Strong Crustal Magnetic Fields Research Articles

Model of Martian Crustal Magnetic Field Based on Equivalent Currents

The UV spectrograph Spectroscopy for the Investigation of Characteristics of the atmosphere of Mars (SPICAM) on board Mars Express made the first observation of auroral-type emission in the cusp region of strong crustal magnetic field on Mars and found that the arc of Martian aurora is very narrow in width, which obviously differs from that of other planets. Based on the observation, we put forward a model of crustal magnetic field on Martian aurora zone through the morphology of Martian aurora. In the model, equivalent currents are proposed, the topology and magnitude of magnetic field generated by these equivalent currents is consistent with that of crustal magnetic field on Martian aurora zone. The morphology of Martian aurora generated through the model matches well with the observations by the Mars Express orbiter.

Variability in ionospheric total electron content at Mars

The Mars Express (MEX) mission includes a multi-purpose radio instrument called the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS). When used in its ionospheric-penetrating subsurface sounder (SS) radar mode, a by-product of the MARSIS observations is the ray-path-integral of electron densities, called the total electron content (TEC). We have used the initial TEC database of approximately 1.2 million TEC values spanning the period June 2005 to September 2007 to study the basic characteristics of TEC morphology and variability. We find quantitative agreement between the TEC values measured and those computed from model simulations of global diurnal behavior. With the basic photo-chemistry of the martian ionosphere a well understood process, it is the departures from average conditions that need specification and modeling. Here we use MARSIS TEC to do this quantitatively. We explore the specification of variability using different ways to define it: standard deviations from sample averages versus departures from control curves.For global studies, we computed the standard deviation (σ in %) of mean values of TEC (in TECU of 1015e−/m2) sorted by latitude, longitude, solar zenith angle (SZA), local time, season, and locations with/without strong crustal magnetic fields (50nT at 150km). For daytime conditions (SZA<75°), the global average 〈TEC〉 is ~6 TECU with σ=~20%, while for nighttime (SZA>105°) 〈TEC〉 is ~0.3 TECU with σ=~75%. Daytime variability is enhanced in the latitude region 0–30°S, a pattern that needs validation by later observations before its source can be identified. Nighttime variability is noticeably larger in regions of strong crustal magnetic fields (B)—an effect noted by previous authors.For regional studies, high resolution latitude patterns of variability in the southern hemisphere – within the longitude sector 150–210° of strong crustal-B values – were computed as percentage changes with respect to zonally-averaged patterns outside the region of interest. We present evidence for the first time of B-fields affecting the variability of the daytime ionosphere by small amounts (~±5%). Under nighttime conditions, the B-field associated variability is ~±20%. The results also reveal an anti-correlation between daytime and nighttime variability ordered by the inclination angle (I) of the B-fields. TEC variability is greater as I approaches vertical at night, but higher during the day (by smaller amounts) where I approaches horizontal patterns.

THE SPIN-DOWN OF PSR J0821-4300 AND PSR J1210-5226: CONFIRMATION OF CENTRAL COMPACT OBJECTS AS ANTI-MAGNETARS

Using XMM-Newton and Chandra, we measure period derivatives for the second and third known pulsars in the class of central compact objects (CCOs) in supernova remnants, proving that these young neutron stars have exceptionally weak dipole magnetic field components. For the 112 ms PSR J0821−4300 in Puppis A, . Its proper motion, μ = 61 ± 9 mas yr−1, was also measured using Chandra. This contributes a kinematic term to the period derivative via the Shklovskii effect, which is subtracted from to derive dipole Bs = 2.9 × 1010 G, a value similar to that of the first measured CCO, PSR J1852+0040 in Kes 79, which has Bs = 3.1 × 1010 G. Antipodal surface hot spots with different temperatures and areas are deduced from the X-ray spectrum and pulse profiles. Paradoxically, such nonuniform surface temperature appears to require strong crustal magnetic fields, probably toroidal or quadrupolar components much stronger than the external dipole. A spectral feature, consisting of either an emission line at ≈0.75 keV or an absorption line at ≈0.46 keV, is modulated in strength with the rotation. It may be due to a cyclotron process in a magnetic field on the surface that is slightly stronger than the dipole deduced from the spin-down. We also timed anew the 424 ms PSR J1210−5226, resolving previous ambiguities about its spin-down rate. Its is (2.22 ± 0.02) × 10−17, corresponding to Bs = 9.8 × 1010 G. This is also compatible with a cyclotron resonance interpretation of its prominent absorption line at 0.7 keV and its harmonics. These results deepen the mystery of the origin and evolution of CCOs: Why are their numerous descendants not evident?

