Regions Of Crustal Magnetic Fields Research Articles

Modeling Ion Transport in the Upper Ionosphere of Mars: Exploring the Effect of Crustal Magnetic Fields

AbstractStatistically ion and electron densities are enhanced above strong crustal magnetic field regions according to measurements made by the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft. Plasma created by ionization of neutrals in the lower ionosphere (where chemistry dominates) flows upward and becomes trapped on closed magnetic field loops. Enhanced ion density in the ionosphere (particularly O2+) is associated with enhanced photochemical escape of atomic oxygen. This paper presents a quasi‐1D multi‐fluid time‐dependent model of the Martian ionosphere for nine ion species. Ionospheric temperatures are adopted but ion densities and velocities (along the field lines) are determined using a numerical solution of the continuity and momentum equations. Diurnal effects are explored by varying photoionization rates. Three crustal field cases are considered: a low altitude closed, a high altitude closed, and a high altitude open field line. Additionally, a case with no crustal field is modeled to provide a comparison between regions with and without crustal fields in the upper Martian ionosphere. Model results show higher ion and electron densities in the crustal field cases than in the purely induced field case. Additionally, we find that densities are generally higher on the closed field lines than on the open field lines, and ion velocities are generally up the field lines, away from the Martian surface. We also find that velocities are larger on the open field line case. We compare modeled density results to MAVEN data and find general agreement. Implications for atmospheric escape, particularly photochemical escape of O, are also discussed.

Gravity wave driven variability in the Mars ionosphere-thermosphere system

Gravity waves (GWs) have been persistently observed in neutral species in the Martian thermosphere. In this paper, we study characteristics of GWs in O2+ and O+ ions in the Mars ionosphere (∼160–200 km) and examine their relation to GWs in the thermosphere. For this purpose, we use O2+, O+, and Ar densities measured by the Neutral Gas and Ion Mass spectrometer (NGIMS) instrument aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. The amplitudes and (horizontal) wavelengths of the GWs are extracted from the density profiles. The results of the present study show that GWs are ubiquitous in the Martian ionosphere-thermosphere system. We found that the amplitudes of GWs in ions are ∼2 times larger than those in neutrals. While the amplitudes in ions reach as high as 70% of the background densities, the most probable amplitude 5%. The amplitudes of the GWs in ions are more on the nightside than on the dayside; more during the 2018 global dust storm period than during nominal dust conditions; and increase with decreasing in solar activity. All these results are qualitatively similar to those observed in neutrals. In addition, amplitudes of the GWs in ions show moderate correlation with those neutrals while the wavelengths show good correlation. The amplitudes of GWs in ions and neutrals show moderate negative correlation with thermospheric temperatures. All these results strongly suggest that the GWs in ions are due to those in neutrals and ion-neutral coupling may be strong in the altitude range of interest. The correlation between the amplitudes of GWs in ions and neutral, however, is local time dependent with weak correlation on the nightside. Other factors such as recombination or particle precipitation may be playing an important role in distorting the wavy nature in ions leading to weaker correlation with neutral waves on the night side. Furthermore, the correlation between GW amplitudes in ions and neutrals is weaker in strong crustal magnetic field regions. This implies that the magnetic fields control the motion of the ions leading to distorting in wavy nature in ions.

Discrete Aurora on the Nightside of Mars: Occurrence Location and Probability

AbstractThis paper represents the first attempt to predict the occurrence location and probability of discrete electron aurora on the nightside of Mars. We run a 3‐D time‐dependent magnetohydrodynamic model to characterize the spatial and temporal dynamics of magnetic field and plasma distributions over the course of one planetary rotation. We perform eight simulation cases under solar minimum quiet‐solar‐wind conditions (four equinox/solstice seasons, each with two interplanetary magnetic field polarities) and in an actual interplanetary coronal mass ejection (ICME) case to assess quiet and space weather situations, respectively. The occurrence of detectable discrete aurora is subject to the combination of the probabilities that (a) the ionosphere is magnetically connected with high altitudes through open field lines and (b) precipitating energy fluxes of >30 eV electrons exceed 0.1 erg/cm2/s. Our results show that during quiet solar activity, discrete aurora occurs likely on small‐scale patches embedded inside strong crustal magnetic field regions (with a magnitude greater than 50 nT at 150 km), and the overall chance across the globe is ∼0.77%. The higher probability over strong crustal field regions is attributed to the stronger magnetic field convergence. Modeling shows the occurrence probability dramatically increases during the ICME event, particularly by more than an order of magnitude in weak crustal field regions. Our model results reasonably agree with NASA Mars Atmosphere and Volatile EvolutioN and Mars Express observations. Our study suggests that nightside discrete electron aurora is not caused by the direct entry of magnetosheath plasma in a cusp‐like process but due to the recycling of nightside magnetospheric electrons.

