We study the average global distribution of the external magnetic field at Mars, and its variability with the upstream solar wind dynamic pressure and interplanetary magnetic field as well as with ...the ambient crustal magnetic field strength. Our approach involves excluding the intrinsic planetary field from the total magnetic field by applying a crustal field model previously derived using low altitude measurements. The distribution of the average external field that remains is statistically analyzed using nearly 8 years of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations and several global, time‐dependent magnetohydrodynamic simulations. Overall consistent results have been obtained from the data and model, which are complementary to each other and cross validate the findings. It is found that the external field is significantly enhanced from the upstream across the bow shock (BS) and further intensifies closer to the planet in the topside ionosphere. It peaks at ∼170 km altitude near the subsolar point, significantly decreasing with increasing solar zenith angle. There is a strong day‐night asymmetry in the external field, with a typical dayside intensity of ∼15–50 nT and a nightside intensity of ∼5–15 nT. Under high solar wind dynamic pressures and IMFs, the external field may be enhanced by a factor of ∼2 everywhere below the BS, on both the dayside and nightside. In addition, our model results suggest that strong crustal fields, which effectively withstand the penetration of the solar wind, reduce the external field at low altitudes.
Plain Language Summary
We use nearly 8 years of satellite observations and several global numerical simulations to analyze the average distribution of the external magnetic field induced in the solar wind‐Mars interaction. Our approach is to separate the intrinsic and external components both in field measurements from the MAVEN spacecraft and in simulation results from a global Mars‐solar wind interaction model. The intrinsic crustal magnetic field is rooted beneath the surface and is organized in the rotating, planet‐fixed reference frame. The external magnetic field is better described in a Sun‐Mars reference frame in light of the complex solar wind interaction with the Mars obstacle (combined magnetosphere and ionosphere). Mixing them together results in the appearance of complex magnetic field distributions and affects the understanding of physical processes. Our exclusion of the intrinsic component using a crustal field model enables us to focus on the distribution of the external magnetic field itself, which is relatively poorly understood. We investigate the variability of the external field distribution due to the changes of the upstream solar wind and magnetic field conditions and the ambient crustal field strength. Our work shows that the average external field distribution follows basic patterns despite complex variabilities.
Key Points
We use Mars Atmosphere and Volatile EvolutioN observations and magnetohydrodynamic simulations to investigate the external magnetic field environment after the crustal field is excluded
The external field has a typical dayside (nightside) intensity of ∼15–50 nT (∼5–15 nT) and peaks at ∼170 km altitude at the subsolar point
The external field intensity can be enhanced by a factor of ∼2 on a global scale below the bow shock during high solar activity
The magnetic field draping pattern in the magnetosheath of Mars is of interest for what it tells us about both the solar wind interaction with the Mars obstacle and the use of the field measured ...there as a proxy for the upstream interplanetary magnetic field (IMF) clock angle. We apply a time‐dependent, global magnetohydrodynamic model toward quantifying the spatial and temporal variations of the magnetic field draping direction on the Martian dayside above 500‐km altitude. The magnetic field and plasma are self‐consistently solved over one Mars rotation period, with the dynamics of the field morphology considered as the result of the rotation of the crustal field orientation. Our results show how the magnetic field direction on the plane perpendicular to the solar wind flow direction gradually departs from the IMF as the solar wind penetrates toward the obstacle and into the tail region. This clock angle departure occurs mainly inside the magnetic pileup region and tailward of the terminator plane, exhibiting significant dawn‐dusk and north‐south asymmetries. Inside the dayside sheath region, the field direction has the greatest departure from the IMF‐perpendicular component direction downstream of the quasi‐parallel bow shock, which for the nominal Parker spiral is over the dawn quadrant. Thus, the best region to obtain an IMF clock angle proxy is within the dayside magnetosheath at sufficiently high altitudes, particularly over subsolar and dusk sectors. Our results illustrate that the crustal field has only a mild influence on the magnetic field draping direction within the magnetosheath region.