Origin of Magnetar-Scale Crustal Field in PSR J1852+0040 and ‘Frozen’ Magnetars

AbstractWe discuss the origin of strong crustal magnetic field in one of central compact objects (CCOs)—a neutron star PSR J1852+0040 in the supernova remnant Kes 79. Taking into account its relatively long present day spin period we conclude that the field could not be generated via a dynamo mechanism. If this neutron star indeed is a magnetar with field submerged during a strong fall-back episode, then it argues against the dynamo field origin in magnetars. Otherwise, Kes 79 is not a close relative of normal magnetars. A discovery of an anti-magnetar with a millisecond period and strong crustal field identifiable, for example, due to large pulse fraction, would be the proof of the dynamo field origin. Existence of such sources is in correspondence with the present standard picture of neutron star unification. However, the fraction of magnetars with submerged fields can be small—few percent of the total number of CCOs.

The Analysis of the Topside Additional Layer of Martian Ionosphere Using MARSIS/Mars Express Data

In this study, the transient second or third layer on the topside of the Martian ionosphere were investigated with the most recently released Mars advanced radar for subsurface and ionospheric sounding/Mars Express data obtained from January 2010 to September 2011 to study the correlation between these topside additional layers and surface magnetic fields, solar zenith angle and solar activities. When examining the zones where the topside layer appeared, the occurrence rate of the topside layer was low at the areas with a strong Martian crustal magnetic field as observed by the Mars global surveyor. The occurrence rate of additional layers on the Martian topside ionosphere decreases as the solar zenith angle increases. However, these layers appeared significantly near the terminator of which solar zenith angle is <TEX>$90^{\circ}$</TEX>. In comparison between F10.7 which is the index of solar activities and the occurrence rate of the topside layer by date, its occurrence rate was higher in 2011 than in 2010 with less solar activities. The result of this study will contribute to better understanding of the environments in the topside of the ionosphere through the correlation between the various conditions regarding the Martian ionosphere and the transient layer.

Stationary flux ropes at the southern terminator of Mars

Flux ropes have long been observed in the upper atmosphere of Venus and more recently at Mars. Here we present magnetic field measurements of flux ropes encountered at the southern terminator of Mars by Mars Global Surveyor and compare them to a flux rope model. This allows several parameters of each rope to be inferred. Remarkably similar flux ropes are met repeatedly at the southern terminator over a period of the Martian year, when strong crustal magnetic fields are upstream of their position, indicating that they are most likely stationary and attached to the upstream crustal fields. A mechanism is described that could produce the observed flux ropes.

ENA diagnostics of auroral activity at Mars

One prominent feature of solar wind—Mars interaction has to do with the presence of strong crustal magnetic fields, especially, in the southern hemisphere. This physical property has set Mars apart from Venus as far as the kinetic effect in detail. In this paper, we review briefly the recent particles-and-fields measurements at Mars in connection to the auroral activity driven by magnetic field reconnection between the crustal magnetic fields and the interplanetary fields. It is suggested that the subsolar ENA (energetic neutral atoms) jets discovered by the NPD instrument of Mars Express could be resulting from charge exchange effect of the shocked solar wind protons at the magnetopause-like regions of the Martian mini-magnetospheres and at the cusp regions of the crustal magnetic fields. It also means that ENA and LENA (low energy neutral atom) detectors could be important experiments for diagnostics of the auroral process unique to Mars, which might contribute to the overall atmospheric loss budget.

