraud & Borovsky, 2008). These rare sub-Alfvénic conditions are most commonly associated with magnetic clouds, which are regions within ICMEs characterized by low plasma density and large-scale looped magnetic fields (Burlaga et al., 1981). In such conditions upstream of a planetary magnetosphere, Alfvén waves emanate from the magnetopause along the interplanetary magnetic field (IMF) at $v_A$. Meanwhile, the plasma frozen into the IMF continues to flow anti-sunward at $v_{sw}$, generating a pair of standing magnetic field structures known as Alfvén wings, which are inclined more steeply relative to the solar wind direction than the magnetotail lobes that form under the super-Alfvénic conditions (Chané et al., 2012; Drell et al., 1965; Neubauer, 1980; Ridley, 2007; Sarantos & Slavin, 2009; Saur et al., 2013). In the sub-Alfvénic conditions of a magnetic cloud, no bow shock is formed upstream of the planet because Alfvén waves may travel upstream, causing the unshocked plasma within the magnetic cloud to interact directly with the magnetopause.

Recent studies combining MHD simulations with Magnetospheric Multiscale (MMS) spacecraft observations have investigated the impact of a sub-Alfvénic magnetic cloud that struck Earth in April 2023. During this event, Alfvén wings connected to the north and south pole of Earth guided the path of open field lines into the tail (Chen et al., 2024; Chen et al., 2025). The upstream IMF during the sub-Alfvénic portion of the magnetic cloud measured by the Wind spacecraft, was largely aligned with the dawn/dusk direction, generating dawnward and duskward Alfvén wings that extend in the ecliptic plane, in contrast to the northern and southern wings typically associated with outer-planetary moon-magnetospheric interactions at objects like Ganymede (Jia & Kivelson, 2021; Kivelson et al., 2004). This Alfvén wing configuration transformed the geometry of field-aligned currents, shifting their ionospheric connections from the magnetotail to the wings themselves (Beedle et al., 2024). Sub-Alfvénic conditions also enhanced dayside magnetic reconnection efficiency (Gurram et al., 2025; Hasegawa et al., 2024) and likely displaced nightside reconnection sites away from local midnight (Burkholder et al., 2024), which impacts the location of aurora at Earth.

As the solar wind propagates outward from the Sun, the average $M_A$ increases, making sub-Alfvénic conditions more likely to occur at Mercury than at other planets in the solar system (Diego et al., 2020; Lai et al., 2024; Liu et al., 2005; Sarantos & Slavin, 2009). Using observations from the MErcury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft and a 3D MHD simulation, Bowers et al. (2025) demonstrated that MESSENGER traversed an Alfvén wing structure during a very low-$M_A$ magnetic cloud, occurring on 30 December 2011 over a ~17 hr period. The upstream field was pointing primarily southward during this event, generating northern and southern Alfvén wings, enabling direct comparisons between Mercury's magnetospheric response and the persistent sub-Alfvénic conditions found at Ganymede. Given that sub-Alfvénic solar wind occurs at 1 AU approximately once every 2 years (Hajra & Tsurutani, 2022), it is likely that Mercury experienced several such events during MESSENGER's 4-year orbital mission.

We present MESSENGER observations of Mercury's altered magnetosphere during another rare sub-Alfvénic interval within a magnetic cloud of an ICME impact that arrived at Mercury on 1 May 2013. These observations show how Mercury's magnetosphere responds to a direct interaction with sub-Alfvénic plasma, providing insights into magnetosphere dynamics under extreme solar conditions. We run a 3D MHD simulation of the event to highlight the global impact on Mercury's magnetospheric configuration, the results of which are in close agreement with MESSENGER observations. Unlike the earlier event described by Bowers et al. (2025), the IMF during this interval was oriented sunward/dawnward. The MHD simulation predicts that asymmetric Alfvén wings formed with a markedly different geometry to those presented in Bowers et al. (2025), revealing the diversity of Alfvén wing configurations at Mercury with implications for their influence on magnetospheric dynamics. Using observations from Solar Orbiter (SO) in the inner heliosphere, we investigate the occurrence rate of sub-Alfvénic conditions at Mercury, providing context for both the MESSENGER mission and the upcoming BepiColombo mission to Mercury. Investigations into these rare sub-Alfvénic magnetospheric interactions at Mercury help us better understand moon–magnetosphere interactions in our solar system and offer insight into
exoplanet-star interactions, particularly for many close-in exoplanets likely immersed in sub-Alfvénic stellar winds (Saur et al., 2013).

## 2. Methodology

### 2.1. Instrumentation

We investigate the impact of a sub-Alfvénic solar wind interval at Mercury by analyzing data obtained by the magnetometer (MAG) (Anderson et al., 2007) and the Fast Imaging Plasma Spectrometer (FIPS) instrument (Andrews et al., 2007) onboard MESSENGER. MAG recorded the three-component magnetic field at a maximum sampling rate of 20 Hz. The data are reported in the aberrated Mercury Solar Magnetospheric MSM' coordinate frame, which is centered on Mercury's magnetic dipole. Mercury's dipole is offset northward by approximately 0.2 $R_M$ from the planet's center of mass (Anderson et al., 2011), where $R_M$ is the radius of Mercury ($1R_M = 2,440$ km). In this frame, $Z_{MSM'}$ points toward Mercury's northern magnetic pole, while $X_{MSM'}$ points in the opposite direction to the solar wind flow in Mercury's rest frame. Due to Mercury's orbital motion, the solar wind does not arrive at the planet exactly anti-sunward, leading to important aberration effects. The aberration angle between Mercury's orbital velocity and the vector that points radially outward from the Sun is calculated daily and is used to determine $X_{MSM'}$, assuming an average solar wind velocity of 400 km/s. This aberration angle varies from 5.6° at aphelion to 8.4° at perihelion. $Y_{MSM'}$ points in the opposite direction to Mercury's orbital motion.

The FIPS instrument measured the energy per unit charge ($E/q$), time of flight, and angle of incidence for ions within an energy range 0.05–13 keV taken over 64 energy bins. The sensor completed a sweep of energies in 10 s near the planet and ~66 s at high altitudes. Energy spectra within this energy range were taken within the instantaneous $1.4\pi$ sr field of view (FOV) of FIPS. MESSENGER's sunshade partially blocked plasma incoming from the anti-sunward direction into the detector, resulting in an effective FOV ~ $1.15\pi$ sr.

