Atmospheric erosion of CO2 -rich Earth-size exoplanets due to coronal mass ejection (CME)-induced ion pick up within close-in habitable zones of active M-type dwarf stars is investigated. Since M s...

Coronal mass ejection (CME) activity of low mass M stars as an important factor for the habitability of terrestrial exoplanets. II. CME-induced ion pick up of Earth-like exoplanets in close-in habitable zones.

Sharp fronts observed by the Chandra satellite between dense cool cluster cores moving with near-sonic velocity through the hotter intergalactic gas, require strong suppression of thermal conductivity across the boundary. This may be due to magnetic fields tangential to the contact surface separating the two plasma components. We point out that a super-Alfvenic motion of a plasma cloud (a core of a merging galaxy) through a weakly magnetized intercluster medium leads to 'magnetic draping', formation of a thin, strongly magnetized boundary layer with a tangential magnetic field. For supersonic cloud motion, M S ≥ 1, magnetic field inside the layer reaches near-equipartition values with thermal pressure. Typical thickness of the layer is ∼L/M 2 A « L, where L is the size of the obstacle (plasma cloud) moving with Alfven Mach number M A >> 1. To a various degree, magnetic draping occurs for both subsonic and supersonic flows, random and ordered magnetic fields and it does not require plasma compressibility. The strongly magnetized layer will thermally isolate the two media and may contribute to the Kelvin-Helmholtz stability of the interface. Similar effects occur for radio bubbles, quasi-spherical expanding cavities blown up by active galactic nucleus jets; in this case, the thickness of the external magnetized layer is smaller, ∼L/M 3 A << L.

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Magnetic draping of merging cores and radio bubbles in clusters of galaxies

[1] We examine the implications of the widely used, force-free, constant-a flux rope model of interplanetary magnetic clouds for the evolution of these mesoscale (fraction 1 AU) structures in the heliosphere, with special emphasis on the inner (≤1 AU) heliosphere. We employ primarily events observed by the Helios 1 and 2 probes between 0.3 and 1 AU in the ascending and maximum phases of solar cycle 21 and by Wind at 1 AU in a similar phase of solar activity cycle. We supplement these data by observations from other spacecraft (e.g., Voyagers 1 and 2, Pioneers 10 and 11, and others). Our data set consists of 130 events. We explore three different approaches. In the first, we work with ensemble averages, binning the results into radial segments of width 0.1 AU in the range 0.3 < r h ≤ 1 AU. Doing this, we find that in the inner heliosphere the modeled average central axial field strength, (B 0 ), varies with heliospheric distance r h as (B 0 ) [nT] = 18.1 ·rh -1.64 [AU], and the average diameter increases quasi-linearly as (D) [AU] = 0.23 r h 1.14 . The orientation of the axis of the underlying magnetic flux tube in our data set is generally found to lie along the east-west direction and in the ecliptic plane at all values of r h , but there is considerable scatter about these average directions. In the second, we monitor the evolution of magnetic clouds in snapshot fashion, using seven spacecraft alignments. The results are in broad agreement with the statistics reported under step 1. In the final approach, we obtain the functional dependence of By and D predicted by an analytic expression for a freely expanding Lundquist flux tube. We find D to vary linearly with r h , broadly similar to that obtained under approach 1. The maximum field strength scales as r h -2 compared to a r h -1.3 dependence obtained from statistics. We compare our findings with those of Bothmer and Schwenn (1998), who used a different methodology. The results obtained form a good background to the forthcoming Solar Terrestrial Relations Observatory (STEREO) and Sentinels missions and to multispacecraft studies of magnetic clouds.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2006JA011940

Consequences of the force-free model of magnetic clouds for their heliospheric evolution

In this review, we discuss first magnetic clouds in the context of other interplanetary causes of geomagnetic storms. We then describe work on the global field line topology of magnetic clouds, focussing on information gained by the use of energetic particles. We then give a summary of theoretical and simulation work on the dynamics of magnetic clouds. In one approach, based on self-similar evolution of radially expanding magnetic flux ropes, the role of electrons is central. A section on the boundaries of magnetic clouds is followed by one on magnetic field line draping around these ejecta, including the formation of a magnetic barrier. In the aspect of the study dealing with the geomagnetic response to magnetic clouds, we discuss effects on the dayside magnetosheath; ionosphere; and nightside magnetosphere at geostationary orbit and beyond, utilizing primarily observations made during Earth's encounter with a magnetic cloud on January, 13 - 14, 1988. A case study is mentioned where solar energetic particles, injected into a magnetic cloud and then guided along its helical field lines, entered the magnetosphere through interconnection of the cloud's field lines with those of Earth. Simulation work on the geomagnetic response to magnetic clouds is briefly reviewed. We finally consider studies specifically correlating magnetic clouds, in isolation or as part of compound streams, with geomagnetic storm activity. Throughout, we indicate areas where further work is needed.