Model of the Crustal Magnetic Field in the Martian Aurora Zone

It is well known that aurorae are prominent on planets with a global magnetic field and occur where open magnetic field lines converge. The UV spectrometer used for investigating the characteristics of the atmosphere of Mars (SPICAM) on board the Mars Express made the first observation of auroral-type emission in the cusp region of the strong crustal magnetic field on Mars and found that the arc of the Martian aurora zone is very narrow in width, which obviously differs from that of other planets. Based on the observation, we put forward a model of a crustal magnetic field on the Martian aurora zone through the morphology of Martian aurorae. In the model, equivalent currents are proposed; the topology and magnitude of the magnetic field generated by these equivalent currents are consistent with that of the crustal magnetic field in the Martian aurora zone. The morphology of the Martian aurora zone generated through the model matches well with the observations made by the Mars Express orbiter.

Ion Energization and Escape on Mars and Venus

Mars and Venus do not have a global magnetic field and as a result solar wind interacts directly with their ionospheres and upper atmospheres. Neutral atoms ionized by solar UV, charge exchange and electron impact, are extracted and scavenged by solar wind providing a significant loss of planetary volatiles. There are different channels and routes through which the ionized planetary matter escapes from the planets. Processes of ion energization driven by direct solar wind forcing and their escape are intimately related. Forces responsible for ion energization in different channels are different and, correspondingly, the effectiveness of escape is also different. Classification of the energization processes and escape channels on Mars and Venus and also their variability with solar wind parameters is the main topic of our review. We will distinguish between classical pickup and ‘mass-loaded’ pickup processes, energization in boundary layer and plasma sheet, polar winds on unmagnetized planets with magnetized ionospheres and enhanced escape flows from localized auroral regions in the regions filled by strong crustal magnetic fields.

Spectral properties, magnetic fields, and dust transport at lunar swirls

Lunar swirls are albedo anomalies associated with strong crustal magnetic fields. Swirls exhibit distinctive spectral properties at both highland and mare locations that are plausibly explained by fine-grained dust sorting. The sorting may result from two processes that are fairly well established on the Moon, but have not been previously considered together. The first process is the vertical electrostatic lofting of charged fine dust. The second process is the development of electrostatic potentials at magnetic anomalies as solar wind protons penetrate more deeply into the magnetic field than electrons. The electrostatic potential can attract or repel charged fine-grained dust that has been lofted. Since the finest fraction of the lunar soil is bright and contributes significantly to the spectral properties of the lunar regolith, the horizontal accumulation or removal of fine dust can change a surface’s spectral properties. This mechanism can explain some of the spectral properties of swirls, accommodates their association with magnetic fields, and permits aspects of weathering by micrometeoroids and the solar wind.

Nightside ionosphere of Mars: Radar soundings by the Mars Express spacecraft

We present results of a survey of the nightside ionosphere of Mars as observed by Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on board the Mars Express spacecraft. The occurrence rate of the nightside ionosphere is studied as a function of solar zenith angle (SZA), magnetic field magnitude, and magnetic field inclination. It is shown that at locations with weak crustal magnetic fields the occurrence rate of the nightside ionosphere decreases with increasing SZA up to about 125°, suggesting that plasma transport from the dayside plays a crucial role in its formation. However, at locations with strong crustal magnetic fields, the dependence on SZA is no longer apparent and the inclination of magnetic field becomes a crucial parameter: the occurrence rate of the nightside ionosphere is more than 4 times larger at locations with nearly vertical magnetic fields as compared to the locations with nearly horizontal magnetic fields. This indicates that impact ionization by precipitating electrons is the main ionization source at these locations. Observed peak electron densities are less than 2 × 104 cm−3 in the vast majority of cases. Lower estimates of altitudes of peak electron densities are mostly between 100 and 150 km.