### 2.2. Event Identification

A total of 69 ICME impact events observed by MESSENGER were compiled by Winslow et al. (2015, 2017). These events were identified based on a set of visual criteria applied to the MAG data set time series, including the detection of an interplanetary shock followed by the ICME sheath and magnetic ejecta regions. During each of these ICME events, MESSENGER made at least one traversal through Mercury's magnetosphere. Given the rarity of sub-Alfvénic conditions at Earth (Hajra & Tsurutani, 2022), Mercury was likely immersed in a sub-Alfvénic magnetic cloud during only a subset of these ICMEs. Determining whether a given ICME contained a sub-Alfvénic magnetic cloud is nontrivial with MESSENGER observations. The lack of an upstream monitor, the diverse spacecraft orbital configurations across MESSENGER's mission, and the complex, event-specific geometry of Alfvén wings all necessitate a careful case-by-case analysis. In this study, we focus on a single ICME event that provides multiple independent lines of evidence for sub-Alfvénic upstream conditions within its magnetic cloud that we discuss in Section 3.1. While additional sub-Alfvénic cases may exist within the ICME event list, this event offers a particularly compelling example that motivates future systematic searches.

## 3. Results

### 3.1. MESSENGER Observations of the ICME Event

In Figures 1a–1c, we show a single ICME impact during which the magnetosphere of Mercury exhibits a number of observable features that strongly suggest sub-Alfvénic conditions. During this ICME event that lasted from 05:54:34 on 1 May 2013 to 00:52:12 on 2 May 2013, MESSENGER made three traversals of Mercury's magnetosphere. At 05:54:34 on 1 May 2013, MESSENGER observed an interplanetary shock as shown by a maroon line toward the left of Figures 1a–1c. This shock was characterized by a sharp increase in $|B|$ from ~30–60 nT. Downstream of the shock, MESSENGER was within the ICME sheath, characterized by a highly variable $|B|$ and enhanced FIPS flux (Figure 1c), suggesting a denser plasma population. MESSENGER then observed a series of inbound bow shock crossings from 06:02:32 to 06:20:27 before traversing Mercury's magnetosphere and crossing back into the upstream ICME at 08:51:39. These inbound and outbound bow shock crossings were identified via the list of boundary crossing intervals throughout the MESSENGER mission provided by Philpott et al. (2020) and are highlighted on Figure 1 by the orange and blue vertical lines,
traversal through Mercury's magnetosphere, $|B|$ returned to the nominal upstream field strength of ~20 nT at 00:52:12 on May 2nd, suggesting that the reverse shock marking the end of the ICME likely occurred while MESSENGER was within Mercury's magnetosphere.

## 3.2. MESSENGER Observations Within the Magnetic Cloud

The plasma and magnetic field properties within a single ICME vary depending on location within the structure. Compared to the ICME sheath region, the magnetic cloud typically exhibits smoother and slightly enhanced $|B|$, significantly lower plasma density, $n$, lower plasma temperatures and comparable bulk speeds (Kilpua et al., 2017; Lavraud & Borovsky, 2008). These factors combine to yield lower plasma $\beta$, higher $v_A$, and lower $M_A$ values within the magnetic cloud of the ICME compared to the ICME sheath. With this in mind, we focus on the second of the three MESSENGER traversals through Mercury's magnetosphere during the ICME event to evaluate the impact of low $M_A$ conditions on the planet's plasma environment. We note that while MESSENGER's third traversal also occurs within the magnetic cloud, $|B|$ was declining before MESSENGER entered the magnetosphere, suggesting that $M_A$ was higher during the third traversal compared to the second.

Figures 1d–1f show MESSENGER observations within the second orbital traversal during the ICME event. The many inbound bow shock crossings (vertical orange lines in Figure 1) occur at much higher altitudes (Figures 1g and 1j) compared to the nominal bow shock position. Following the final inbound bow shock crossing, MESSENGER sampled the magnetosheath, characterized by a highly variable magnetic field and hotter, denser plasma populations compared to the upstream conditions. FIPS counts within the upstream solar wind are typically very low due to the limited FOV of the instrument which largely excludes the cold, anti-sunward plasma populations characteristic of the solar wind. The plasma heating and density increase across the shock yield higher FIPS fluxes within the magnetosheath (Gershman et al., 2012, 2013), as shown in the green-shaded region in Figure 1f. MESSENGER observations are consistent with a planetary magnetosheath from 13:30 to 14:50. However, from 14:50 to 15:34, FIPS counts decrease dramatically, and $|B|$ is much smoother compared to the magnetosheath region. This period, highlighted as the yellow region in Figures 1d–1f, is indicative of unshocked plasma, suggesting that the bow shock has retreated sufficiently upstream to no longer significantly shock the magnetic cloud. Moreover, $|B|$ within this period drops to ~97 nT, which is similar to the upstream values, further suggesting that MESSENGER measured unshocked plasma within the magnetic cloud near the typical magnetopause location (Figures 1g and 1j).

This transition from shocked to unshocked plasma near the planet's magnetopause was also observed during the recent sub-Alfvénic magnetic cloud impact at Earth (Chen et al., 2024). During this event, MMS was within the terrestrial magnetosheath, while simultaneous measurements from OMNI upstream of the bow shock measured a minimum $M_A$ of ~0.6 for ~2 hr within the magnetic cloud of the ICME. As the upstream conditions transitioned from super-to sub-Alfvénic as measured by OMNI, MMS measurements evolved from shocked to unshocked solar wind plasma just upstream of the terrestrial magnetopause, consistent with similar observations presented in this study at Mercury. However, Mercury did not possess an upstream monitor during the MESSENGER mission, making direct measurements of the upstream $M_A$ conditions during this transition impossible. Furthermore, the FOV constraints of FIPS also make direct estimates of $n_{sw}$ challenging, so we are unable to estimate solar wind plasma moments during this event using MESSENGER observations in the same manner as those presented at Earth in Chen et al. (2024).

