First observations of an ICME compressing Mercury’s dayside magnetosphere to the surface
Mercury is the only planet in the inner solar system, other than Earth, that possesses a dynamo-generated global, albeit weak, magnetic field. As a consequence of this weak field and the planet’s proximity to the Sun, the magnetosphere of Mercury is highly dynamic, especially during times of interplanetary coronal mass ejections (ICMEs), which travel at high speeds compared to the solar wind and carry mass and magnetic field from the Sun. It has been long hypothesized[2, 3] that the magnetic flux on Mercury’s dayside may be completely eroded or compressed below the surface during extreme conditions. Here we report the first observations of an ICME compressing Mercury’s dayside magnetosphere below the surface, accompanied by a 40 increase in the peak planetary field and a 300 increase in the magnetotail field. These phenomena have not been observed at any planet to date, and they provide evidence that Mercury sometimes behaves as an unmagnetized airless body. The ICME plasma interacts directly with Mercury’s surface, weathering the regolith and sputtering particles into the exosphere. The collapse of Mercury’s dayside magnetosphere has important implications for the habitability of close-in exoplanets around M dwarf stars, as such events may significantly contribute to planetary atmospheric loss in these systems.
Institute for the Study of Earth, Ocean, and Space, University of New Hampshire, Durham, NH, USA
Department of Earth, Ocean and Atmospheric Sciences, The University of British Columbia, Vancouver, BC, Canada.
Planetary Science Institute, Tucson, AZ, USA.
The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA.
Department of Earth Sciences, University of Oregon, Eugene, OR, USA.
Mercury’s weak internal field, described by a dipole with a moment of 190 nT (where is Mercury’s radius), offset 0.2 northward from the geographic equator[5, 6, 7, 8], does not present a large obstacle to the solar wind. During average solar wind conditions, the magnetopause subsolar stand-off distance, , is 1.45 from the planet’s center, and the bow shock subsolar stand-off distance is 1.96 . These distances are largely controlled by solar wind parameters and are most strongly affected during ICMEs when conditions are extreme. Observations from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft indicate that the magnetopause was likely below the planet’s surface during 30 of ICME-affected MESSENGER orbits.
Here we examine an ICME event at Mercury that shows unambiguous signatures of magnetopause compression well below the planetary dayside surface. The ICME was launched from the Sun on 11/29/2013, reached Mercury at 14:25:35 UT on 11/30/2013, and was observed in situ by the Magnetometer onboard MESSENGER (Figure 1a). The ICME, and specifically the magnetic cloud substructure (beginning at 16:13:12 UT), affected two consecutive orbits around Mercury. Because the magnetospheric perturbation was strongest during the first orbit, we analyze this orbit (orbit 2577) in detail (Figure 1a). The magnetic field direction in the magnetic cloud was predominantly southward, however, it had a northward component for 30 mins prior to MESSENGER crossing into Mercury’s magnetosphere on this orbit. Using the modified Newtonian approximation described in Winslow et al., we estimated the ram pressure, , to be 260 nPa from the inbound magnetopause crossing on orbit 2577. In comparison, the average at Mercury is 14 nPa, nearly twenty times lower. From the ICME launch time at the Sun and the arrival time at Mercury, we calculated the Sun-to-Mercury transit speed to be 800 km s, which together with the estimate yields a plasma number density of 240 cm, compared to 100 cm on average.
The most remarkable effect of this ICME on Mercury was the compression of the dayside magnetopause below the surface (Figure 1 and Extended Data Figure 1) resulting from this very high . During noon-midnight orbits, MESSENGER regularly spent 20 minutes sampling Mercury’s low-latitude dayside magnetosphere, usually entering the magnetosphere at latitudes close to the equator. Even though MESSENGER was in a noon-midnight orbit during this ICME, the location of the dayside magnetopause crossing into the magnetosphere occurred at a high latitude of 84 N, a longitude of 50, and an altitude of only 400 km (Figures 1b,c, 2, and Extended Data Figure 2). Prior to this crossing, MESSENGER was at altitudes less than 400 km at more southern latitudes and the magnetosphere was not encountered. Less than 1 minute after crossing the magnetopause, MESSENGER crossed into the nightside magnetosphere. Thus unlike typical noon-midnight orbits, MESSENGER spent less than 1 minute in the dayside after crossing the magnetopause, which explains the lack of a cusp crossing. No dayside northward magnetic field direction was observed in the magnetosphere (Figure 1b,c and Extended Data Figure 1). The field lines that MESSENGER sampled immediately after entering the magnetosphere are southward and sunward pointing, consistent with field lines in the high northern hemisphere close to and threading the night-side (Figure 3).
