High-resolution Magnetic Field Data Research Articles

Fleeting Small-scale Surface Magnetic Fields Build the Quiet-Sun Corona

Arch-like loop structures filled with million Kelvin hot plasma form the building blocks of the quiet-Sun corona. Both high-resolution observations and magnetoconvection simulations show the ubiquitous presence of magnetic fields on the solar surface on small spatial scales of ∼100 km. However, the question of how exactly these quiet-Sun coronal loops originate from the photosphere and how the magnetic energy from the surface is channeled to heat the overlying atmosphere is a long-standing puzzle. Here we report high-resolution photospheric magnetic field and coronal data acquired during the second science perihelion of Solar Orbiter that reveal a highly dynamic magnetic landscape underlying the observed quiet-Sun corona. We found that coronal loops often connect to surface regions that harbor fleeting weaker, mixed-polarity magnetic field patches structured on small spatial scales, and that coronal disturbances could emerge from these areas. We suggest that weaker magnetic fields with fluxes as low as 1015 Mx and/or those that evolve on timescales less than 5 minutes are crucial to understanding the coronal structuring and dynamics.

A Machine Learning–Based Approach to Time-series Wave Identification in the Solar Wind

The Wind spacecraft has yielded several decades of high-resolution magnetic field data, a large fraction of which displays small-scale structures. In particular, the solar wind is full of wavelike fluctuations that appear in both the field magnitude and its components. The nature of these fluctuations can be tied to the properties of other structures in the solar wind, such as shocks, that have implications for the time evolution of the solar wind. As such, having a large collection of wave events would facilitate further study of the effects that these fluctuations have on solar wind evolution. Given the large volume of magnetic field data available, machine learning is the most practical approach to classifying the myriad small-scale structures observed. To this end, a subset of Wind data is labeled and used as a training set for a multibranch 1D convolutional neural network aimed at classifying circularly polarized wave modes. Using this algorithm, a preliminary statistical study of 1 yr of data is performed, yielding about 300,000 wave intervals out of about 5,000,000 solar wind intervals. The wave intervals come about more often in the fast solar wind and at higher temperatures, and the number of waves per day is highly periodic. This machine learning–based approach to wave detection has the potential to be a powerful, inexpensive way to catalog waves throughout decades of spacecraft data.

Low-frequency Waves Due to Newborn Interstellar Pickup Ions Observed from 43 to 47 au by the Voyager 1 Spacecraft

Interstellar neutral atoms enter the heliosphere at a relatively slow speed corresponding to the motion of the Sun through the local interstellar medium, which is approximately 25 km s−1. Neutral hydrogen atoms enter from the approximate location of the Voyager spacecraft and are eventually ionized primarily by collision with thermal solar wind ions. An earlier analysis by Hollick et al. examined low-frequency magnetic waves observed by the Voyager spacecraft from launch through 1990 that are thought to arise from the scattering of newborn interstellar pickup H+ and He+. We report an analysis of Voyager 1 observations in 1991, which is the last year of high-resolution magnetic field data that are publicly available, and find 70 examples of low-frequency waves with the characteristics that suggest excitation by pickup H+ and 10 examples of waves consistent with excitation by pickup He+. We find a particularly dense cluster of observations at the tail end of what is thought to be a Merged Interaction Region (MIR) that was previously studied by Burlaga & Ness using Voyager 2 observations. This is not unexpected if the MIR is followed by a large rarefaction region, as they tend to be regions of reduced turbulence levels that permit the growth of the waves over the long time periods that are generally required of this instability.

Insight into the formation mechanism of the Medusae fossae formation on Mars from magnetic field data

The Medusae Fossae Formation (MFF) is one of the largest unconformable deposits on Mars. The origin of the MFF is unknown, but several formation processes have been proposed, including volcanic, wind, and ice-related mechanisms. An insight into the nature and the origin of the MFF may be ascertained by examining the degree of magnetization carried by the MFF. Here we examine the high-resolution nighttime magnetic field data from the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft over the MFF to ascertain the degree of magnetization carried by the MFF. By showing small modulations in the magnetic field induced by impact craters superposed on the MFF, we demonstrate that the MFF is strongly magnetized close to Apollinaris Mons. Distal to Apollinaris Mons, the MFF displays a relatively lower degree of magnetization. We show that the MFF most likely got magnetized via thermal remanent magnetization, indicating that the MFF near Apollinaris Mons was emplaced as a high-temperature volcanic product. We propose that the decline in remanent magnetization |B| with distance from Apollinaris Mons could be related to lower emplacement temperature. We combine the magnetic field constraints with previous observations of the MFF and propose that the MFF was likely emplaced as a volcanic deposit.