## 3.3. Estimate of $M_A$ Within the Magnetic Cloud

To approximate $M_A$ within the magnetic cloud, we determine a proxy for $M_A$ just upstream of the bow shock based on MAG data surrounding boundaries within Mercury's plasma environment in the same manner described in Winslow et al. (2017), Bowers et al. (2025). This method assumes a pressure balance between the upstream solar wind ($P_{SW}$) and Mercury's dayside magnetosphere ($P_M$). We assume $P_M$ is composed only of the magnetic pressure term supplied by Mercury's dipole field ($P_M = P_M^{mag}$). We further assume that $P_{SW}$ is mostly composed of a dynamic pressure term ($P_{SW}^{dyn}$) and a magnetic pressure term, $P_{SW}^{mag}$, while neglecting the thermal pressure for simplicity, yielding:

$$P_{SW}^{dyn} + P_{SW}^{mag} = P_M^{mag} \quad (1)$$
$P_M$ balances the portion of solar wind dynamic pressure that acts on the dayside magnetopause, and therefore $P_{SW}^{dyn}$ depends on the angle between Mercury's magnetopause normal and the solar wind velocity, $\theta_{MP}$, via the expression below:

$$P_{SW}^{dyn} = \rho v_{SW}^2 * K \cos^2 \theta_{MP} \quad (2)$$

Here, $\rho$ and $v_{SW}$ are the mass density and velocity of the solar wind, respectively. The scaling factor, $K$, is set to 0.88 based on numerical modeling of gas dynamics and acts as a drag coefficient of plasma around a planetary obstacle within a supersonic flow (Spreiter et al., 1966). Combining Equations 1 and 2 allows for an estimate of the upstream $M_A$ conditions using only MAG observations:

$$M_A = \frac{v_{SW}}{v_A} = \frac{\sqrt{\frac{P_M^{mag} - P_{SW}^{mag}}{\rho * K \cos^2 \theta}}}{\frac{B_{IMF}}{\sqrt{\mu_0 \rho}}} = \sqrt{\frac{\mu_0 (P_M^{mag} - P_{SW}^{mag})}{B_{IMF}^2 * K \cos^2 \theta}} \quad (3)$$

The estimate of $P_{SW}^{dyn}$ described in Equation 2 is only valid for small values of $\theta_{MP}$ ($\theta_{MP} < 60^{\circ}$) because at larger angles, the thermal component in the pressure balance system is no longer negligible. As a result, our estimate for $M_A$ is derived from the dayside boundary crossings along MESSENGER's outbound trajectory. We determine $B_{IMF}$ by averaging $|B|$ within the unshocked plasma region (yellow shaded region in Figures 1d–1f), yielding $B_{IMF} = 97$ nT. $P_{SW}^{mag}$ is then determined from $B_{IMF}$ ($P_{SW}^{mag} = \frac{B_{IMF}^2}{2\mu_0}$). Similarly, $P_M^{mag}$ is estimated from measurements 1 min inside the innermost magnetopause crossing along the outbound trajectory. The outbound magnetopause crossing interval was 16:44:46 to 16:50:14, as identified by the MESSENGER boundary crossing list described in Philpott et al. (2020). This calculation only considers the component of the magnetospheric magnetic field that is tangential to the nominal magnetopause surface determined by Winslow et al. (2013) based on the form provided by Shue et al. (1997) and shown as the inner dashed line in Figures 1g, 1h, and 1j. This tangential field was calculated with respect to the magnetopause surface assuming a consistent flaring angle of the magnetopause, which is determined by the $\alpha$ parameter in the magnetopause fit ($\alpha = 0.5$). We consider only the top 20% of the tangential $|B|$ measurements in the region to avoid underestimating the true pressure at the magnetopause by including diamagnetic depression effects that provide magnetospheric thermal pressure to balance the solar wind (Winslow et al., 2017). This procedure yields a $P_M^{mag}$ value of 8.6 nPa. To estimate $\theta_{MP}$, we first scale the empirical magnetopause model of Shue et al. (1997) to intersect the observed magnetopause crossing, assuming a constant flaring angle. We then calculate the normal vector to the resulting curve at the point of intersection. The angle $\theta_{MP}$ is defined as the angle between this magnetopause normal and the upstream plasma flow direction, which is assumed to be along the $-X_{MSM'}$ axis. On the dayside magnetopause within the magnetic cloud, we find $\theta_{MP} = 21^{\circ}$. Using this pressure-balance approach along MESSENGER's outbound trajectory, we estimate $M_A \approx 0.9$.

### 3.4. Caveats

It is important to note that the estimate for $M_A$ within the magnetic cloud of $\approx 0.9$ may be an overestimate of the true $M_A$ value. The pressure balance technique described above accounts for a possible thermal pressure component within Mercury's magnetosphere by considering only the top 20% of $|B|$ measurements just inside the magnetopause. This approach raises the magnetic pressure, $P_M^{mag}$, which may lead to an overestimated $M_A$ value. Furthermore, we assume that the thermal pressure within the solar wind is negligible. A non-negligible thermal pressure within the solar wind would decrease our estimate for $P_{dyn}^{SW}$, resulting in a lower $M_A$ value. Moreover, our approach to calculate $P_M^{mag}$ assumes that the average magnetopause shape is a good approximation for the measured magnetopause during the magnetic cloud, which may not be appropriate given the extreme magnetospheric conditions imposed by the ICME (Lai et al., 2024). A lower $P_{dyn}^{SW}$ value would lead to a higher flaring angle of the magnetopause than the nominal fit of $\alpha = 0.5$. For example, if we consider a flared magnetopause with $\alpha = 0.8$, our estimated value for $M_A \approx 0.8$. Furthermore, our estimate of $P_M^{mag}$ was made along the dayside magnetopause, which was $\sim 1.5$ hr after the unshocked solar wind interval. It is possible that the $M_A$ value increased within this time period, resulting in an overestimate of $M_A$ during MESSENGER's nightside traversal.
Indeed, the presence of magnetosheath plasma upstream of the dayside magnetopause (17:15–18:00) suggests that $M_A$ had increased compared to that during the unshocked solar wind region. While determining a precise value for $M_A$ during this interval is challenging, the presence of unshocked solar wind plasma near the magnetopause and this pressure balance estimation technique strongly suggest that the upstream conditions at Mercury were sub-Alfvénic for a significant portion of the magnetic cloud.