The Vertical Extent of Enhanced Densities in Cusp‐Like Regions of the Ionosphere of Mars

AbstractUnusually large values of peak electron density in the ionosphere of Mars are occasionally observed in cusp‐like regions of crustal magnetic field. These features are observed as atypical examples of oblique echoes in ionograms. However, the vertical extent over which electron density values are enhanced has not yet been determined. Furthermore, the physical processes responsible for these striking features are not yet understood. Here we analyze ionograms from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument on Mars Express to better constrain the spatial distribution of electron density in and around these rare ionospheric features. Data from orbits 2359, 2491, 4469, and 4513 are examined. When peak electron density is unusually large, electron density values from the peak to 165 km are increased by 30% ± 10% and the electron density at the spacecraft (∼300–400 km) is diminished. We investigate whether the central portion of an oblique echo is equivalent to a nadir echo, which would permit the reliable determination of an electron density profile in instances of unusually large peak electron densities. We find that uncertainties in the location of the center of an oblique echo render this approach unfeasible. We also report that 86.5 μs should be subtracted from travel time values reported in current MARSIS data products released prior to 2019.

Mars' Ionopause: A Matter of Pressures

AbstractThis study assesses under what circumstances the Martian ionopause is formed on the dayside, both in regions where there are strong crustal magnetic fields and areas where these fields are small (<30 nT). Multiple data sets from three MAVEN dayside deep dip campaigns are utilized between periapsis and 600–1,000 km, as well as solar wind observations from Mars Express. The ionopause is identified as a sudden decrease of the electron density with increasing altitude and a simultaneous increase of the electron temperature and variability below 400 km. This is a physically robust approach as the electron temperature is a key parameter in determining the structure of the ionospheric profile, and, therefore, also a strong indicator of the ionopause location. We find that 36% (54%) of the electron density profiles over strong (weak) crustal magnetic field regions had an ionopause event. We also evaluate the roles of ionospheric thermal and magnetic pressures on the ionopause formation as well as the presence of solar wind particles, H+, down to the location of the ionopause. We found that the topside ionosphere is typically magnetized at mostly all altitudes. The ionopause, if formed, occurs where the total ionospheric pressure (magnetic + thermal) equals the upstream solar wind dynamic pressure. Moreover, the lower edge of the ionopause coincides with the altitude where the solar wind flow stops: The thermal pressure suffers a significant reduction with increasing altitude and the solar wind proton density has a prominent increase.

The Effects of Solar Wind Dynamic Pressure on the Structure of the Topside Ionosphere of Mars

AbstractWe use Mars Atmosphere and Volatile EvolutioN observations of the upstream solar wind, and Mars Express observations of ionospheric electron densities and magnetic fields, to study how the topside ionosphere (>320 km) of Mars is affected by variations in solar wind dynamic pressure. We find that high solar wind dynamic pressures result in the topside ionosphere being depleted of plasma at all solar zenith angles, coincident with increased induced magnetic field strengths. The depletion of topside plasma in response to high solar wind dynamic pressures is observed in both weak and strong crustal magnetic field regions. Taken together, our results suggest that high solar wind dynamic pressures lead to ionospheric compression, increased ion escape, and reduced day‐to‐night plasma transport in the high‐altitude nightside ionosphere.

The Statistical Characteristics of Small‐Scale Ionospheric Irregularities Observed in the Martian Ionosphere

AbstractSmall‐scale ionospheric irregularities in magnetic field strength and ionospheric density have recently been discovered at Mars by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Studying these irregularities provides insight into the interaction between the thermosphere and ionosphere and allows for comparative aeronomy between Mars, Earth, and other planets. This paper reports the first statistical study utilizing 3.5 years of MAVEN in situ measurements to characterize these irregularities and to provide maps of where these irregularities are most likely to be observed. Irregularities are most likely to be observed at dawn and dusk, between solar zenith angles of 90° and 150°, although events are observed all the way to midnight. Irregularities occur primarily outside of the crustal magnetic field regions, between altitudes of ∼130 and 160 km, when ionospheric dynamo conditions exist (magnetized electrons and unmagnetized ions). The irregularities appear to be MHD‐like in nature (an interesting difference to their electrostatic terrestrial counterparts), characterized by variations in absolute magnetic field strength of 5–15% of the background field and perpendicular to magnetic field length scales of 5–20 km. Although MAVEN cannot accurately resolve the associated density variations, we estimate these variations to typically be between 10% and 75% of the background ionospheric density. When present, irregularity events appear to occur with equal likelihood above and below the ionospheric peak. At Earth, the occurrence of irregularity events exhibits strong seasonal dependencies; the MAVEN data set is currently too small to identify any conclusive trends as a function of Mars season.

