Magnetospheric Magnetic Field Lines Research Articles

Prediction and Understanding of Soft-proton Contamination in XMM-Newton: A Machine Learning Approach

Abstract One of the major and unfortunately unforeseen sources of background for the current generation of X-ray telescopes are few tens to hundreds of keV (soft) protons concentrated by the mirrors. One such telescope is the European Space Agency’s (ESA) X-ray Multi-Mirror Mission (XMM-Newton). Its observing time lost due to background contamination is about 40%. This loss of observing time affects all the major broad science goals of this observatory, ranging from cosmology to astrophysics of neutron stars and black holes. The soft-proton background could dramatically impact future large X-ray missions such as the ESA planned Athena mission (http://www.the-athena-x-ray-observatory.eu/). Physical processes that trigger this background are still poorly understood. We use a machine learning (ML) approach to delineate related important parameters and to develop a model to predict the background contamination using 12 yr of XMM-Newton observations. As predictors we use the location of the satellite and solar and geomagnetic activity parameters. We revealed that the contamination is most strongly related to the distance in the southern direction, Z (XMM-Newton observations were in the southern hemisphere), the solar wind radial velocity, and the location on the magnetospheric magnetic field lines. We derived simple empirical models for the first two individual predictors and an ML model that utilizes an ensemble of the predictors (Extra-Trees Regressor) and gives better performance. Based on our analysis, future missions should minimize observations during times associated with high solar wind speed and avoid closed magnetic field lines, especially at the dusk flank region in the southern hemisphere.

Energetic particle impact on X‐ray imaging with XMM‐Newton

AbstractSince energetic protons (∼100 keV) hinder space‐based X‐ray imaging, there is a need to characterize the proton environments encountered by a soft X‐ray mission when planning missions and operations in the near‐Earth environment. Impacts range from enhanced noise in the images to damage to CCD detectors. The high‐apogee (17.9 RE), inclined (40°), and elliptical orbit of European Space Agency's XMM‐Newton mission frequently passes through the Earth's Van Allen radiation belts on closed magnetospheric magnetic field lines, the magnetotail lobes on open magnetic field lines, and into the magnetosheath and solar wind on open solar magnetic field lines, four regions with very different energetic proton environments. We use XMM‐Newton measurements from 2000 to 2010 to survey the occurrence of proton strikes or “soft proton flares” on the European Photon Imaging Camera pn‐junction CCD detector. Proton flares affect ∼55% of all measurements. Rates vary from 50%at large radial distances in the solar wind and magnetosheath to 25% in the high‐latitude magnetotail lobes and increase to 66%of the measurements on closed low‐latitude magnetospheric magnetic field lines.

Magnetospheric Boundary Layer Structure and Dynamics as Seen From Cluster and Double Star Measurements

In this review, we discuss the structure and dynamics of the magnetospheric Low-Latitude Boundary Layer (LLBL) based on recent results from multi-satellite missions Cluster and Double Star. This boundary layer, adjacent to the magnetopause on the magnetospheric side, usually consists of a mixture of plasma of magnetospheric and magnetosheath origins, and plays an important role in the transfer of mass and energy from the solar wind into the magnetosphere and subsequent magnetospheric dynamics. During southward Interplanetary Magnetic Field (IMF) conditions, this boundary layer is generally considered to be formed as a result of the reconnection process between the IMF and magnetospheric magnetic field lines at the dayside magnetopause, and the structure and plasma properties inside the LLBL can be understood in terms of the time history since the reconnection process. During northward IMF conditions, the LLBL is usually thicker, and has more complex structure and topology. Recent observations confirm that the LLBL observed at the dayside can be formed by single lobe reconnection, dual lobe reconnection, or by sequential dual lobe reconnection, as well as partially by localized cross-field diffusion. The LLBL magnetic topology and plasma signatures inside the different sub-layers formed by these processes are discussed in this review. The role of the Kelvin-Helmholtz instability in the formation of the LLBL at the flank magnetopause is also discussed. Overall, we conclude that the LLBL observed at the flanks can be formed by the combination of processes, (dual) lobe reconnection and plasma mixing due to non-linear Kelvin-Helmholtz waves.