Multiscale Features of Magnetic Field Fluctuations and Field-Aligned Currents in the Polar Ionosphere: A Case Study

Complexity is a typical feature of space plasmas that may involve the formation of multiscale coherent magnetic and plasma structures. The winding features (pseudo-polarization) of magnetic field fluctuations at different spatial scales are a useful quantity in this framework for investigating complexity in space plasma. Indeed, a strong link between pseudo-polarization, magnetic/plasma structures, turbulence and dissipation exists. We present some preliminary results on the link between the polarization of the magnetic field fluctuations and the structure of field-aligned currents in the high-latitude ionosphere. This study is based on high-resolution (50 Hz) magnetic field data collected on board the European Space Agency Swarm constellation. The results show the existence of a clear link between the multiscale coarse-grained structure of pseudo-polarization and intensity of the field-aligned currents, supporting the recent findings according to which turbulence may be capable of generating multiscale filamentary current structures in the auroral ionosphere. This feature is also examined theoretically, along with its significance for the rate of energy deposition and heating in the polar regions.

Statistical Analysis of Magnetic Field Fluctuations in Coronal Mass Ejection-Driven Sheath Regions

We report a statistical analysis of magnetic field fluctuations in 79 coronal mass ejection- (CME-) driven sheath regions that were observed in the near-Earth solar wind. Wind high-resolution magnetic field data were used to investigate 2 h regions adjacent to the shock and ejecta leading edge (Near-Shock and Near-LE regions, respectively), and the results were compared with a 2 h region upstream of the shock. The inertial-range spectral indices in the sheaths are found to be mostly steeper than the Kolmogorov −5/3 index and steeper than in the solar wind ahead. We did not find indications of an f−1 spectrum, implying that magnetic fluctuation properties in CME sheaths differ significantly from planetary magnetosheaths and that CME-driven shocks do not reset the solar wind turbulence, as appears to happen downstream of planetary bow shocks. However, our study suggests that new compressible fluctuations are generated in the sheath for a wide variety of shock/upstream conditions. Fluctuation properties particularly differed between the Near-Shock region and the solar wind ahead. A strong positive correlation in the mean magnetic compressibility was found between the upstream and downstream regions, but the compressibility values in the sheaths were similar to those in the slow solar wind (<0.2), regardless of the value in the preceding wind. However, we did not find clear correlations between the inertial-range spectral indices in the sheaths and shock/preceding solar wind properties, nor with the mean normalized fluctuation amplitudes. Correlations were also considerably lower in the Near-LE region than in the Near-Shock region. Intermittency was also considerably higher in the sheath than in the upstream wind according to several proxies, particularly so in the Near-Shock region. Fluctuations in the sheath exhibit larger rotations than upstream, implying the presence of strong current sheets in the sheath that can add to intermittency.

Energy Conversion between Ions and Electrons through Ion Cyclotron Waves and Embedded Ion-scale Rotational Discontinuity in Collisionless Space Plasmas

Abstract Wave–particle interaction is a fundamental process in collisionless plasma, which results in the redistribution of energy between plasma waves and particle species. The analysis of high-resolution Magnetospheric Multiscale plasma and magnetic field data directly reveals the energy exchange between electromagnetic energy, particle bulk kinetic energy, and thermal kinetic energy in magnetosheath turbulence. This work focuses on the energy transfer associated with ion cyclotron waves (ICWs) and embedded rotational discontinuity (RD). We find that (1) the particle kinetic energy of ions is converted into electromagnetic energy; (2) the electrons are gaining energy from electromagnetic fields, having significant electron heating in the parallel direction around the RD; (3) the ICWs and RD connect and redistribute energy between ions and electrons in the postshock downstream sheath region; and (4) the interactions between pressure tensor and strain tensor redistribute the ion and electron bulk and thermal kinetic energies, but less significantly than direct field–particle interaction by one order of magnitude in the ICW turbulence with weak compressibility, in the sense that , .