Plain Language Summary
According to the classic magnetic field draping theory, when the solar wind plasma encounters unmagnetized planetary bodies, the entrained interplanetary magnetic field (IMF) would pile up and drape around as the flow is diverted. Under this approximation, the draped field lines maintain an orientation similar to the upstream IMF in the plane perpendicular to the solar wind flow direction. However, the real morphology of the magnetic field draping at Mars has been poorly understood. In this study, we apply a state‐of‐the‐art global model to investigate the degree of distortion of the draped field lines when the complex Mars‐solar wind interaction is self‐consistently accounted for. Our results illustrate that when the IMF penetrates the magnetosheath edge into lower altitudes, the magnetic field lines may be so distorted and bent that their directions significantly deviate from the expectation from the classic field draping scenario. Our study reinforces the need to change any remaining notion of Mars in field line draping as a nonmagnetic planet. Moreover, this work presents a practical approach for inferring the IMF direction when direct measurements of the pristine solar wind are not available.
Key Points
The clock angle of the field within the dayside magnetosheath is a reasonable proxy for the IMF
The magnetic field clock angle departure increases with decreasing altitude and increasing SZA
The draping direction departure is the greatest downstream of the quasi‐parallel bow shock
A spherical harmonic model of the magnetic field of Jupiter is obtained from vector magnetic field observations acquired by the Juno spacecraft during its first nine polar orbits about the planet. ...Observations acquired during eight of these orbits provide the first truly global coverage of Jupiter's magnetic field with a coarse longitudinal separation of ~45 deg between perijoves. The magnetic field is represented with a degree 20 spherical harmonic model for the planetary ("internal") field, combined with a simple model of the magnetodisc for the field ("external") due to distributed magnetospheric currents. Partial solution of the underdetermined inverse problem using generalized inverse techniques yields a model ("Juno Reference Model through Perijove 9") of the planetary magnetic field with spherical harmonic coefficients well determined through degree and order 10, providing the first detailed view of a planetary dynamo beyond Earth.
The near lunar surface contains small‐scale magnetic field structures that provide a natural test bed for observing plasmas with a non‐zero Hall electric field, as well as potentially facilitating ...electron‐only reconnection. This study presents observational evidence of magnetized electrons as well as demagnetized ions when THEMIS‐ARTEMIS probe B reached an altitude of ∼15 km above the lunar surface. Additionally, observations suggest the presence of a field line topology change and traversal of a closed magnetic field structure containing solar wind electrons, suggestive of magnetic reconnection having occurred at some point between the solar wind interplanetary magnetic field and a lunar crustal magnetic field. Thus, the observations presented here are consistent with previous studies that predict prominent Hall electric fields near lunar crustal magnetic fields and further suggest that the solar wind interplanetary magnetic field may reconnect with lunar crustal magnetic fields, most likely via electron‐only reconnection.
Plain Language Summary
While interactions between the solar wind and the Earth's magnetosphere have been well studied, there is still much to be learned by studying the interactions between the solar wind and the small‐scale lunar magnetic fields. Due to the small‐scale nature of the lunar magnetic fields, previous studies have suggested that the ions do not respond in the same manner as the electrons. The resulting effects lead to an electric field near regions of lunar magnetic fields. This study presents observational evidence of the aforementioned phenomena. Additionally, the spacecraft observations also suggest that magnetic reconnection, or the breaking of the lunar magnetic field lines and reconnection to the magnetic field in the solar wind, was occurring between the solar wind and the lunar magnetic fields.
Key Points
Observations suggest magnetic reconnection occurs between the solar wind IMF and lunar crustal magnetic fields
Electron pitch angle and velocity distributions suggest the spacecraft traversed a closed magnetic topology containing solar wind electrons
We report in‐situ observations of demagnetized ions and associated Hall electric fields near the lunar surface
The two main sources of the magnetic field in the Martian ionosphere are the solar wind interaction with the planet, and, mainly in the southern hemisphere, remnant crustal magnetization. The ...magnetic fields measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) and Mars Global Surveyor spacecraft displayed a wide range of spatial scales, from the global (i.e., L ≈ 103 km) to mesoscale (L ≈ 102 km) to small‐scale (L < 10 km). Hamil et al. (2022) used MAVEN magnetometer and Langmuir Probe data to study these structures and suggested that they might be advected into the ionosphere from the solar wind and magnetosheath. In the current study, we apply a Fourier analysis to the fields and interpret the resulting power spectral density profiles versus frequency. The power spectral density function found from MAVEN data resembles that of the solar wind magnetic field (or interplanetary magnetic field) (i.e., power law with an index of about −2), but shifted upward in frequency by a factor of about 100. From a comparison of ionospheric power spectra with solar wind power spectra, we deduce that plasma, carrying a magnetic field with it moves from the magnetic pile‐up region downward into the ionosphere with speeds of roughly tens of meters per second. The derived power spectra in the ionosphere, in addition to the basic power law shape, show hints of extra power at a spatial scale of about 10 km, and this might be due to the creation of a magnetic structure within the ionosphere itself.