On the collective curvature radiation

The paper deals with the one possible mechanism of the pulsar radio emission, i.e., with the collective curvature radiation of the relativistic particle stream moving along the curved magnetospheric magnetic field lines. It is shown that the electromagnetic wave containing one cylindrical harmonic exp{is{\phi}} can not be radiated by the curvature radiation mechanism, that corresponds to radiation of a charged particle moving along curved magnetic field lines. The point is that the particle in vacuum radiates the triplex of harmonics (s, s \pm 1), so for the collective curvature radiation the wave polarization is very important and cannot be fixed a priori. For this reason the polarization of real unstable waves must be determined directly from the solution of wave equations for the media. Its electromagnetic properties should be described by the dielectric permittivity tensor \^{\epsilon}({\omega},k,r), that contains the information on the reaction on all possible types of radiation.

Periodic variations of geomagnetic activity indices

Geomagnetic activity is generated by the interaction of the solar wind with the magnetosphere in connection with energy and mass transfer. Most observations suggest that the solar wind energy is transferred to the magnetosphere by the reconnection process at the dayside magnetopause, where oppositely directed southward IMF and northward magnetospheric magnetic field lines merge. Yearly averages of geomagnetic activity indices Ap for the years 1965 to 2009 are compared to the respective averages of v and Bx, By, Bz, where v is the solar wind velocity and Bx, By, Bz are the southward, northward and north-south component of inter- planetary magnetic field (IMF). The correlation of these quantities is known to be rather good. Comparing the averages of Ap with v and Bx, By, Bz separately we find that, during the declining phase of the solar cycle, v and during the ascending phase Bx have more influence on Ap. Analysing the daily variation of ap in an analogue manner suggesting that during phases of the solar cycle, when ap depends more on the IMF- Bx, By, Bz, components, whereas during phases when ap increases as v increases.

Interball contribution to the high-altitude cusp observations

The polar cusps have traditionally been described as narrow funnel-shaped regions of magnetospheric magnetic field lines directly connected to magnetosheath, allowing the magnetosheath plasma to precipitate into the ionosphere. However, recent observations and theoretical considerations revealed that the formation of the cusp cannot be treated separately from the processes along the whole dayside magnetopause and that the plasma in regions like cleft or low-latitude boundary layer is of the same origin. Our review of statistical results as well as numerous case studies identified the anti-parallel merging at the magnetopause as the principal source of the magnetosheath plasma in all altitudes. Since effective merging requires a low plasma speed at the reconnection spot, we have found that the magnetopause shape and especially its indentation at the outer cusp is a very important part of the whole process. The plasma is slowed down in this indentation and arising multiscale turbulent processes enhance the reconnection rate.

Computing magnetospheric mass density from field line resonances in a realistic magnetic field geometry

Ultra‐low‐frequency (ULF) field line resonances can be used to infer the mass density along magnetospheric magnetic field lines. By specifying how mass density is distributed along the magnetic field (usually a power law as a function of distance from the Earth) and a dipole magnetic field geometry, the MHD standing wave equation can be analytically solved and mass density inferred from observed field line eigenfrequencies. However, the geometry of the Earth's magnetic field can deviate significantly from a dipole, even at relatively low L shells and on the dayside magnetosphere. This study investigates the importance of including a realistic magnetic field geometry when computing plasma mass density from observed field line eigenfrequencies. A generalized version of the toroidal mode MHD standing wave equation is solved using the Tsyganenko (2002a, 2002b) empirical magnetic field model (T01). The results are compared to those found using a dipole. We find that assuming a dipole magnetic field geometry results in an overestimation of mass density. The overestimation is larger for more disturbed levels of geomagnetic activity. Our results have important implications for the inference of heavy ions in the magnetosphere. Namely, an increase in heavy ion concentration as a result of enhanced geomagnetic activity will be exaggerated unless the proper magnetic field geometry is taken into account when calculating mass density from field line eigenfrequencies.