The established shape and location of Mercury’s magnetopause determined empirically provide further confirmation of the compression well below the planetary surface. The Shue et al. magnetopause model has been shown to fit Mercury’s average magnetopause shape well, under both nominal and extreme solar wind conditions[9, 10]. For the inbound and outbound aberrated magnetopause crossings on this orbit, the best-fit model yields an of 0.69 , i.e. a value that is well below the surface (Figure 2). Thus, the evidence clearly implies the utter absence of a dayside magnetosphere in this case (Figure 3).
Past studies using magnetohydrodynamic simulations of the Mercury-solar wind interaction have predicted such magnetospheric compression to the surface. Even taking into account induction effects in their simulations, Jia et al. found that the magnetopause stand-off distance was reduced to 1.04 for a of 132 nPa, which is sufficient for the magnetopause to reach some of the southern dayside surface. Given the much higher of 260 nPa estimated for the ICME in our study, it is unsurprising that it collapsed Mercury’s dayside magnetosphere.
Another remarkable effect of the ICME on Mercury’s magnetosphere is the 40 increase in the observed spacecraft closest-approach field strength compared to orbits prior to, and after the ICME (Figure 1a). We investigated whether this increased field strength could be attributed to an induction effect resulting from magnetospheric compression. Annual induction signals have been observed in MESSENGER data and larger signals are expected during times of extreme compression. Self-similar compression of the magnetosphere, resulting in a change in subsolar distance from to at or below the surface could result in an increase in the internal dipole moment of up to 25, similar to the observed increase in the field strength. However, an induction effect would increase the magnitude of all three components close to the planet, which is not observed. Extended Data Figure 1 shows that the increase in the field is almost entirely due to an increase in , which persists throughout the orbit. In contrast, the component is almost identical on both orbit 2577 and the orbit prior to the ICME arrival, and decreases slightly close to the planet. In the tail, the field magnitude is enhanced by a factor of 4 on orbit 2577 compared to 2576, suggesting that very different tail conditions, not induction, dominate the observed signal.
Despite the enhancement in the magnetotail field during this event, we find no evidence for flaring of the tail (see Methods). This, together with the calculated tail flux of MWb (see Methods), consistent with the average value of MWb observed at Mercury, shows that the increase in the magnetotail field strength is not caused by flux-loading of the tail. The absence of flaring implies that Dungey cycle magnetospheric convection did not substantially enhance the tail field, which is supported by the fact that there are only a few flux transfer events (FTEs)[17, 18]. These FTEs are observed shortly after the inbound magnetopause at high latitudes, indicating some high-latitude reconnection. Low-latitude reconnection is unlikely because of the lack of dayside magnetosphere. However, observations by the Fast Imaging Plasma Spectrometer (FIPS) onboard MESSENGER indicate that plasma is present in the southern tail lobe (Extended Data Figure 2), likely convected into the magnetosphere through the plasma mantle or via lobe convection driven by high-latitude reconnection.
The calculated magnetotail current sheet density of 1000 nA m for orbit 2577 (see Methods and Extended Data Figure 5) is an order of magnitude higher than during average solar wind at Mercury. It has been statistically established at Earth, and also recently at Mercury, that an increase in increases the tail lobe field and therefore the magnetic pressure in the tail. The increased magnetic pressure in the northern and southern tail lobes causes the current sheet to thin and compresses the total current into a smaller volume thereby increasing the current density. The strong magnetotail current is likely the main source of the increase in the intrinsic planetary field at closest-approach resulting from the strengthened component.
Recent observations of the exospheric Na emission pattern at Mercury detected diffused equatorial Na at times of ICME passage, in contrast to the more commonly observed two-peak pattern at high latitudes in both hemispheres. The major driver of Na surface release is particle precipitation, thus it is expected that during average conditions most of the Na emission is in the cusp regions where the magnetic field is weakest and particle precipitation is common[23, 8, 24, 25]. However, compression of Mercury’s dayside magnetosphere to the surface allows ICME plasma to directly impact the dayside at low latitudes, thereby sputtering Na into the exosphere. Our observation of such compression, and the previously predicted rate of occurrence for compression to the surface of of ICME cases, provides evidence for a mechanism of Na-generation at equatorial latitudes. Due to the collapse of the dayside magnetosphere (and likely the exosphere) and the complete dominance of ICME particles on Mercury’s dayside, the Na number densities estimated from FIPS observations show no dayside equatorial Na on orbit 2577 (Extended Data Figure 2). However, the following two orbits (Extended Data Figures 3 and 4) show increased densities over the pre-ICME levels on most of the dayside, indicating sputtering by the high density ICME.