Episodic detachment of Martian crustal magnetic fields leading to bulk atmospheric plasma escape

We present an analysis of magnetic field and suprathermal electron measurements from the Mars Global Surveyor (MGS) spacecraft that reveals isolated magnetic structures filled with Martian atmospheric plasma located downstream from strong crustal magnetic fields with respect to the flowing solar wind. The structures are characterized by magnetic field enhancements and rotations characteristic of magnetic flux ropes, and characteristic ionospheric electron energy distributions with angular distributions distinct from surrounding regions. These observations indicate that significant amounts of atmosphere are intermittently being carried away from Mars by a bulk removal process: the top portions of crustal field loops are stretched through interaction with the solar wind and detach via magnetic reconnection. This process occurs frequently and may account for as much as 10% of the total present‐day ion escape from Mars.

Effects of Martian crustal magnetic field on its ionosphere

The effect of the Martian crustal magnetic field is one of the hot topics of the study of Martian ionosphere. The studies on this topic are summarized in this paper. Main data of the Martian ionosphere were resulted from radio occultation experiments. According to the observations, the electron density scale height and the peak electron density of the Martian ionosphere are influenced by its crustal magnetic field. The strong horizontal magnetic field prevents the vertical diffusion of the plasma and makes the electron density scale height in the topside ionosphere close to that in the photo equilibrium region. In the cusp-like regions with strong vertical magnetic field, the enhanced vertical diffusion leads to a larger electron density scale height in the diffusion equilibrium region. The observation of radio occultation experiment onboard Mars Global Surveyor (MGS) showed that the averaged peak electron density observed in the southern hemisphere with strong crustal magnetic field was slightly larger than that in the northern hemisphere with weak crustal magnetic field. The Mars advanced radar for subsurface and ionosphere sounding (MARSIS) onboard Mars Express (MEX) was the first topside sounder to be used to observe Martian ionosphere. The MARSIS results confirmed that the enhancement of the peak electron density occurred in cusp-like regions with open field lines, and the amount of the enhancement was much larger than that observed by the radio occultation experiment. There are two possible mechanisms for the peak electron density enhancement in the cusp-like crustal magnetic field regions: One is the precipitation of the energetic particles and the other is the heating by the waves excited by plasma instabilities. It’s difficult to determine which one is the key mechanism for the peak electron density enhancement. Based on these studies, several interesting problems on the Martian ionosphere and plasma environment are presented.

Magnetic anomalies near Apollinaris Patera and the Medusae Fossae Formation in Lucus Planum, Mars

The nature of strong martian crustal field sources is investigated by mapping and modeling of Mars Global Surveyor magnetometer data near Apollinaris Patera, a previously proposed volcanic source, supplemented by large-scale correlative studies. Regional mapping yields evidence for positive correlations of orbital anomalies with both Apollinaris Patera and Lucus Planum, a nearby probable extrusive pyroclastic flow deposit that is mapped as part of the Medusae Fossae Formation. Iterative forward modeling of the Apollinaris Patera magnetic anomaly assuming a source model consisting of one or more uniformly magnetized near-surface disks indicates that the source is centered approximately on the construct with a scale size several times larger and comparable to that of the Apollinaris Patera free-air gravity anomaly. A significantly lower rms deviation is obtained using a two-disk model that favors a concentration of magnetization near the construct itself. Estimates for the dipole moment per unit area of the Lucus Planum source together with maximum thicknesses of ∼3 km based on topographic and radar sounding data lead to an estimated minimum magnetization intensity of ∼50 A/m within the pyroclastic deposits. Intensities of this magnitude are similar to those obtained experimentally for Fe-rich Mars analog basalts that cooled in an oxidizing (high fO 2) environment in the presence of a strong (⩾10 μT) surface field. Further evidence for the need for an oxidizing environment is provided by a broad spatial correlation of the locations of phyllosilicate exposures identified to date using Mars Express OMEGA data with areas containing strong crustal magnetic fields and valley networks in the Noachian-aged southern highlands. This indicates that the presence of liquid water, which is a major crustal oxidant, was an important factor in the formation of strong magnetic sources. The evidence discussed here for magnetic sources associated with relatively young volcanic units suggests that a martian dynamo existed during the late Noachian/early Hesperian, after the last major basin-forming impacts and the formation of the northern lowlands.