Hall effect on magnetic reconnection at the Earth's magnetopause

We analyze in situ observations of magnetic reconnection at the Earth magnetopause to estimate the importance of the Hall current during the merging of interplanetary and magnetospheric magnetic field lines. The reconnection process is studied through numerical simulations, integrating the Hall MHD equations in 2.5 dimensions. A large influence of the Hall effect is found, which can be measured by a significant increase of the reconnection rate.

Ionospheric signatures of split reconnection X‐lines during conditions of IMF Bz < 0 and |By| ∼ |Bz|: Evidence for the antiparallel merging hypothesis

We present measurements of the convection electric field in the dayside ionosphere during extended intervals when the interplanetary magnetic field (IMF) has a negative Bz component and a significant By component (|By| ∼ |Bz|). Under these conditions, distinct differences in the ionospheric projection of the magnetopause reconnection X‐line are predicted by the antiparallel and subsolar merging hypotheses. Close to solstice, in the winter hemisphere the antiparallel merging hypothesis predicts two ionospheric merging lines which are significantly separated either side of noon. This prediction is not expected from any subsolar merging hypothesis (e.g., component merging). Using the method of global convection mapping with line‐of‐sight velocity data from the SuperDARN HF radar network, we present evidence of two distinct merging regions on the dayside magnetopause under these IMF and seasonal conditions. The two merging regions manifest themselves in the ionosphere as two regions of convection flow with a strong poleward component separated by a region of convection flow with either a weak poleward or equatorward component corresponding to an adiaroic portion of the polar cap boundary. The spatiotemporal nature of the actual convection scenario can be complicated, but the predicted pattern represents a recurring feature of the convection electric field under these IMF and seasonal conditions. These observations lend support to the hypothesis that antiparallel merging is the dominant form of magnetopause reconnection during these steady state IMF intervals. This suggests that the magnetic shear between magnetosheath and magnetospheric magnetic field lines plays the major role in determining the location of reconnection on the dayside magnetopause.

O+ observations in the cusp: Implications for dayside magnetic field topology

Under conditions of sustained northward interplanetary magnetic field (IMF), magnetospheric magnetic field lines on the dayside may undergo multiple reconnection, producing closed flux tubes filled with solar wind plasma. Ionospheric O+ beams with energies of ∼1 keV/e that are propagating parallel to the magnetic field in the northern cusp region are potential evidence for this multiple reconnection process. Using observations from the Polar spacecraft, we present evidence of parallel propagating O+ in the northern cusp. A statistical study of these events indicates that they are very rare in the Polar data. Therefore, under the assumption that parallel propagating O+ beams in the northern cusp are evidence of multiple reconnection, the occurrence frequency of multiple reconnection on the dayside under northward IMF conditions appears to be small. Thus this process is insufficient to account for the nearly continuous population of solar wind plasma on closed field lines in the magnetosphere.

High-altitude cusp: INTERBALL observation

The cusp is a funnel-shaped part of the Earth's magnetosphere where the magnetospheric magnetic field lines are directly interconnected with the magnetosheath ones. The magnetic field configuration allows the magnetosheath plasma to precipitate toward the ionosphere. This feature is used for the determination of the cusp position in low altitudes. However, it is often difficult to distinguish different plasma populations in high altitudes. The cusp region is bounded with the low-latitude boundary layer (LLBL), entry layer or cleft on the equatorward side, and by the plasma mantle on the poleward side, but the plasma and magnetic field parameters are similar in all these regions. We have used the INTERBALL-1 and MAGION-4 satellites to study the topology and dynamics of high-altitude cusp regions under quiet solar wind conditions but different directions of the interplanetary magnetic field (IMF). Two-point event studies have shown that (1) the topology of the magnetic field in the high-altitude cusp is controlled by the IMF direction, (2) the cusp plasma source is located near the tailward boundary of the cusp during northward IMF, and (3) magnetosheath fluctuations are correlated with the fluctuations of the cusp precipitation.