Plain Language Summary
The two main sources of magnetic field for the Martian ionosphere are the solar wind magnetic fields (i.e., the interplanetary magnetic field), and, particularly in the southern hemisphere, remnant crustal magnetization. The magnetic fields measured in the ionosphere by the magnetometers onboard the Mars Atmosphere and Volatile EvolutioN spacecraft and the Mars Global Surveyor display a wide range of spatial scales. In this study, we use a Fourier analysis of the magnetic field to quantify the field strength as a function of scale‐size. The fourier analysis represents a function of time (or some other variable) as the sum over a wide range of sinusoidal functions of different frequencies. We find that the results of the Fourier analysis for the ionospheric magnetic field have much in common with the Fourier analysis of the interplanetary magnetic field. The study exploits this similarity to probe the sources of the ionosphere's magnetic structure.
Key Points
A wide range of spatial scales of magnetic structure has been observed in the dayside ionosphere by the Mars Atmosphere and Volatile EvolutioN magnetometer
A Fourier analysis of the ionospheric magnetic structure suggests that the power spectrum is like that of the interplanetary magnetic field
Simple magnetohydrodynamical theory shows that plasma in the Martian dayside ionosphere flows downward with speeds of tens of meters per second
We present results from a set of numerical simulations aimed at exploring the mechanism of coronal mass ejection (CME) suppression in active stars by an overlying large-scale magnetic field. We use a ...state-of-the-art 3D magnetohydrodynamic code that considers a self-consistent coupling between an Alfvén wave-driven stellar wind solution, and a first-principles CME model based on the eruption of a flux rope anchored to a mixed-polarity region. By replicating the driving conditions used in simulations of strong solar CMEs, we show that a large-scale dipolar magnetic field of 75 G is able to fully confine eruptions within the stellar corona. Our simulations also consider CMEs exceeding the magnetic energy used in solar studies, which are able to escape the large-scale magnetic field confinement. The analysis includes a qualitative and quantitative description of the simulated CMEs and their dynamics, which reveals a drastic reduction of the radial speed caused by the overlying magnetic field. With the aid of recent observational studies, we place our numerical results in the context of solar and stellar flaring events. In this way, we find that this particular large-scale magnetic field configuration establishes a suppression threshold around ∼3 × 1032 erg in the CME kinetic energy. Extending the solar flare-CME relations to other stars, such CME kinetic energies could be typically achieved during erupting flaring events with total energies larger than 6 × 1032 erg (GOES class ∼X70).
In this paper, we present a comprehensive study of the evolutionary phases of a major M6.6 long duration event with special emphasize on its pre-flare phase. The event occurred in NOAA 12371 on 2015 ...June 22. A remarkable aspect of the event was an active pre-flare phase lasting for about an hour during which a hot EUV coronal channel was in the build-up stage and displayed cospatial hard X-ray (HXR) emission up to energies of 25 keV. This is the first evidence of the HXR coronal channel. The coronal magnetic field configuration based on nonlinear-force-free-field modeling clearly exhibited a magnetic flux rope (MFR) oriented along the polarity inversion line (PIL) and cospatial with the coronal channel. We observed significant changes in the AR's photospheric magnetic field during an extended period of 42 hr in the form of rotation of sunspots, moving magnetic features, and flux cancellation along the PIL. Prior to the flare onset, the MFR underwent a slow rise phase ( 14 km s−1) for 12 minutes, which we attribute to the faster build-up and activation of the MFR by tether-cutting reconnection occurring at multiple locations along the MFR itself. The sudden transition in the kinematic evolution of the MFR from the phase of slow to fast rise ( 109 km s−1 with acceleration 110 m s−2) precisely divides the pre-flare and impulsive phase of the flare, which points toward the feedback process between the early dynamics of the eruption and the strength of the flare magnetic reconnection.