Finally, the disappearance of Mercury’s dayside magnetosphere may have important implications for exoplanetary systems. M dwarfs, the most common type of star, can have high occurrence rates of stellar flares and are known to typically host exoplanets. It is unknown whether the flare-to-CME ratio of M dwarfs is similar to that of the Sun. However, because the habitable zone of M dwarfs is 10 times closer to the star than in our solar system, exoplanets in these zones are susceptible to possibly very frequent magnetic eruptive events. ICMEs at close proximity to the host star are likely to carry higher than that observed in this study, due to the increased ICME speed and density close to the star. We thus suggest that a habitable exoplanet around an M dwarf might require an intrinsic planetary magnetic field substantially stronger than that of Mercury to not lose its atmosphere over geologic timescales. It has also been suggested that tidally locked planets may have inherently weaker magnetic moments, so that for close-in exoplanets stellar-flare activity and associated ICME interactions may significantly hinder the emergence and development of life.
0.1 MESSENGER’s orbit, lack of dayside magnetosphere, and peak field.
MESSENGER’s highly eccentric orbit around Mercury (between March 2011 and April 2015) had periapsis altitudes between 200-500 km and apoapsis altitudes of 10,000-12,000 km. This allowed the spacecraft to spend a significant fraction of each orbit both in the solar wind and in the magnetosphere.
MESSENGER orbits 2576 (the orbit just prior to the ICME arrival) and 2577 (the first orbit affected by the ICME in question) were aligned in time on their equator crossing times (Extended Data Figure 1) to compare the magnetic field data before and after the ICME arrival. The increase in field strength in the nightside magnetosphere after the ICME arrival is evident from orbit 2576 to 2577, with peak field strengths of 405 nT and 575 nT on orbits 2576 and 2577, respectively. Furthermore, the almost complete absence of the dayside field on orbit 2577 compared to orbit 2576 is striking: the dayside northern hemisphere dipole field on orbit 2576 is seen via downward pointing (negative ), anti-sunward (negative ) field, that is absent on orbit 2577.
0.2 Lack of tail flaring.
Two lines of evidence support the inference of a magnetotail lacking substantial flaring during the passage of the ICME. First, the topology of the tail current sheet can be inferred from the direction of the field lines. A high amplitude of the and components relative to the component imply that the tail is flared and can suggest tail loading. We calculated () in different regions of the tail (following Slavin et al.), but no tail loading events were detected. We also calculated an average (and ) over the tail region for orbit 2577. For orbit 2576 (i.e., prior to the ICME arrival) the average was 0.27 whereas on orbit 2577 the average was 0.13 ( was even lower). This results from the dominant and large amplitude on orbit 2577. The large static/thermal pressure in the sheath compressing the tail likely contributes to the observed lack of tail flaring at this time.
Second, the shape of the magnetopause boundary in the tail also provides information on the tail geometry. We fit the Shue et al. magnetopause model to the magnetopause boundary crossings inbound (high latitude northern hemisphere) and outbound (southern hemisphere) on this orbit to obtain the shape of the magnetopause, and therefore the magnetotail on this orbit (see next section for more details). The initial fit yielded a slightly flared (; indicates no flaring) magnetopause due to the location of the outbound magnetopause crossing in the southern hemisphere. However, tilting of the tail southward due to non-radial ICME flows can cause the outbound magnetopause crossing to be observed farther south than it would be if there were no tail tilting, which can be mistaken for flaring of the magnetotail in an empirical fit.
We confirmed that the tail is actually tilted southward and not flared by comparing the location of the magnetic equator on this orbit with the average magnetic equator crossing location. Mercury’s magnetic equator is observed on average at km, whereas on orbit 2577 the northernmost and southernmost crossings of the magnetic equator occurred at km and km respectively, both south of the average value.
The southward tilting of the magnetotail implies southward tilting of the field lines. To check that this is the case on this orbit, we calculated in the tail and found a downward (i.e., southward) deflection of the field lines, in line with a southward tail tilting.