### 3.5. Estimate for $n_{sw}$, $v_{sw}$, $T$ Within the Magnetic Cloud

Our estimate for $M_A$ does not provide any constraints on $v_{sw}$ or $n_{sw}$ of the ICME. To estimate $v_{sw}$, we use the same technique described in Winslow et al. (2017), Bowers et al. (2025) in which we identify the launch time of the CME via coronagraph observations made by the Solar and Heliospheric Observatory (SOHO) and Solar TErrestrial Relations Observatory (STEREO). A list of automatically detected CME events was compiled by the Computer Aided CME Tracking software (CACTus) from coronagraph observations of SOHO, STEREO A, and STEREO B (Brueckner et al., 1995; Robbrecht et al., 2009).

To determine the most probable candidate CME from the CACTus database (http://secchi.nrl.navy.mil/cactus/; Robbrecht et al. (2009)), we search entries for CME eruptions between 49 and 9 hr prior to the ICME detection by MESSENGER. This corresponds to a time window from April 29 at 04:54:34 to April 30 at 20:54:34. This time range is derived from the range of CME speeds observed at Earth, which are between 300 and 1,500 km/s (Cane & Richardson, 2003; Guo et al., 2010). During this ICME event, STEREO A was nearly radially aligned with Mercury, at 135° west of the Sun-Earth line, while STEREO B was located westward of the Sun-Mercury line, at 85° west of the Sun-Mercury line and 142° east of the Sun-Earth line. As a result, the best candidate CME in the coronagraph database must satisfy the following conditions: it must occur within the specified time frame, appear as a forward halo CME in STEREO A observations, an east-limb CME in STEREO B observations, and a west-limb CME in SOHO observations. Only one CME meets these criteria, with the coronagraph image from STEREO B capturing an east-limb CME that initiated at 02:20:45 on April 30. We then compute the time elapsed between the CME's onset and MESSENGER's observation of the magnetic cloud at 08:51:10 on May 1st. Assuming that the CME experiences neither acceleration nor deceleration during transit, we estimate the velocity of the magnetic ejecta ($v_{SW}$) to be 483 km/s. It is important to note that ICMEs do undergo some acceleration in the lower corona due to magnetic reconnection (Zhuang et al., 2022), which suggests that our calculated value of $v_{SW}$ may slightly underestimate the true speed. Using these derived values of $v_{SW}$ and $M_A$, we estimate the plasma density to be $n = 15.7 \text{ cm}^{-3}$, significantly lower than the average solar wind density, which is approximately 58 $\text{cm}^{-3}$ at this heliocentric distance based on a best-fit line from PSP data (Sun et al., 2022). The ICME temperature, $T$, was estimated based on the average magnetic cloud temperature at Earth: $T = 8.7 \times 10^4 \text{ K}$ (Gopalswamy et al., 2015).

### 3.6. MHD Model Inputs

To gain a global perspective on the magnetic cloud's impact on Mercury's magnetosphere, we run two 3D Block Adaptive Tree Solar wind Roe-type Upwind (BATSRUS) simulations of Mercury's plasma environment. The BATSRUS code is part of the Space Weather Modeling Framework (Gombosi et al., 2021; Tóth et al., 2012) and has been used in previous studies investigating Alfvén wing formation in planetary environments (Bowers et al., 2025; Chen et al., 2025; Ridley, 2007). We utilize the same BATSRUS model as applied to Mercury's magnetospheric environment described in Jia et al. (2015), Jia et al. (2019), Bowers et al. (2025). The code solves the MHD equations, which govern the conservation of mass, momentum, and energy, alongside Faraday's law, to model the interaction between the solar wind and Mercury's plasma environment. The model represents Mercury's conductive, iron core as a spherically symmetric object with very high conductivity ($10^5 \text{ S/m}$) within the iron core ($r < 0.8 R_M$) and very low ($10^{-7} \text{ S/m}$) conductivity within the regolith ($0.8 R_M \le r \le 1 R_M$). This conductivity profile couples to the space environment by governing the diffusion of magnetic field lines attached to Mercury (Jia et al., 2015).

Table 1 shows two sets of upstream conditions that were input into the BATSRUS model. For the magnetic cloud simulation, the magnetic field is the average of MESSENGER observations within the yellow region highlighted in Figure 1. The remaining plasma parameters were derived from the methodologies discussed in Sections 3.3 and 3.4. We also run a BATSRUS simulation of Mercury's magnetosphere under nominal upstream conditions to provide a useful comparison with the extreme, sub-Alfvénic conditions within the magnetic cloud. We determine
<table><caption>**Table 1**
Inputs to BATSRUS Model for the Upstream Conditions at Mercury Within the Magnetic Cloud (Purple) and Under Nominal Conditions (Orange)</caption><thead><tr><th colspan="2">Input Variable</th></tr></thead><tbody><tr><td colspan="2">Run 1: Magnetic Cloud</td></tr><tr><td>B<sup>X</sup><sub>IMF</sub></td><td>70 nT</td></tr><tr><td>B<sup>Y</sup><sub>IMF</sub></td><td>-63 nT</td></tr><tr><td>B<sup>Z</sup><sub>IMF</sub></td><td>22 nT</td></tr><tr><td>|B<sub>IMF</sub>|</td><td>97 nT</td></tr><tr><td>n<sub>sw</sub></td><td>15.7 cm<sup>-3</sup></td></tr><tr><td>v<sub>sw</sub></td><td>483 km/s</td></tr><tr><td>T</td><td>8.7 &times; 10<sup>4</sup> K</td></tr><tr><td>M<sub>A</sub><sup>a</sup></td><td>0.9</td></tr><tr><td>&beta;<sup>a</sup></td><td>0.005</td></tr><tr><td colspan="2">Run 2: Nominal Solar Wind Conditions</td></tr><tr><td>B<sup>X</sup><sub>IMF</sub></td><td>16 nT</td></tr><tr><td>B<sup>Y</sup><sub>IMF</sub></td><td>-13 nT</td></tr><tr><td>B<sup>Z</sup><sub>IMF</sub></td><td>7 nT</td></tr><tr><td>|B<sub>IMF</sub>|</td><td>22 nT</td></tr><tr><td>n<sub>sw</sub></td><td>47 cm<sup>-3</sup></td></tr><tr><td>v<sub>sw</sub></td><td>366 km/s</td></tr><tr><td>T</td><td>8.7 &times; 10<sup>4</sup> K</td></tr><tr><td>M<sub>A</sub><sup>a</sup></td><td>5.9</td></tr><tr><td>&beta;<sup>a</sup></td><td>0.36</td></tr></tbody></table>