Understanding the Role of α Particles in Oblique Heliospheric Shock Oscillations

AbstractRecent observations by DSCOVR provide high temporal resolution (50 samples per second) magnetic vector field data that allows investigating the details of oblique heliospheric shock oscillations. It was found that some of these shocks exhibit magnetic oscillations, both downstream and upstream of the shock front. The DSCOVR/MAG magnetic field data are supplemented by an extensive database of low Mach number (M < 3) low‐β (<1) shock data observed by Wind albeit with lower temporal resolution. Motivated by the observations, we use the 2.5D hybrid model of the oblique shocks with α particles in addition to kinetic protons and electron fluid. We model the properties of the oblique shocks for a number of typical parameters found in observations and study the effects of the shock parameters and the relative α particle abundances on the properties of the shock magnetic field, density, and velocity oscillations. We find the α particles “surf” on the shock front and produce a wake of density oscillations. We examine the details of the phase space of the ions as well as the ion velocity distribution functions in various parts of the shock and study their nonthermal properties. We determine the effects of the α particle kinetic properties and abundances on the structure and dynamics of the shock downstream oscillations for a range of parameters relevant to low Mach number low‐β heliospheric shocks.

Electromagnetic Waves around the Proton Cyclotron Frequency in the Sheath Regions of Interplanetary Magnetic Clouds: STEREO Observations

Abstract The compressed and turbulent sheath regions of interplanetary magnetic clouds (IMCs) provide a natural laboratory to study electromagnetic waves (EMWs) around the proton cyclotron frequency f cp. Based on the Morlet wavelet spectral analysis, the repeated filtering analysis and the minimum variance analysis of high-resolution magnetic field data from the STEREO spacecraft, 81 EMW events are identified in the sheath regions of six IMCs. These EMWs are all transverse, almost circularly polarized, and quasi-parallel propagating along the background magnetic field . They can be left-handed (LH) or right-handed (RH) polarized in the spacecraft frame, where the occurrence rate of the LH-polarized EMWs is higher than that of RH-polarized ones, consistent with previous observations in the solar wind. Also, a comparative analysis of polarization sense of these EMWs has been made in the spacecraft and plasma frames. Our results show that more than half of EMW events suffer a polarization reversal from the spacecraft to plasma frames, which are deduced to propagate inward relative to the solar wind flow. Others are outward-propagating waves. In the plasma frame, the outward-propagating LH-EMWs and inward-propagating RH-EMWs have relatively higher occurrence rates than the inward-propagating LH-EMWs and outward-propagating RH-EMWs, respectively. Furthermore, in the plasma frame all the frequencies of LH-EMWs are below f cp, but the RH-EMW frequencies can exceed f cp. These results are helpful in understanding the physical properties of EMWs and their roles in the sheath regions of IMCs.

Solar wind and interplanetary shock parameters near Saturn's orbit (∼10 AU)

In this work, in-situ 1-h averaged plasma and magnetic field spacecraft data are used to characterize solar wind conditions near Saturn's orbit. Further, high resolution solar wind interplanetary data are used to identify and to calculate interplanetary shock parameters. When only plasma or magnetic field data are available, identified discontinuities were classified as possible shocks. Solar wind parameters at 10 AU decrease as expected from the inner heliosphere averages. For instance, solar wind dynamic pressure is ∼2% and ∼40% of its value at 1 and 5 AU, respectively. Wave magnetosonic speed is similar to 5 AU, but it is only about 60% of the 1 AU value. From analysis of high resolution plasma and magnetic field data, only a small number of shocks (13) were identified, because of scarce data availability. On the other hand, a larger number of possible shocks (60) were identified. Most of the confirmed shocks (∼75%) were fast forward shocks, while ∼25% were reverse shocks. Mach numbers ranged from 1.2 to 3.5 (average 2.2). Shock normal angle was on average 68° with oblique to perpendicular propagation to the magnetic field. From solar wind and interplanetary shock parameters analyzed in this work, and comparison with results from other works at different radial distances, it is concluded that conditions for shock occurrence and shock strength seem to increase from 1 to 5 AU and then level off or decrease near ∼10 AU, with shock strength at 10 AU comparable to shock strength at 1 AU. These results are of interest for interplanetary dynamics and for solar wind-planetary magnetosphere coupling.