Context.
Explaining the currently observed magnetic fields in galaxies requires relatively strong seeding in the early Universe. One of the current theories proposes that magnetic seeds on the order ...of μG were expelled by supernova (SN) explosions after primordial fields of nG strength or weaker were amplified in stellar interiors.
Aims.
In this work, we take a closer look at this theory and calculate the maximum magnetic energy that can be injected in the interstellar medium by a stellar cluster of mass
M
cl
based on what is currently known about stellar magnetism.
Methods.
We consider early-type stars and adopt either a Salpeter or a top-heavy initial mass function. For their magnetic fields, we adopt either a Gaussian or a bimodal distribution. The Gaussian model assumes that all massive stars are magnetized with 10
3
< ⟨
B
*
⟩< 10
4
G, while the bimodal, consistent with observations of Milky Way stars, assumes only 5 − 10% of OB stars have 10
3
< ⟨
B
*
⟩< 10
4
G, while the rest have 10 < ⟨
B
*
⟩< 10
2
G. We ignore the effect of magnetic diffusion and assume no losses of magnetic energy.
Results.
We find that the maximum magnetic energy that can be injected by a stellar population is between 10
−10
and 10
−7
times the total SN energy. The highest end of these estimates is about five orders of magnitude lower than what is usually employed in cosmological simulations, where about 10
−2
of the SN energy is injected as magnetic.
Conclusions.
Pure advection of the stellar magnetic field by SN explosions is a good candidate for seeding a dynamo, but not enough to magnetize galaxies. Assuming SNe as the main mechanism for galactic magnetization, the magnetic field cannot exceed an intensity of 10
−7
G in the best-case scenario for a population of 10
5
solar masses in a superbubble of 300 pc radius, while more typical values are between 10
−10
and 10
−9
G. Therefore, other scenarios for galactic magnetization at high redshift need to be explored.
Over the last decade there has been mounting evidence that the strength of the Sun's polar magnetic fields during a solar cycle minimum is the best predictor of the amplitude of the next solar cycle. ...Surface flux transport models can be used to extend these predictions by evolving the Sun's surface magnetic field to obtain an earlier prediction for the strength of the polar fields, and thus the amplitude of the next cycle. In 2016, our Advective Flux Transport (AFT) model was used to do this, producing an early prediction for Solar Cycle 25. At that time, AFT predicted that Cycle 25 will be similar in strength to the Cycle 24, with an uncertainty of about 15%. AFT also predicted that the polar fields in the southern hemisphere would weaken in late 2016 and into 2017 before recovering. That AFT prediction was based on the magnetic field configuration at the end of January 2016. We now have two more years of observations. We examine the accuracy of the 2016 AFT prediction and find that the new observations track well with AFT's predictions for the last 2 years. We show that the southern relapse did in fact occur, though the timing was off by several months. We propose a possible cause for the southern relapse and discuss the reason for the offset in timing. Finally, we provide an updated AFT prediction for Solar Cycle 25 that includes solar observations through January of 2018.
Plain Language Summary
After the exceptionally weak Solar Cycle 24 (SC24), there is considerable interest in accurately predicting the amplitude of the coming Solar Cycle 25 (SC25). In 2016, the Advective Flux Transport (AFT) Model was used to make such a prediction. We now have two additional years of solar data. Here we compare the results of the previous prediction to the observations that have since occurred. We then use the additional two years of data to create an updated prediction, with a much smaller uncertainty. We predict that SC25 will be about slightly smaller (∼95%) the strength of SC24, making it the weakest solar cycle in the last hundred years. We also predict that,like SC24, SC25 will be preceded by a long extended solar minimum. Finally, these results indicate that we are now in the midst of a Modern Gleissberg Minimum.
Key Points
Cycle 25 will be slightly weaker than Cycle 24, making it the weakest cycle in the last hundred years
Weak cycles are preceded by long extended minima; we may not reach the Cycle 24/25 minimum until 2021
We are currently (beginning with Cycle 24) in the midst of a Gleissberg cycle minimum
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