Note. Within the magnetic cloud, $B_{\text{IMF}}^{X}$, $B_{\text{IMF}}^{Y}$, $B_{\text{IMF}}^{Z}$, and $|B_{\text{IMF}}|$ are the 30-min average within the upstream region (yellow shaded area in Figure 1). $n_{sw}$ is the estimate for the upstream density. $v_{sw}$ is the estimate for the velocity of the ICME. <sup>a</sup>Next to the $M_{A}$ and $\beta$ values indicate dimensionless variables calculated from the other input parameters and are not standalone inputs into the model. $|B_{\text{IMF}}|$, $n_{sw}$, $v_{sw}$, under nominal conditions are calculated as the average at 0.36 AU from the fit of PSP measurements within the inner-heliosphere as detailed in Sun et al. (2022), while $T$ is kept consistent between the two runs. The orientation of $B$ under nominal conditions is the same as the magnetic cloud, but scaled to the nominal $|B|$.

$|B_{\text{IMF}}|$, $n_{sw}$, and $v_{sw}$ for nominal conditions via the fits determined by Sun et al. (2022) based on PSP observations in the inner heliosphere at 0.36 AU. The orientation of $B_{\text{IMF}}$ and $T$ for the nominal BATSRUS run were kept consistent with that in the magnetic cloud.

## 3.7. Comparisons Between MESSENGER Observations and MHD Simulations

Figure 2 provides a comparison of Mercury's magnetosphere along MESSENGER's path, both between MAG data (cyan, black, and gray lines) and orbital traces through BATSRUS simulations (violet and orange lines). The comparison between MAG measurements of the magnetic cloud (cyan) and the orbital trace through the simulated magnetosphere from Run 1 (violet) provides a one-to-one data-model comparison, as the simulation was specifically tailored to the upstream conditions during this MESSENGER orbit. We also compare a BATSRUS simulation with typical solar wind conditions (Run 2, orange lines) with data from the orbits surrounding the ICME. Note that we do not consider the orbits immediately before and after the orbit within the magnetic cloud as they took place within the ICME itself (Figure 1) and are not indicative of the nominal solar wind conditions. Instead, we consider five orbits before the start of the ICME (from 04-29 12:28:41 to 05-01 04:29:08) and five orbits after the end of the ICME (from 05-02 04:29:30 to 05-03 20:29:59). These surrounding orbits traverse a similar spatial region, minimizing the influence of orbital sampling effects (Figures 2e-2g). To plot these data on the same horizontal axis, we determine the time between a measurement and the most recent apoapse of the spacecraft. This allows us to compare time series measurements among neighboring orbits that sample very similar spatial regimes but are separated in time. MAG measurements during each of these orbits are shown in gray in Figure 2. The black lines in Figure 2 represent the average value of each measurement across all 10 of the surrounding orbits. Using the technique described in Section 3.3 and Bowers et al. (2025), the $M_{A}$ estimate for the 10 surrounding orbits ranged from 2.5 to 14.1, with an average $M_{A}$ value of 6.1, which is similar to that used in Run 2.

MAG measurements within the magnetic cloud largely align with the predicted magnetospheric environment from Run 1 under sub-Alfvénic conditions. In particular, the observations and the model show strong agreement throughout MESSENGER's transit of the nightside magnetosphere (~150-225 min after apoapse), where conditions deviate significantly from the nominal state. A detailed comparison of Mercury's magnetospheric shape

under sub- and super-Alfvénic conditions is provided in Section 3.8. Differences between the MAG observations during the magnetic cloud and Run 1 provide valuable insights. During the initial phase of the magnetic cloud (~80-120 min after apoapse), MESSENGER was in the magnetosheath (Figure 1), where it measured shocked solar wind plasma, characterized by greater variability and higher field strength than the unshocked solar wind. In contrast, $M_{A}$ within Run 1 is constant and sub-Alfvénic, resulting in the absence of a bow shock or magnetosheath plasma and producing a steady $|B|$ signature (Figure 2d). The measured $|B|$ exceeds model predictions during MESSENGER's nightside traversal as well (~175-225 min after apoapse), suggesting the actual upstream $|B|$ may, at times, exceed what was predicted in Run 1. These differences highlight the variable nature of the magnetic cloud, which cannot be fully captured by a simulation with a non-variable input as we have used here. Without an upstream monitor at Mercury, dynamic simulations reflecting the shifting upstream solar conditions are not possible. That being said, the general close agreement between the MHD model, both on Mercury's nightside and dayside magnetosphere, suggests that our estimation techniques for the upstream conditions are valid and the model run plausibly captures the large-scale configuration of Mercury's magnetosphere.

Unlike Run 1, Run 2 exhibits a well-defined inbound bow shock crossing at ~95 min after apoapse, where $|B|$ increases from 22 to 48 nT, and a clear outbound crossing at ~265 min, where $|B|$ drops from 80 back to 22 nT.
<figure><img src="image_3.png" /><figcaption>**Figure 2.** The magnetic field time series of multiple MESSENGER orbits and orbital traces of the two MHD runs detailed in Table 1 demonstrating that Mercury's magnetosphere within the magnetic cloud is best described by the sub-Alfvénic model run (Run 1). Panels (a–c) display the $X_{MSM'}$, $Y_{MSM'}$, $Z_{MSM'}$ coordinates of the magnetic field in nT. Panel d shows the magnitude of the magnetic field, $|B|$ in nT. MAG data during the magnetic cloud are shown in cyan, and MAG data from the five orbits before and five orbits after the CME impact are shown in gray. The average of all the surrounding orbits at each time step is shown in black. The orbital trace for Run 1 is shown in violet, and the orbital trace for Run 2 is shown in orange. The time range for the orbit within the magnetic cloud is from 2013-05-01 04:29:10 to 2013-05-01 12:29:12. The bottom horizontal axis is the minutes from each orbit's apoapsis. Panels (e–g) show MESSENGER's orbital trajectory plotted during the time series above in the XZ/XY/YZ$_{MSM'}$ planes. The orbit during the magnetic cloud impact is colored by the minutes from the apoapsis. The outer and inner dashed black lines represent the nominal bow shock and magnetopause distances based on Winslow et al. (2013) fits to curves derived in Shue et al. (1997).</figcaption></figure>