Internal structure of flux transfer events produced by the onset of merging at a single X line

We present a new, higher‐order, two‐dimensional, magnetohydrodynamic model for the detailed internal structure of bubbles produced by the onset of merging along a single subsolar X line. The key feature of the model is to implement a nonlinear filter to postprocess results provided by the pseudospectral element method. The devised nonlinear filter (1) eliminates only the high‐frequency oscillations, (2) retains conservation laws, and (3) enforces a total variation diminished criterion to guarantee the elimination of spurious oscillations near discontinuities. For realistic parameters, the model predicts that satellites which remain in the magnetosheath will observe asymmetric bipolar magnetic field signatures normal to the magnetopause, magnetic field strength increases, density and temperature decreases, and velocities opposite the motion of the events. Satellites in the magnetosheath which enter the events observe asymmetric bipolar signatures normal to the magnetopause, magnetic field strength decreases, densities intermediate between those in the magnetosheath and magneto‐sphere, temperature increases, and velocities in the direction of event motion. Satellites initially located just outside the the magnetopause observe greatly depressed densities and enhanced temperatures (but no significant bipolar signatures normal to the magnetopause) followed by velocity jets of magnetosheath‐like plasma on northward magnetospheric magnetic field lines. Satellites in the magnetosphere which enter the events observe depressed magnetic field strengths, enhanced densities and temperatures, and accelerated flows surrounding a core region of accelerated magnetosheath‐like plasma. Neither they nor satellites which remain in the magnetosphere observe any significant bipolar signatures normal to the nominal magnetopause.

A large statistical study of the entry of interplanetary magnetic field Y‐component into the magnetosphere

We used 5 years of GOES‐6 and 6 years of GOES‐7 data to correlate the y‐component of the geosynchronous magnetic field with the y‐component of IMF. One motivation is that the subsolar merging model and the antiparallel model predict distinctly different patterns for By in the magnetosphere as a function of IMF By. Although both models predict that nightside magnetospheric magnetic field will tilt in the direction of the IMF By, the antiparallel merging model predicts that magnetospheric magnetic field lines in the vicinity of local noon will tilt in the direction of the IMF, where as the subsolar model does not. The correlation coefficients between the geosynchronous data and the IMF By peak at both noon (0.61) and midnight (0.50), favoring the antiparallel model. These results were obtained with 1789 (noon) and 1312 (midnight) data points. The slopes of the regression lines indicate 29% IMF By entry at noon and 79% at midnight. The local‐time distribution of the slopes varies smoothly from midnight peak to a broad minimum near noon.

Solar wind dynamic pressure variations and transient magnetospheric signatures

Contrary to the prevailing popular view, we find that some transient ground events with bipolar north‐south signatures are related to variations in solar wind dynamic pressure and not necessarily to magnetic merging. We present simultaneous solar wind plasma observations for two previously reported transient ground events observed at dayside auroral latitudes. During the first event, originally reported by Lanzerotti et al. [1987], conjugate ground magnetometers recorded north‐south magnetic field deflections accompanied by strong deflections in the east‐west and vertical directions. The second event was reported by Todd et al. [1986], who noted ground radar observations indicating strong northward then southward ionospheric flows. The events were associated with the postulated signatures of patchy, sporadic, merging of magnetosheath and magnetospheric magnetic field lines at the dayside magnetopause, known as flux transfer events. Conversely, we demonstrate that the event reported by Lanzerotti et al. was accompanied by a sharp increase in solar wind dynamic pressure, a magnetospheric compression, and a consequent ringing of the magnetospheric magnetic field. The event reported by Todd et al. was associated with a brief but sharp increase in the solar wind dynamic pressure.