Steep, transient density gradients in the Martian ionosphere similar to the ionopause at Venus

With the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft, the electron density can be measured by two methods: from the excitation of local plasma oscillations and from remote sounding. A study of the local electron density versus time for 1664 orbits revealed that in 132 orbits very sharp gradients in the electron density occurred that are similar to the ionopause boundary commonly observed at Venus. In 40 of these cases, remote sounding data have also confirmed identical locations of steep ionopause‐like density gradients. Measurements from the Analyzer of Space Plasma and Energetic Atoms (ASPERA‐3) electron spectrometer and ion mass analyzer instruments (also on Mars Express) verify that these sharp decreases in the electron density occur somewhere between the end of the region where ionospheric photoelectrons are dominant and the magnetosheath. Combined studies of the two experiments reveal that the steep density gradients define a boundary where the magnetic fields change from open to closed. This study shows that, although the individual cases are from a wide range of altitudes, the average altitude of the boundary as a function of solar zenith angle is almost constant. The average altitude is approximately 500 km up to solar zenith angles of 60°, after which it shows a slight increase. The average thickness of the boundary is about 22 km according to remote sounding measurements. The altitude of the steep gradients shows an increase at locations with strong crustal magnetic fields.

Plasma boundary variability at Mars as observed by Mars Global Surveyor and Mars Express

Abstract. We have used Mars Express (MEX) and Mars Global Surveyor (MGS) simultaneous and non-simultaneous measurements to study the Martian plasma environment. In particular, we have derived quantitative expressions for the altitude of the terminator bow shock (BS) and magnetic pileup boundary (MPB) as functions of solar wind dynamic pressure, crustal magnetic fields and solar EUV flux. We have also studied the influence of the interplanetary magnetic field (IMF) direction. Through simultaneous two-spacecraft case studies we have shown that the dynamic pressure has a strong influence on the location and shape of these boundaries, which is also confirmed through a large statistical study. A higher dynamic pressure pushes the boundaries downward. The IMF direction has a weaker but still significant influence on both boundaries and causes them to move outward in the hemisphere of locally upward electric field. However, the MPB in the Southern Hemisphere is found to actually move inward when the electric field is directed locally upward. The crustal magnetic fields in the Southern Hemisphere have a strong influence on the MPB and cause it to move to higher altitudes over strong crustal magnetic fields. The influence of the crustal magnetic fields on the BS is more ambiguous since there are few crossings over the strongest crustal fields, but there appears to be at least a small trend of a higher BS for stronger crustal fields. An increased solar EUV flux has been found to cause the BS to move outward and the MPB to move inward.

A plasma flow velocity boundary at Mars from the disappearance of electron plasma oscillations

The Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft is capable of measuring ionospheric electron density by the use of two main methods: remote radar sounding and from the excitation of local plasma oscillations. The frequency of the locally excited electron plasma oscillations is used to measure the local electron density. However, plasma oscillations are not observed when the plasma flow velocity is higher than about 160 km/s, which occurs mainly in the solar wind and magnetosheath. As a consequence, in many passes, there is a sudden disappearance of the plasma oscillations as the spacecraft enters into the magnetosheath. This fact allows us to identify a flow velocity boundary on the dayside, between the ionosphere of Mars and the shocked solar wind. This paper summarizes the results of the measurement of 552 orbits mostly over a period from August 4, 2005 to August 17, 2007. The boundary points found using MARSIS have been verified by measurements from the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) Electron Spectrometer (ELS) instrument on Mars Express. The average position of the flow velocity boundary is compared to flow velocity simulations computed using hybrid model and other boundaries. The boundary altitude is slightly lower than the magnetic pile-up boundary determined using Phobos 2 and Mars Global Surveyor (MGS) crossings, but it is in good agreement with the induced magnetospheric boundary determined by ASPERA-3. Investigation of the effect of the crustal magnetic field revealed that the flow velocity boundary is raised at the locations with strong crustal magnetic fields.