These shock crossings occur in a spatial region consistent with the average bow shock shape determined by Shue et al. (1997) and Winslow et al. (2013) (Figures 2e–2g). Upstream variability among the surrounding orbits causes slight differences in the timing of the bow shock crossings, leading to a smoothing effect when averaging them together (black curve). On average, the inbound bow shock location across the surrounding orbits occurs at ~105 min after apoapsis, with the outbound crossing at ~265 min, closely matching the timing in Run 2. Outside the magnetosphere (~100–260 min after apoapsis), significant discrepancies arise between Run 2 and the surrounding orbits due to variations in $B_{IMF}$ across different orbits. However, within Mercury's magnetosphere (~164–255 min after apoapsis), Run 2 closely aligns with the orbit-averaged results. This suggests that the average large-scale magnetospheric configuration across 10 orbits does not deviate significantly from Run 2, providing a representative baseline for comparison with the extreme conditions observed in the magnetic cloud.
the magnetotail under super-Alfvénic conditions imposed by the $-B_Y$ component of the upstream IMF. This near-planet twist is broadly consistent with findings from Romanelli et al. (2022), who reported an average of ~3° twist at 2 $R_M$ in response to the direction of IMF $B_Y$ at Mercury and Sun, Slavin, Dewey, et al. (2020), Sun, Slavin, Smith, et al. (2020) who reported a 4° twist of Mercury's magnetotail during an ICME event. In contrast, the sub-Alfvénic configuration shows a dawnward displacement of the central current sheet by approximately 1 $R_M$ and a pronounced counter-clockwise tilt of around 40°. Further investigation into the influence of Alfvén Mach number ($M_A$) on Mercury's magnetotail orientation may help uncover the physical drivers of tail twisting. Understanding these mechanisms has broader implications for how IMF $B_Y$ affects not only Mercury, but also the terrestrial magnetotail (Kaymaz et al., 1994) and Martian magnetotail (DiBraccio et al., 2018, 2022).

### 3.9. Geometry of Alfvén Wings

Within the magnetic cloud, the IMF was predominantly aligned along the $+X_{MSM'}/-Y_{MSM'}$ direction, with a slight $+B_Z$ component (Table 1). This small $+B_Z$ component causes the Alfvénic interaction to deviate from the $XY_{MSM'}$ plane, instead aligning along the IMF clock angle, defined as $\phi = \tan^{-1}(B_Z/B_Y)$. A 2D projection of Mercury's magnetosphere along this tilted plane is shown for both sub-Alfvénic (Figure 3c) and super-Alfvénic (Figure 3f) upstream conditions. The orientation of this tilted plane is indicated by the cyan dashed lines in Figures 3d and 3g.

In the sub-Alfvénic regime imposed by Run 1, two distinct Alfvén wings form: a dawnside wing connected to the southern magnetic pole and an anti-sunward wing connected to the northern pole (Figure 3c). The orientation of these wings in the $MSM'$ coordinate system is governed by the Elsässer variables (Elsässer, 1950), also known as Alfvén characteristics (Neubauer, 1980, 1998):

$$ \mathbf{c}_{\mathbf{A}}^{\pm} = (\mathbf{v}_{\text{sw}} \pm \mathbf{v}_{\mathbf{A}})/(|\mathbf{v}_{\text{sw}} \pm \mathbf{v}_{\mathbf{A}}|) \quad (4) $$

The Alfvén characteristics represent the superposition of the bulk flow velocity and the group velocity of the Alfvén wave moving parallel and anti-parallel to the magnetic field. Here, the negative characteristic $\mathbf{c}_{\mathbf{A}}^-$ defines the dawn wing, while the positive characteristic $\mathbf{c}_{\mathbf{A}}^+$ defines anti-sunward wing. For the Run 1 upstream conditions in Table 1, the characteristic vectors in $MSM'$ coordinates are: $\mathbf{c}_{\mathbf{A}}^+ = [-0.26, -0.91, 0.32]$ and $\mathbf{c}_{\mathbf{A}}^- = [-0.92, 0.37, -0.13]$, corresponding to the dawnward and anti-sunward wings, respectively. These characteristics are highlighted as green arrows in Figure 3c. The dawnward wing is highly inclined relative to the upstream solar wind velocity, visible as a broad region of dawnward-pointing magnetic flux originating from the southern pole (Figure 3c). In contrast, the anti-sunward wing is closely aligned with the $-X_{MSM'}$ direction, with a small duskward component.

## 4. Discussion

### 4.1. Comparisons With Other Alfvén Wing Configurations

The global MHD simulation under sub-Alfvénic conditions (Run 1) reveals an overall structure consistent with an asymmetric Alfvén wing geometry. During MESSENGER's passage through Mercury's near-space environment, the spacecraft observed clear evidence of a significant departure from typical magnetospheric conditions, which closely match the distortions predicted by the model. In this section, we explore how the simulated Alfvén wing configuration shapes the plasma environment around Mercury and compare this structure to other known Alfvén wing systems.

The orientation of the IMF governs the geometry of planetary Alfvén wings, which in turn shapes spacecraft observations through the magnetosphere. While previous studies at Earth and Mercury, as well as at the moons of Jupiter and Saturn, have reported nearly symmetric Alfvén wings associated with strong southward or $-B_Y$ upstream magnetic field orientations (Beedle et al., 2024; Bowers et al., 2025; Burkholder et al., 2024; Chen et al., 2024, 2025; Kivelson et al., 2004), our simulation results present a different scenario. The asymmetry between the two wings with respect to the solar wind flow direction shown in Figures 3b–3d provides a compelling case study into Alfvén wing physics and their role in shaping magnetospheric dynamics.