Magnetic clouds and the quiet-storm effect at Earth

We use observations from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, in orbit around Mercury, to investigate interplanetary coronal mass ejections (ICMEs) near 0.3 AU. MESSENGER is the first spacecraft since Helios 1 and 2 in the 1980s to make in situ measurements of the interplanetary medium at heliocentric distances < 0.5 AU. As such, it presents a unique opportunity for observing the innermost heliosphere. It also allows for observations of ICMEs well within 1 AU to study their evolution as they expand and propagate outward, interacting with the solar wind. We catalog ICME events observed by the MESSENGER Magnetometer between 2011 and 2014 and present statistical analyses of ICME properties at Mercury. In addition, using existing data sets of ICMEs at 1 AU, we investigate key ICME property changes from Mercury to 1 AU. We find good agreement with previous studies for the magnetic field strength dependence on distance, and we also find evidence that ICME deceleration continues past the orbit of Mercury. This paper describes the database of ICMEs from MESSENGER orbital observations around Mercury, which is publicly available through the supporting information (Table S1) associated with this manuscript and the Virtual Energetic Particle Observatory. Our ICME database will prove particularly useful for multipoint spacecraft studies of recent ICMEs, as well as for model validation of ICME properties.

Interplanetary coronal mass ejections from MESSENGER orbital observations at Mercury

We present an ideal MHD theory to describe for the first time the “magnetic barrier” (or “depletion layer”) of that class of interplanetary ejecta called magnetic clouds. By “magnetic barrier” we mean that region of the sheath where the magnetic pressure is comparable to, or larger than, the gas pressure and where, therefore, the effects of the magnetic field on the flow are substantial. We model magnetic clouds as cylindrical flux ropes. We consider three cases: one steady state and two nonsteady situations. The two nonsteady situations correspond to (1) a self-similarly expanding magnetic cloud, and (2) to a nonexpanding magnetic cloud which has a net bulk motion with respect to the medium at infinity. In all cases the cloud drives an interplanetary shock ahead of it. We describe an algorithm to integrate the MHD equations in which the behavior of the sum of the magnetic and plasma pressure is prescribed. We assume here that the sum of the magnetic and plasma pressure is constant along any line normal to the magnetic cloud boundary. We find that in steady state the cloud boundary cannot be a tangential discontinuity, that is, a finite magnetic barrier thickness can only be obtained with a reconnecting cloud boundary. In general, the magnetic barriers of magnetic clouds are thick, that is, they are a substantial fraction of the cloud's sheath. In steady state and the nonsteady case (situation 2, above), their width depends inversely on the Alfven Mach number. The non-steady state (situation 1) has similarities with the problem of solar wind flow around the terrestrial magnetosphere. In particular, the barrier thickness in this case is proportional to the inverse square of the Alfven Mach number. This work should be useful in the interpretation of data from the sheath region ahead of magnetic clouds driving interplanetary shocks.

Ideal MHD flow behind interplanetary shocks driven by magnetic clouds

Magnetosheath parameters and reconnection: a case study for the near-cusp region and the equatorial flank

The terrestrial magnetopause region shows a big variety of different structures and properties depending on different situations, mainly in the free solar wind. In a recent paper, Song et al. (1993) reported a multi-instrument study on magnetopause parameters on November 1, 1978, 1510–1530 UT. We complement this descriptive study by examining this event in terms of a reconnection model. For doing this we use a shock-type reconnection model which allows for magnetic fields of arbitrary strength and skew and for arbitrary plasma flows on either side of the reconnection layer as well as for different densities. Thus we are able to model the magnetopause structure within a big parameter regime. We show that the entire sequence of regions and discontinuities detected on November 1, 1978, 1510–1530 UT, and reported by Song et al. (1993) agrees with the magnetopause structure predicted by our reconnection model. However, we point out that some features of the data need additional assumptions, which make the problem open for future study.

Structure of the subsolar magnetopause on November 1, 1978, 1510–1530 UT: An effect of reconnection?

We study the process of magnetic field line reconnection at the Earth’s dayside magnetopause from the theoretical point of view and compare model predictions with observations made by the ISEE 2 spacecraft on its outbound magnetopause crossing on September 11, 1979. During the process of magnetic field line reconnection all types of magnetohydrodynamic waves can be excited. An analytical treatment of this process is performed in a generalized Petschek–type model. In this approach all wave structures are considered to be steepened and/or sufficiently thin so that they can be described as discontinuities. A direct comparison is made between the predicted structures and spacecraft data, and we show the influence of heavy ions on the mass density, and correspondingly the structure of the reconnection layer.

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Gerhard A. Bachmaier

Papers

Ideal MHD flow behind interplanetary shocks driven by magnetic clouds

Magnetosheath parameters and reconnection: a case study for the near-cusp region and the equatorial flank

Structure of the subsolar magnetopause on November 1, 1978, 1510–1530 UT: An effect of reconnection?

Petschek-type reconnection at the magnetopause: A comparison of theory with data from ISEE 2 on September 11, 1979