Further supporting the tilting of the tail is the fact that there were strong non-radial flows in the ICME. Although we do not have solar wind velocity and density data for this ICME at Mercury, the ICME was observed in longitudinal conjunction by STEREO A. The solar wind velocity at STEREO A shows strong excursions both north and south from radial flow (i.e. away from the ecliptic) during this ICME passage. We determined , as described in Anderson et al. and found it to range between and at STEREO A, larger than than the average non-radial solar wind flow at Mercury of . As shown in Anderson et al., an average non-radial flow at Mercury is able to cause tail tilting. Given that we observe the magnetic equator southward of the average location, we can assume that at the time of the orbit in question MESSENGER is in the portion of the ICME that has a southward directed flow, with an average of . We thus assume during this time at Mercury, which is also supported by the observed degree of field line tilting in the tail as mentioned above. We correct the outbound magnetopause and bow shock crossing points for this extra aberration, by rotating the points clockwise by . Fitting the Shue et al. model to the corrected magnetopause location yields a boundary with a non-flared magnetotail, i.e. a magnetopause with a flaring angle of , consistent with the direct observations of extremely low and ratios at this time in the tail.
0.3 Empirical model fits to magnetopause and bow shock.
Similarly to Winslow et al.[9, 10], we fit empirical models to the magnetopause and bow shock crossing points to characterize the magnetospheric boundaries during orbit 2577. For the bow shock, we used a conic section given by
where is the focus point, is the eccentricity, is the focal parameter, and . For the magnetopause, we used the empirical model by Shue et al.
where is the distance from the dipole center and is the flaring parameter that governs how open the magnetotail is.
We used these models to fit to our ICME-affected bow shock and magnetopause crossing locations, using a grid search method that minimized the root mean square (RMS) residual of the perpendicular distance of the observed boundary crossing from the model boundary. Although not the focus of the paper, these fits indicate that the bow shock was also substantially compressed on orbit 2577, having an estimated subsolar stand-off distance of with best-fit model parameters of , , and .
For the model fits to the boundary locations, we used the instantaneous boundary crossings nearest to the magnetosphere for both the inbound and outbound crossings, and fit the inbound and outbound points with one model. It should be noted that fitting the inbound and outbound bow shock/magnetopause crossings with one bow shock/magnetopause model assumes that the solar wind conditions do not change substantially in the hour between the inbound and outbound observations. This assumption is not unreasonable for an ICME magnetic cloud passage, during which the solar wind plasma and magnetic field parameters are expected to vary more slowly than under average solar wind conditions.
0.4 Tail flux and tail current.
We calculated the total magnetic flux in the tail from , where , the cross-sectional radius of the tail, is km and was calculated from the location of the midpoint of the outbound magnetopause crossing in the southern hemisphere (after correcting for the southward tilt of the tail). The error bar reflects the extent of the magnetopause layer. was found to be nT and was calculated by averaging the magnitude of the magnetic field over 2 minutes prior to the outbound magnetopause crossing, with the uncertainty being the standard deviation of the field over the averaging period. This yields a total magnetic flux in the tail of MWb, consistent with the average value of MWb observed at Mercury.
Tail current sheet (TCS) crossings were identified via a clear rotation in the field direction, specifically a rotation in from sunward in the northern lobe to antisunward in the southern lobe, accompanied by a decrease in the total field strength and an increase in the high-frequency variability in the field. The magnetic field data from orbit 2576 (Extended Data Figure 5, top left panel) show no evidence for a TCS crossing: changes smoothly along the orbit as expected for a mainly dipolar internal field, and there is no decrease in near the equator crossing. Assuming that any TCS current flows mainly in the -direction, the current density, is proportional to . We computed numerically along the orbit, and smoothed the estimate with a 30 second running mean. The resulting for orbit 2576 confirms no TCS crossing, i.e., on this orbit the magnetic equator crossing was planetward of the TCS. In contrast Orbit 2577 (Extended Data Figure 5, right) has a very different signature. The field magnitude in the tail lobe is 125 nT, 4 times that for the previous orbit. A pronounced dip in accompanies a reversal in near the magnetic equator crossing. The inferred current density, shows a clear peak near = 0 km, reaching a maximum value over 1000 nA m. This confirms that the tail current moved closer to the planet relative to the previous orbit and that the current density is particularly intense.
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Support for this work was provided by NASA grant NNX15AW31G. R. M. W. acknowledges support from NASA grant NNX15AW31G and NSF grant AGS1622352. N. L. acknowledges support from NASA grants NNX15AB87G and NNX13AP52G. C. L. J., M. A. A. and L P. acknowledge support from the Natural Sciences and Engineering Research Council of Canada.
R. M. W. developed the paper concept, performed the data analysis, interpretation and manuscript preparation. N. L., C. F., B. J. A., C. P, and N. A. S. contributed to data interpretation and general scientific evaluation. C. L. J. conducted the induction calculation. C. L. J. and M. A. A. calculated the magnetotail current and assisted with figure preparation. L. P. assisted with MESSENGER FIPS data analysis and figure preparation.
The authors declare that they have no competing interests.
Correspondence and requests for materials should be addressed to R.M.W.