Figures 4a–4c highlight several simulated features of this asymmetric Alfvén wing geometry. Within both the dawnward and anti-sunward Alfvén wings, the bulk flow velocity in the $X_{MSM'}$ plane is significantly reduced
<figure><img src="image_4.png" /><figcaption>**Figure 4.** BATSRUS simulation of Mercury's magnetosphere under sub-Alfvénic conditions described by Run 1. Panels (a, d) shows the results of the BATSRUS simulation Run 1 shown in the $XZ_{MSM'}$ planes. Panel (b, e) are cut of Mercury's magnetosphere taken along the plane determined by the clock angle of the upstream field (shown as the gray dashed line in panels (c, f)). Panels (c, f) shows Mercury's magnetosphere looking sunward from behind the planet at $X_{MSM'} = -2R_M$. Panels (a–c) show ion density in cm$^{-3}$, panels (d–f) show the plasma bulk velocity in the $X_{MSM'}$ direction ($u_x$), and panels (g–i) show the plasma bulk velocity in the $Y_{MSM'}$ direction ($u_y$).</figcaption></figure>

(Figure 4e), consistent with expectations for both planetary-solar wind and moon-magnetosphere Alfvén wing configurations. In the anti-sunward wing, we see a clear plasma density depletion (Figure 4a), reminiscent of the cavity behind the northern lobe under super-Alfvénic conditions. However, this depletion does not extend directly behind Mercury along the $-X_{MSM'}$ axis. Instead, it protrudes duskward, deviating away from the geometric shadow of the planet (Figure 4b). This tilted plasma wake is caused by the dawnward Alfvén wing deflecting the upstream solar wind plasma toward Mercury. As this deflected plasma impacts Mercury's surface, it leaves behind a plasma-depleted region that extends along this tilted path. This effect is clearly shown in Figures 4g and 4h, where the plasma velocity has a significant duskward ($+u_y$) component on the dayside and dawnside terminator of Mercury, deflecting plasma asymmetrically behind the planet. This configuration resembles predictions for the moon-magnetospheric interaction at Triton, where Alfvén wings form as the moon orbits within Neptune's magnetosphere. In that case, the relatively small angle between the upstream magnetic field and the sub-Alfvénic flow direction, $\Theta$, over the moon generates a pair of Alfvén wings, with one wing extending upstream of the moon (Liuzzo et al., 2021). While the displacement of the wake is most evident when one of the Alfvén wings has an upstream component, Simon et al. (2022) demonstrated that plasma wake deflections also occur if $\Theta$ falls below a critical angle that depends on $M_A$, and the Alfvénic interaction strength of the moon.

Sub-Alfvénic conditions also have important implications for the circulation of magnetic flux within Mercury's magnetosphere via magnetic reconnection. The MHD simulation indicates that MESSENGER likely crossed near the nightside current sheet formed between the two Alfvén wings, as evidenced by a brief transition in $B_X$ from negative to positive around 190 min after apoapsis (Figure 3). The magnetic field measured by MESSENGER during this interval is highly variable and features a series of rotations coinciding with peaks in field magnitude. This suggests the presence of magnetic flux ropes, possibly generated by nightside magnetic reconnection (DiBraccio et al., 2015; Imber & Slavin, 2017; Smith et al., 2017; Sun, Slavin, Smith, et al., 2020). In this sub-Alfvénic case, the reconnection site appears to be located much further south along the MESSENGER trajectory compared to the super-Alfvénic case (Figure 3). This current sheet is also asymmetric with respect to plasma density (Figure 4a), with much lower densities on the northern side of the current sheet compared to the southern side. Similar behavior has been simulated at Earth during the sub-Alfvénic ICME impact in April 2023, where a
strong negative IMF $B_y$ component shifted nightside reconnection toward the dawnside, concentrating flux closure in that region (Burkholder et al., 2024). Future studies combining MHD and kinetic simulations, such as those conducted by Li et al. (2024), with spacecraft observations of reconnection signatures are required to better understand the influence of sub-Alfvénic conditions on magnetic flux circulation at Mercury.

Studies of this rare magnetospheric configuration driven by sub-Alfvénic conditions provide valuable insights into exoplanetary magnetospheric interactions with their host star, many of which orbit within 0.1 AU where the stellar wind is sub-Alfvénic (Schneider et al., 2011). The asymmetric Alfvén wings observed in this event are representative of the stellar wind conditions commonly found upstream of such exoplanets, where the magnetic field has a significant component aligned with the stellar wind flow. This upstream configuration allows for the possibility that one of the Alfvén characteristics points back toward the host star, enabling electromagnetic coupling between the planet and the star (Saur et al., 2013). In Mercury's case, this coupling would take place if either Alfvén characteristic has a positive $X_{MSM}$ component. Although this condition was not satisfied in the example shown here, small adjustments to the upstream parameters used in Run 1 would cause the positive Alfvén characteristic to shift sunward. For instance, using the same IMF orientation as in Run 1, an $M_A$ value of 0.64 would satisfy the above condition. Similarly, if we hold the $M_A$ fixed at the value used in Run 1, a 24° counter-clockwise rotation in the $XY_{MSM'}$ plane of the upstream IMF would also lead to an Alfvén characteristic that satisfies this condition. However, for electromagnetic coupling to occur, this condition must be sustained for tens of hours, which is very unlikely at Mercury's heliocentric distance. That being said, these rare intervals of sub-Alfvénic solar wind provide brief windows into the global magnetic structure that may be typical for many known exoplanets.

## 4.2. Frequency of Sub-Alfvénic Intervals at Mercury

The average $M_A$ is lower at Mercury compared to any other planet in the solar system, making it the ideal planet to study the impact of low $M_A$ conditions on a planetary magnetosphere. However, sub-Alfvénic periods remain quite rare at Mercury. Hajra and Tsurutani (2022) surveyed the near-Earth solar wind environment across multiple spacecraft missions spanning more than four solar cycles (1973–2020), and found that, on average, the solar wind becomes sub-Alfvénic for approximately once every 2 years, but this frequency varies significantly with solar cycle. For example, there were 20 sub-Alfvénic intervals at Earth during solar cycle 23 (1996–2008), with four sub-Alfvénic events occurring in 2000 and 2002. The majority (83%) of these intervals occurred within magnetic clouds, with the remaining 17% occurring during the trailing portion of high-speed streams (HSSs). It is reasonable to assume that if the magnetic cloud is sub-Alfvénic at Earth, it was also sub-Alfvénic at Mercury's orbit as $M_A$ increases with heliocentric distance, so it is likely that the upstream conditions at Mercury are sub-Alfvénic at least once every 2 years on average.