Solar Wind Turbulence Studies Using MMS Fast Plasma Investigation Data

Abstract Studies of solar wind turbulence traditionally employ high-resolution magnetic field data, but high-resolution measurements of ion and electron moments have been possible only recently. We report the first turbulence studies of ion and electron velocity moments accumulated in pristine solar wind by the Fast Plasma Investigation (FPI) instrument on board the Magnetospheric Multiscale Mission. Use of these data is made possible by a novel implementation of a frequency domain Hampel filter, described herein. After presenting procedures for processing of the data, we discuss statistical properties of solar wind turbulence extending into the kinetic range. Magnetic field fluctuations dominate electron and ion-velocity fluctuation spectra throughout the energy-containing and inertial ranges. However, a multispacecraft analysis indicates that at scales shorter than the ion inertial length, electron velocity fluctuations become larger than ion-velocity and magnetic field fluctuations. The kurtosis of ion-velocity peaks around a few ion inertial lengths and returns to a near Gaussian value at sub-ion scales.

Energy Conversion and Collisionless Plasma Dissipation Channels in the Turbulent Magnetosheath Observed by the Magnetospheric Multiscale Mission

Abstract Analysis of high-resolution Magnetospheric Multiscale Mission plasma and magnetic field data directly reveals the exchanges of energy between electromagnetic and flow energy and between microscopic flows and random kinetic energy in the inhomogeneous turbulent magnetosheath. The computed rates of exchange are based on exact results from the collisionless Vlasov model of plasma dynamics, without appeal to viscous or other closures. The description includes analyses of several structures observed in intervals of burst mode data in the magnetosheath, revealing pathways of energy exchange at sub-ion scales. Time-series of the work done by the electromagnetic field, and the pressure–stress interaction, enable description of the pathways to dissipation in this low-collisionality plasma. This method does not require any specific mechanism for its application, such as reconnection or a selected mode, although with increased experience it will be useful for distinguishing between proposed possibilities.

Global characteristics of auroral Hall currents derived from the Swarm constellation: dependences on season and IMF orientation

Abstract. On the basis of field-aligned currents (FACs) and Hall currents derived from high-resolution magnetic field data of the Swarm constellation, the average characteristics of these two current systems in the auroral regions are comprehensively investigated by statistical methods. This is the first study considering both current types determined simultaneously by the same spacecraft in both hemispheres. The FAC distribution, derived from the novel Swarm dual-spacecraft approach, reveals the well-known features of Region 1 (R1) and Region 2 (R2) FACs. At high latitudes, Region 0 (R0) FACs appear on the dayside. Their flow direction, up or down, depends on the orientation of the interplanetary magnetic field (IMF) By component. Of particular interest is the distribution of auroral Hall currents. The prominent auroral electrojets are found to be closely controlled by the solar wind input, but we find no dependence of their intensity on the IMF By orientation. The eastward electrojet is about 1.5 times stronger in local summer than in winter. Conversely, the westward electrojet shows less dependence on season. As to higher latitudes, part of the electrojet current is closed over the polar cap. Here the seasonal variation of conductivity mainly controls the current density. During local summer of the Northern Hemisphere, there is a clear channeling of return currents over the polar cap. For positive (negative) IMF By a dominant eastward (westward) Hall current circuit is formed from the afternoon (morning) electrojet towards the dawn side (dusk side) polar cap return current. The direction of polar cap Hall currents in the noon sector depends directly on the orientation of the IMF By. This is true for both signs of the IMF Bz component. Comparable Hall current distributions can be observed in the Southern Hemisphere but for opposite IMF By signs. Around the midnight sector the westward substorm electrojet is dominating. As expected, it is highly dependent on magnetic activity, but it shows only little response to season and IMF By polarity. An important finding is that all the IMF By dependences of FACs and Hall currents practically disappear in the dark winter hemisphere.