Statistical analysis of the location of the Martian magnetic pileup boundary and bow shock and the influence of crustal magnetic fields

We use the data set from the magnetometer and electron reflectometer instruments on board the Mars Global Surveyor spacecraft to show that the crustal magnetic fields of Mars affect the location of the magnetic pileup boundary (MPB) and bow shock (BS) globally. We search for crossings of the MPB and BS in the data that were observed over the first 16 months of the mission. To identify the influence of the crustal magnetic fields, all crossings are extrapolated to the terminator plane in order to remove the solar zenith angle (SZA) dependence, and to make it possible to compare crossings independently of location. The MPB crossings that were observed over regions on Mars, which contain strong crustal magnetic fields, are on average located further out than crossings observed over regions with weak crustal fields. This is shown in three separate longitude intervals. We also find that the dayside BS crossings observed over the southern hemisphere of Mars are on average located further out than the BS crossings observed over the northern hemisphere, possibly because of the influence of the crustal fields. We also study the magnetic field strength and its variation at the inside of the MPB and their dependence on the SZA and altitude. We find that the magnitude of the magnetic field in the MPB is closely linked to the altitude of the MPB, with the magnitude increasing as the MPB is observed closer to the planet.

Influence of IMF draping direction and crustal magnetic field location on Martian ion beams

Data from the Ion Mass Analyzer (IMA) sensor of the ASPERA-3 instrument suite onboard Mars Express and data from the Magnetometer/Electron Reflectometer (MAG/ER) on Mars Global Surveyor have been analyzed to determine whether ion beam events (IBEs) are correlated with the direction of the draped interplanetary magnetic field (IMF) or the proximity of strong crustal magnetic fields to the subsolar point. We examined 150 IBEs and found that they are organized by IMF draping direction. However, no clear dependence on the subsolar longitude of the strongest magnetic anomaly is evident, making it uncertain whether crustal magnetic fields have an effect on the formation of the beams. We also examined data from the IMA sensor of the ASPERA-4 instrument suite on Venus Express and found that IBEs are observed at Venus as well, which indicates the morphology of the Martian and Venusian magnetotails are similar.

Antipodal effects of lunar basin-forming impacts: Initial 3D simulations and comparisons with observations

3D simulations of basin-scale lunar impacts are carried out to investigate: (a) the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins; and (b) the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the South Pole-Aitken (SPA) basin. The simulations demonstrate that a basin-forming impact produces a massive, hot, partially ionized cloud of vapor and melt that expands thermally around the Moon, converging near the basin antipode approximately 1 h after the impact for typical impact parameters. In agreement with previous work, analytic calculations of the interaction of this vapor–melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region. The time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode. The latter produce antipodal shock stresses within the range of 5–25 GPa where stable shock remanent magnetization (SRM) of lunar soils has been found experimentally to occur. Calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally. This suggests that pre-existing ejecta materials, which would also contain abundant metallic iron remanence carriers, may be important anomaly sources, a possibility that is consistent with enhanced magnetic anomalies observed peripheral to SPA. The latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick SPA ejecta mantle occurring near the antipodes of the Imbrium and Serenitatis impacts. Together with converging seismic compressional waves, these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of SPA near the Imbrium antipode, allowing the ascent of magma with enhanced KREEP concentrations.