MAVEN Observations of the Effects of Crustal Magnetic Fields on Electron Density and Temperature in the Martian Dayside Ionosphere.

Mars lacks a global magnetic field but possesses concentrated regions of crustal magnetic field that influence the planet's interaction with the solar wind and the structure of the Martian ionosphere. In this study we survey 17 months of MAVEN Langmuir Probe and Waves dayside electron density and temperature measurements to study how these quantities are affected in regions with strong crustal magnetic fields. Above 200 km altitude, we find that regions of strong crustal magnetic fields feature cooler electron temperatures and enhanced electron densities compared to regions with little or no crustal magnetic field. Neutral densities and temperatures are not significantly affected. Closed field lines on which electrons can be trapped are more prevalent in strong crustal field regions than elsewhere. Trapped on closed field lines, electrons are protected against loss processes involving the solar wind. This would lead to longer plasma lifetimes, higher densities, and lower temperatures.

Interpreting Mars ionospheric anomalies over crustal magnetic field regions using a 2‐D ionospheric model

AbstractThe spatially inhomogeneous, small‐scale crustal magnetic fields of Mars influence the escape of planetary atmospheric species and the interaction of the solar wind with the ionosphere. Understanding the plasma response to crustal magnetic field regions can therefore provide insight to ionospheric structure and dynamics. To date, several localized spatial structures in ionospheric properties that have been observed over regions of varying magnetic field at Mars have yet to be explained. In this study, a two‐dimensional ionospheric model is used to simulate the effects of field‐aligned plasma transport in regions of strong crustal magnetic fields. Resulting spatial and diurnal plasma distributions are analyzed and found to agree with observations from several spacecraft and offer compelling interpretations for many of the anomalous ionospheric behaviors observed at or near regions of strong crustal magnetic fields on Mars.

Particle‐in‐cell simulations of the solar wind interaction with lunar crustal magnetic anomalies: Magnetic cusp regions

As the solar wind is incident upon the lunar surface, it will occasionally encounter lunar crustal remanent magnetic fields. These magnetic fields are small‐scale, highly non‐dipolar, have strengths up to hundreds of nanotesla, and typically interact with the solar wind in a kinetic fashion. Simulations, theoretical analyses, and spacecraft observations have shown that crustal fields can reflect solar wind protons via a combination of magnetic and electrostatic reflection; however, analyses of surface properties have suggested that protons may still access the lunar surface in the cusp regions of crustal magnetic fields. In this first report from a planned series of studies, we use a 11/2‐dimensional, electrostatic particle‐in‐cell code to model the self‐consistent interaction between the solar wind, the cusp regions of lunar crustal remanent magnetic fields, and the lunar surface. We describe the self‐consistent electrostatic environment within crustal cusp regions and discuss the implications of this work for the role that crustal fields may play regulating space weathering of the lunar surface via proton bombardment.

Ion Acceleration and Outflow from Mars and Venus: An Overview

Solar wind forcing of Mars and Venus results in outflow and escape of ionospheric ions. Observations show that the replenishment of ionospheric ions starts in the dayside at low altitudes (≈300–800 km), ions moving at a low velocity (5–10 km/s) in the direction of the external/ magnetosheath flow. At high altitudes, in the inner magnetosheath and in the central tail, ions may be accelerated up to keV energies. However, the dominating energization and outflow process, applicable for the inner magnetosphere of Mars and Venus, leads to outflow at energies ≈5–20 eV. The aim of this overview is to analyze ion acceleration processes associated with the outflow and escape of ionospheric ions from Mars and Venus. Qualitatively, ion acceleration may be divided in two categories: In the first category we find a processes denoted “planetary wind”, the result of e.g. ambipolar diffusion, wave enhanced planetary wind, and mass-loaded ion pickup. In the second category we find ion pickup, current sheet acceleration, wave acceleration, and parallel electric fields, the latter above Martian crustal magnetic field regions. Both categories involve mass loading. Highly mass-loaded ion energization may lead to a low-velocity bulk flow—A consequence of energy and momentum conservation. It is therefore not self-evident what group, or what processes are connected with the low-energy outflow of ionospheric ions from Mars. Experimental and theoretical findings on ionospheric ion acceleration and outflow from Mars and Venus are discussed in this report.