EMIC waves observed at geosynchronous orbit under quiet geomagnetic conditions (Kp ≤ 1)

AbstractWe statistically study the local time distribution of the helium band electromagnetic ion cyclotron (EMIC) waves observed at geosynchronous orbit when geomagnetic activity was low (Kp ≤ 1). In order to identify the geosynchronous EMIC waves, we use high time resolution magnetic field data acquired from GOES 10, 11, and 12 over a 2 year period from 2007 and 2008 and examine the local time distribution of EMIC wave events. Unlike previous studies, which reported high EMIC wave occurrence in the postnoon sector with a peak around 1500–1600 magnetic local time (MLT) during magnetically disturbed times (i.e., storm and/or substorm), we observed that quiet time EMIC waves mostly occur in a region from morning (∼0600 MLT) to afternoon (∼1600 MLT) with a peak around 1100–1200 MLT. To investigate whether the quiet time EMIC wave occurrence has a causal relationship with magnetospheric convection enhancement or solar wind dynamic pressure variations, we performed a superposed epoch analysis of solar wind parameters (solar wind speed, density, dynamic pressure, and interplanetary magnetic field Bz) and geomagnetic indices (AE and SYM‐H). From the superposed epoch analysis we found that solar wind dynamic pressure variation is a more important parameter than AE and SYM‐H for quiet time EMIC wave occurrence.

Magnetic mapping of river channel and palaeochannel deposits - an example from Teetulpa, South Australia

SUMMARY High-resolution, low-level aeromagnetic surveys of the Teetulpa gold field in the Nackara Arc, South Australia, map the distribution of magnetic minerals in the alluvial cover, in the form of linear anomalies with a dendritic pattern typical of drainage systems. These anomalies are not evident in the regional aeromagnetic data flown at wider line spacing and higher elevation. Combination of the high resolution magnetic field data with mapping of present day drainage is an important input to gold exploration of the area. The magnetic anomalies can be modelled and inverted, and this might provide quantitative information to indirectly target and evaluate gold resources. Sampling and statistical analysis of relationships between the gold and magnetic minerals within the alluvium are required to form the basis for any such study.

Noise features of the CHAMP vector magnetometer in the 1–25 Hz frequency range

CHAMP (CHAllenging Minisatellite Payload), a German small satellite mission to study the Earth's gravity field, magnetic field and upper atmosphere, ended in space on 19 September 2010. Thanks to the good quality of the satellite and to several altitude maneuvers, the satellite provided continuous and reliable observations including house-keeping data at different processing levels for more than 10 years. Among them the high-resolution vector magnetic field data, provided by the FGM (FluxGate vector Magnetometer) at 50Hz sample rate, are one of the most important data products. The 50Hz data allows us to investigate all signals from DC to 25Hz. Here we report on noise features found in the frequency range above 1Hz. There are predominantly three types of disturbances found regularly in the vector magnetic field data. One is at ∼2.1Hz and harmonic signals, an other is an 8Hz single frequency signal. The features of these two signals are characterized and analyzed, which helps us to trace the signals to the source of disturbance. The 2.1Hz harmonic signals mainly depend on the incident angle of sunlight, which points at the star tracker instrument ASC (Advanced Stellar Compass), as the signal depends on the level of blinding. The 8Hz sinusoidal signal is continuously present and its amplitude is controlled by the temperature inside of the satellite. For higher temperatures the signal gets smaller, and it practically disappears above 20°C. Only the FGM x-axis is affected by the 8Hz signal. The third artificial signal affects the FGM y-axis. A frequency sweep from the Nyquist frequency down to zero and up again occurs when the readings of the FGM y-axis go through zero. This is probably related to a ringing of the analog-to-digital converter at the sign change. The amplitudes of all these noise features are generally below 50pT.

Improved interpretation of solar wind ion measurements via high-resolution magnetic field data

The Wind spacecraft’s Faraday cups (FC) continue to produce high-quality, in situ observations of thermal protons (i.e., ionized hydrogen) and α-particles (i.e., fully ionized helium) in the solar wind. By fitting a Wind/FC ion spectrum with a model velocity distribution function (VDF) for each particle species, values for density, bulk velocity, and temperature can be inferred. Incorporating measurements of the background magnetic field from the Wind Magnetic Field Investigation (MFI) allows perpendicular and parallel temperature components to be separated. Prior implementations of this analysis averaged the higher-cadence Wind/MFI measurements to match that of the Wind/FC ion spectra. However, this article summarizes recent and extensive revisions to the analysis software that, among other things, eliminate such averaging and thereby account for variations in the direction of the magnetic field over the time taken to measure the ions. A statistical comparison reveals that the old version consistently underestimates the temperature anisotropy of ion VDF’s: averaging over fluctuations in the magnetic field essentially blurs the perpendicular and parallel temperature components, which makes the plasma seem artificially more isotropic. The new version not only provides a more accurate dataset of ion parameters (which is well suited to the study of microkinetic phenomena), it also demonstrates a novel technique for jointly processing particle and field data. Such methods are crucial to heliophysics as wave-particle interactions are increasingly seen as playing an important role in the dynamics of the solar wind and similar space plasmas.