Energetic magnetospheric ions at the dayside magnetopause: Leakage or merging?

We elaborate upon the leakage model for the escape of energetic magnetospheric particles into the magnetosheath. Unlike the merging model, no interconnection (or merging) of magnetospheric and magnetosheath magnetic field lines is required. Because outer magnetospheric energetic particle drift paths intersect the magnetopause, the leakage model requires the continual escape of ions at postnoon local times and electrons at prenoon local times, regardless of solar wind conditions. It also predicts a division between dawnward and duskward streaming ions at the point where most magnetosheath magnetic field lines make their closest approach to the magnetopause, typically near 1500 LT. Like the merging model, the leakage model predicts equatorward streaming just inside the magnetopause. We study the motion of an escaping energetic ion at a planar magnetopause to show that, without scattering, ions must move dawnward and northward in a duskward magnetosheath magnetic field and dawnward and southward in a dawnward magnetosheath magnetic field. Scattering permits some ions to move duskward. We present new observations of streaming ions outside the dayside magnetopause made by the Charge Composition Explorer satellite, a part of the Active Magnetospheric Particle Tracer Explorers program. To place these observations in context, we have performed a statistical study of previous particle observations both inside and outside the dayside magnetopause. The ensemble of observations indicates that energetic magnetospheric ions of all species continually escape from the dayside magnetosphere and stream along magnetosheath magnetic field lines, even when no merging is expected. The magnetosheath magnetic field controls the direction in which the ions stream: they move away from the magnetosphere. The results of this work indicate that energetic particle observations at the dayside magnetopause need not be taken as evidence for merging of magnetosheath and magnetospheric magnetic field lines.

Field line topology determined by energetic particles during a possible magnetopause reconnection event

The September 8, 1978, crossing of the magnetopause by ISEE 1 and 2 has been studied in the light of energetic ion and electron data in order to derive the magnetic field topology. The plasma and magnetic field observations during this crossing provided evidence for the occurrence of plasma acceleration intrinsic to reconnection between interplanetary and magnetospheric magnetic field lines. The energetic particle observations result in two different pictures of the open and closed field lines within the magnetosphere, depending on either the ion or electron observations. According to the ion measurements, the high‐speed flow can be placed exclusively on open field lines. The electrons, however, exhibit trapped distributions during these times. This apparent dilemma can be resolved by invoking magnetic mirroring of the electrons off the increased magnetosheath field, as shown in a companion paper by Daly and Fritz (1982).

Magnetospheric substorms: Some problems and controversies

This description of the present status of work on magnetospheric substorms is based on the talks and discussions that occurred at the Rice University‐AGU Conference on Magnetospheric Substorms. Various types of new data were presented, including, for example, Isis 2 scanning photometer observations of the aurora from above and lunar shadowing observations made from the Apollo 15 subsatellite. The subject of greatest controversy at the conference was the substorm growth phase: gaps in the network of ground magnetometer stations can result in the expansion phase of one substorm being mistaken for the growth phase of another, and there was considerable debate over whether this observational ambiguity could invalidate popular ideas about the growth phase and its signatures. Correlations between magnetospheric phenomena and the direction of the interplanetary magnetic field constituted another area of considerable discussion: the picture appears complicated, with different magnetospheric phenomena correlating with different components of the interplanetary magnetic field. Two areas in which previous controversies have been closed for some time now are the question of whether the magnetosphere is open or closed and the general pattern of the average magnetospheric electric field: it now seems generally agreed that there is direct connection between interplanetary and magnetospheric magnetic field lines, although the details of the connection process remain uncertain, and electric‐field measurements from balloons and satellites have convincingly verified the existence of systematic magnetospheric convection, with dawn‐to‐dusk electric fields over the polar caps. A variety of theoretical approaches to the substorm problem are being pursued, with, as yet, no commonly accepted unifying concept.