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.

Observations of aurorae by SPICAM ultraviolet spectrograph on board Mars Express: Simultaneous ASPERA‐3 and MARSIS measurements

We present a new set of observations of Martian aurorae obtained by Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) on board Mars Express (MEX). Using nadir viewing, several auroral events have been identified on the Martian nightside, all near regions of crustal magnetic fields. For most of these events, two to three consecutive events with variable intensities and separated by a few seconds to several tens of seconds have been observed, whereas simultaneous observations with Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and Analyzer of Space Plasma and Energetic Atoms (ASPERA‐3) have been possible. In this paper, we present the data set for these events and discuss the possible correlation between the measured UV emission by SPICAM, the measured downward and/or upward flux of electrons by ASPERA‐3 and the total electron content recorded by MARSIS. Despite the limited coverage of SPICAM ultraviolet spectrograph (UVS) on the Martian nightside (essentially in regions of high crustal magnetic fields), there is however a very good correlation between the regions with the locally smallest probability to be on closed crustal magnetic field lines, as derived from Mars Global Surveyor/Electron Reflectometer (MGS/MAG‐ER), and the position of an aurora event. This suggests that the crustal magnetic fields, when organized into cusp‐like structure, can trigger the few aurorae identified by SPICAM UVS. It confirms also the good probability, in the cases where SPICAM UVS measured UV emissions, that the increase in the measured total electron content by MARSIS and the simultaneous measured precipitating electron flux by the ASPERA‐3/Electron Spectrometer may be related to each other.

Evidence for collisionless magnetic reconnection at Mars

Using data from Mars Global Surveyor (MGS) in combination with Particle‐In‐Cell (PIC) simulations of reconnection, we present the first direct evidence of collisionless magnetic reconnection at Mars. The evidence indicates that the spacecraft passed through the diffusion region where reconnection is initiated and observed the magnetic field signatures of differential electron and ion motion – the Hall magnetic field – that uniquely indicate the reconnection process. These are the first such in‐situ reconnection observations at an astronomical body other than the Earth. Reconnection may be the source of Mars' recently discovered auroral activity and the changing boundaries of the closed regions of crustal magnetic field.

Role of plasma waves in Mars' atmospheric loss

Recent observations of plasma waves, electron fluxes, and ion fluxes in Mars' ionosphere indicate that ion heating may have had a significant impact on Mars' atmospheric loss. We discuss two energy sources of plasma waves: the solar wind interaction with Mars and field‐aligned currents in regions of crustal magnetic fields. These plasma waves can damp through cyclotron resonance with the O+ population in the ionosphere leading to heating and subsequent O+ escape supporting the ∼1025 atoms s−1 (∼0.4 kg/s) O+ outflow indicated by present‐day observations. A stronger solar wind and O+ source of ∼4 Gyr ago could support losses of ∼100 kg/s, enough to strip Mars' atmosphere or 10 m of water in a ∼0.3 Gyr period. The observational evidence for ion heating is, with current data sets, largely circumstantial so we suggest needed observations.

The IMF pile-up regions near the Earth and Venus: lessons for the solar wind - Mars interaction

The Mars Global Surveyor mission has revealed that localized crustal paleomagnetic anomalies are a common feature of the Southern Hemisphere of Mars. The magnetometer measured small-scale magnetic fields associated with many individual magnetic anomalies have magnitudes ranging from hundreds to thousands nT at altitude above 120 km. That makes Mars globally different from both Venus and Earth. The data collected by Lunar Prospector near the Moon were interpreted as evidence that above regions of inferred strong surface magnetic fields on the Moon the SW flow is deflected, and a small-scale mini-magnetosphere exists under some circumstances. With a factor of 100 stronger magnetic fields at Mars and a lower SW dynamic pressure, those conditions offer the opportunity for a larger size of small `magnetospheres' which can be formed by the crustal magnetic fields. Outside the regions of the magnetic anomalies, the SW/Mars interaction is Venus-like. Thus, at Mars the distinguishing feature of the magnetic field pile-up boundary most likely varies from Venus-like to Earth-like above the crustal magnetic field regions. The observational data regarding the IMF pile-up regions near Venus and the Earth are initially reviewed. As long as the SW/Mars interaction remains like that at Venus, the IMF penetrates deep into the Martian ionosphere under the `overpressure' conditions. Results of numerical simulations and theoretical expectations regarding the temporal evolution of the IMF inside the Venus ionosphere and appearance of superthermal electrons are also reviewed and assessed.