Although Mercury has never possessed a dedicated upstream monitor, the recent Parker Solar Probe (PSP) (Fox et al., 2016) and Solar Orbiter (SO) (Müller et al., 2020) missions have provided the latest view of the solar wind environment in the inner heliosphere, allowing us to explore the occurrence rate of sub-Alfvénic conditions within the Mercury environment. The orbits of these spacecraft span from 1 AU to within Mercury's perihelion (0.3 AU). Figure 5 shows a 2-D histogram of SO measurements for $|B|$, $n_{sw}$, $v_{sw}$ and $M_A$, spanning heliocentric distances from 0.3 AU to 1 AU. Magnetic field data were obtained from the Solar Orbiter Magnetometer (Horbury et al., 2020), while plasma moments were derived from the Solar Wind Analyzer instrument suite (Owen et al., 2020). We utilize SO observations in the solar wind in the inner heliosphere from 30 December 2021, and 01 July 2024. We note that plasma and magnetic field instrumentation onboard SO was not continuously in operation during this period, and there are many small data gaps in the data. The black lines represent best-fit trends for each parameter as a function of heliocentric distance ($r_H$ in AU). For comparison, the green lines show empirical trends based on Parker Solar Probe (PSP) observations between 01 October 2018 and 01 March 2020, as reported by Sun et al. (2022). Both PSP and SO observations agree that $|B|$ and $n_{sw}$ are inversely proportional to $r_H$ while $v_{sw}$ has only a slight dependence on $r_H$. As a result, $M_A$ values within Mercury's orbit, indicated by the gray dashed lines in Figure 5d, are smaller compared to 1 AU.

The differences between the SO (black) and PSP (green) curves likely reflect the solar cycle phase during each observation period. The equations presented in Sun et al. (2022) were derived from PSP observations during solar minimum (Figure 5e) while those derived via SO observations were taken during the rising portion toward solar maximum. The $M_A$ values derived from SO observations are lower than those from PSP, primarily due to higher
Similar sunspot numbers during the SO observation period and MESSENGER's orbital lifetime (Figure 5e) suggest that the SO data are a reasonable representation of average conditions during MESSENGER's mission. These observations suggest that, ~2.5 times per year, extreme conditions will cause a series of sub-Alfvénic intervals upstream of Mercury, resulting in ~10 intervals of sub-Alfvénic conditions within MESSENGER's orbital lifetime. MHD simulations of the terrestrial magnetosphere show that Alfvén wing-like structures also form under slightly super-Alfvénic conditions ($1 < M_A < 1.5$) (Ridley, 2007; Wilder et al., 2015), suggesting the frequency of Alfvén wing formation at Mercury may be higher than we estimate here. Based on missing bow shock crossings from MESSENGER (i.e., orbits in which the bow shock was not visually identified), Lugaz et al. (2016) estimated that the solar wind was sub-Alfvénic ~2.2 times per year during the spacecraft's orbital phase, in good agreement with our initial estimation.

At this estimated rate of sub-Alfvénic intervals at Mercury, the upcoming dual-spacecraft BepiColombo mission will encounter 2–3 sub-Alfvénic intervals during its 1-year nominal mission phase, which could be increased to 8–9 intervals throughout its 3.5 years extended mission phase (Milillo et al., 2020). Furthermore, both spacecraft carry comprehensive plasma science instruments capable of characterizing plasma populations in the solar wind and Mercury's magnetosphere (Baumjohann et al., 2020; Heyner et al., 2021). This enables Mio to directly measure solar wind properties and derive dynamic estimates of $M_A$ from plasma moments, providing a more detailed picture of the upstream conditions than the single pressure balance-based estimate used in our study. Future multi-spacecraft analysis of Mercury's magnetosphere to sub-Alfvénic conditions will provide an opportunity to explore a planet's dynamic response to the rare transition from super-to sub-Alfvénic upstream conditions. These observations should be coupled with MHD and/or kinetic simulations of Mercury's magnetosphere to obtain a more global understanding of the variety of ways in which magnetospheric processes are altered under these extreme conditions.

## 5. Summary

We present MESSENGER observations of Mercury's magnetosphere during a rare interval of sub-Alfvénic upstream solar wind conditions, driven by an ICME that impacted the planet on 1 May 2013. These observations reveal a distorted magnetotail with a depleted southern lobe. To interpret these observations, we employ a 3D MHD simulation of Mercury's response to the derived sub-Alfvénic upstream conditions. The simulation predicts that Mercury's magnetosphere underwent a global reconfiguration during this event, including the formation of asymmetric Alfvén wings which redirect magnetic flux and plasma, distort the magnetotail, and generate a tilted plasma wake. The close agreement between the simulation and MESSENGER's trajectory through the magnetosphere suggests that the observed magnetospheric configuration is consistent with this Alfvén wing geometry. Based on SO observations, we estimate that such sub-Alfvénic conditions occur about 2.5 times per year near solar maximum, suggesting that Mercury likely encountered these rare upstream conditions multiple times during MESSENGER's orbital phase and will do so again during the BepiColombo mission. Investigations into this rare upstream regime provide valuable analogs for moon-magnetosphere interactions and star-planet interactions in exoplanetary environments under similar conditions.

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

In accordance with the AGU data policy, MESSENGER data are publicly available through the Planetary Plasma Interactions Node of the Planetary Data System (Korth, 2021). Observations from Solar Orbiter are available at the Solar Orbiter Archive (SOAR; https://soar.esac.esa.int/soar/). The results of the BATSRUS model run during the magnetic cloud can be downloaded via (Bowers, 2025). This worked utilized a number of open python packages such as sunpy (Mumford et al., 2020; The SunPy Community et al., 2020), matplotlib (Hunter, 2007), numpy (Harris et al., 2020), scipy (Virtanen et al., 2020), and pandas (pandas development team, 2020).
Acknowledgments

Work at DIAS was supported by Taighte Éireann—Research Ireland: Laureate Consolidator award SOLMEX (CFB and CMJ); President of Ireland award 18/FRL/ 6199 (MJR, CMJ); Postgraduate Scholarship GOIPG/2024/3906 (DMH), 22/FFP-P/11545 (CMJ, HH, SC). RMD is supported by NASA's Discovery Data Analysis Program Grant 80NSSC24K0059. HH work at DIAS was supported by the DIAS Research Fellowship in Astrophysics. XJ acknowledges support from NASA Early Career Fellow startup Grant 80NSSC20K1286 and NASA Discovery Data Analysis Program through Grant 80NSSC21K1012. L.A.H is supported through a Royal Society-Research Ireland University Research Fellowship. WJS's contributions were supported by NASA Discovery Data Analysis Program Grant 80NSSC24K1777 and the National Science Foundation under Grant 2321595.

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