THEMIS observations of unusual bow shock motion attending a transient magnetospheric event

We present a multipoint case study of solar wind and magnetospheric observations during a transient magnetospheric compression at 23:19 UT on 15 October 2008. We use high time resolution magnetic field and plasma data from the THEMIS and GOES 11/12 spacecraft to show that this transient event corresponded to an abrupt rotation in the IMF orientation, a change in the location of the foreshock, and transient outward bow shock motion. We employ results from a global hybrid code model to reconcile the observations indicating transient inward magnetopause motion with the outward bow shock motion.

Mirror Mode Waves Downstream of a Stream Interaction Region Forward Shock near 1 AU

Mirror mode waves in the solar wind are typically observed not as quasi-periodic sinusoidal signatures, but as trains of nonperiodic structures of two types: magnetic "peaks" and magnetic "dips." Some trains of long durations have been called mirror mode storms. In this work we report mirror mode waves downstream of a stream interaction region (SIR) forward shock observed near 1 AU on 7 May 2007 with Solar Terrestrial Relations Observatory (STEREO) and Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. The high-resolution magnetic-field data (0.125-second resolution) from STEREO are scanned to search for magnetic dips and peaks (or upgoing magnetic "mesas") in the solar wind. STEREO-A observes a mirror mode storm: the appearance of mirror mode waves (mainly magnetic peaks and upgoing mesas) is simultaneous with the entry into a high-density, high-temperature, and high plasma beta accompanied by a depressed field region; the magnetic dips survive in the lower plasma-beta region. STEREO-B observes mirror mode waves (mainly magnetic peaks) with different amplitudes and asymmetric forms, which can survive in a low plasma beta region. THEMIS-D, which was located in the solar wind, also observes mirror mode waves (mainly magnetic peaks and upgoing mesas) as well as an enhanced ion temperature anisotropy (T (aSyen)a parts per thousand 3T (ayen)). The enhanced ion temperature anisotropy and high plasma beta satisfy the mirror-instability criterion. These observations of STEREO and THEMIS-D show that mirror mode waves can be excited downstream of a SIR forward shock near 1 AU.

Estimating magnetic dike parameters using a non‐linear constrained inversion technique: an example from the Särna area, west central Sweden

ABSTRACTIn this paper, we describe a non‐linear constrained inversion technique for 2D interpretation of high resolution magnetic field data along flight lines using a simple dike model. We first estimate the strike direction of a quasi 2D structure based on the eigenvector corresponding to the minimum eigenvalue of the pseudogravity gradient tensor derived from gridded, low‐pass filtered magnetic field anomalies, assuming that the magnetization direction is known. Then the measured magnetic field can be transformed into the strike coordinate system and all magnetic dike parameters – horizontal position, depth to the top, dip angle, width and susceptibility contrast – can be estimated by non‐linear least squares inversion of the high resolution magnetic field data along the flight lines.We use the Levenberg‐Marquardt algorithm together with the trust‐region‐reflective method enabling users to define inequality constraints on model parameters such that the estimated parameters are always in a trust region. Assuming that the maximum of the calculated gzz (vertical gradient of the pseudogravity field) is approximately located above the causative body, data points enclosed by a window, along the profile, centred at the maximum of gzz are used in the inversion scheme for estimating the dike parameters. The size of the window is increased until it exceeds a predefined limit. Then the solution corresponding to the minimum data fit error is chosen as the most reliable one.Using synthetic data we study the effect of random noise and interfering sources on the estimated models and we apply our method to a new aeromagnetic data set from the Särna area, west central Sweden including constraints from laboratory measurements on rock samples from the area.