H. K. Biernat

The effect of tidal locking on the magnetospheric and atmospheric evolution of "Hot Jupiters"

We study the interaction between the planetary magnetosphere and atmosphere of the close-in extrasolar planets HD 209458b and OGLE-TR-56b with the stellar wind during the evolution of their host stars. Recent astrophysical observations of solar-like stars indicate that the radiation and particle environments of young stars are orders of magnitudes larger than for stars with ages comparable to the sun (∼4.6 Gyr). We model the interaction for the present and for early evolutionary stages, showing that it is possible that Hot Jupiters have an ionosphere-stellar wind interaction like Venus, Our study suggests that the internal magnetic field of exoplanets orbiting close to their host stars may be very weak due to tidal locking. The magnetic moments can be less than one tenth of the value presently observed for the rapidly rotating planet Jupiter. We find that the stronger stellar wind of younger solar-type stars compresses the magnetosphere to a standoff distance at which the ionized part of the upper atmosphere, hydrodynamically expanded by the XUV-flux, builds an obstacle. Because of a much larger stellar wind particle flux during the first ∼0.5 Gyr after the host stars arrived on the Zero-Age-Main-Sequence, Hot Jupiters may have not been protected by their intrinsic magnetic fields, even if one neglects the effect of tidal locking. In such a case, the unshielded upper atmosphere will be affected by different ionization and non-thermal ion loss processes. This contributes to the estimated neutral hydrogen loss rates of about ≥10 10 g/s of the observed expanded exosphere of HD 209458b (Vidal-Madjar et al. 2003) and will be an ionized part of the estimated upper energy-limited neutral hydrogen loss rates of about 10 12 g/s (Lammer et al. 2003a).

Roche lobe effects on the atmospheric loss from "Hot Jupiters"

Context. A study of the mass loss enhancement for very close “Hot Jupiters” due to the gravitational field of the host star is presented. Aims. The influence of the proximity to a planet of the Roche lobe boundary on the critical temperature for blow-off conditions for estimating the increase of the mass loss rate through hydrodynamic blow-off for close-in exoplanets is investigated. Methods. We consider the gravitational potential for a star and a planet along the line that joins their mass centers and the energy balance equation for an evaporating planetary atmosphere including the effect of the stellar tidal force on atmospheric escape. Results. By studying the effect of the Roche lobe on the atmospheric loss from short-periodic gas giants we derived reasonably accurate approximate formulas to estimate atmospheric loss enhancement due to the action of tidal forces on a “Hot Jupiter” and to calculate the critical temperature for the onset of “geometrical blow-off”, which are valid for any physical values of the Roche lobe radial distance. Using these formulas, we found that the stellar tidal forces can enhance the hydrodynamic evaporation rate from TreS-1 and OGLE-TR-56b by about 2 fold, while for HD 209458b we found an enhancement of about 50%. For similar exoplanets which are closer to their host star than OGLE-TR-56b, the mass loss enhancement can be even larger. Moreover, we showed that the effect of the Roche lobe allows “Hot Jupiters” to reach blow-off conditions at temperatures which are less than expected due to the stellar X-ray and EUV heating.

Determining the mass loss limit for close-in exoplanets: what can we learn from transit observations?

Aims. We study the possible atmospheric mass loss from 57 known transiting exoplanets around F, G, K, and M-type stars over evolutionary timescales. For stellar wind induced mass loss studies, we estimate the position of the pressure balance boundary between Coronal Mass Ejection (CME) and stellar wind ram pressures and the planetary ionosphere pressure for non- or weakly magnetized gas giants at close orbits. Methods. The thermal mass loss of atomic hydrogen is calculated by a mass loss equation where we consider a realistic heating efficiency, a radius-scaling law and a mass loss enhancement factor due to stellar tidal forces. The model takes into account the temporal evolution of the stellar EUV flux by applying power laws for F, G, K, and M-type stars. The planetary ionopause obstacle, which is an important factor for ion pick-up escape from non- or weakly magnetized gas giants is estimated by applying empirical power-laws. Results. By assuming a realistic heating efficiency of about 10–25% we found that WASP-12b may have lost about 6–12% of its mass during its lifetime. A few transiting low density gas giants at similar orbital location, like WASP-13b, WASP-15b, CoRoT-1b or CoRoT-5b may have lost up to 1–4% of their initial mass. All other transiting exoplanets in our sample experience negligible thermal loss (≤1%) during their lifetime. We found that the ionospheric pressure can balance the impinging dense stellar wind and average CME plasma flows at distances which are above the visual radius of “Hot Jupiters”, resulting in mass losses <2% over evolutionary timescales. The ram pressure of fast CMEs cannot be balanced by the ionospheric plasma pressure for orbital distances between 0.02–0.1 AU. Therefore, collisions of fast CMEs with hot gas giants should result in large atmospheric losses which may influence the mass evolution of gas giants with masses <MJup. Depending on the stellar luminosity spectral type, planetary density, heating efficiency, orbital distance, and the related Roche lobe effect, we expect that at distances between 0.015–0.02 AU, Jupiter-class and sub-Jupiter-class exoplanets can lose several percent of their initial mass. At orbital distances ≤0.015 AU, low density hot gas giants in orbits around solar type stars may even evaporate down to their coresize, while low density Neptune-class objects can lose their hydrogen envelopes at orbital distances ≤0.02 AU.

STEREO and Wind observations of a fast ICME flank triggering a prolonged geomagnetic storm on 5-7 April 2010

On 5 April 2010 an interplanetary (IP) shock was detected by the Wind spacecraft ahead of Earth, followed by a fast (average speed 650 km/s) IP coronal mass ejection (ICME). During the subsequent moderate geomagnetic storm (minimum Dst = -72 nT, maximum Kp=8-), communication with the Galaxy 15 satellite was lost. We link images from STEREO/SECCHI to the near-Earth in situ observations and show that the ICME did not decelerate much between Sun and Earth. The ICME flank was responsible for a long storm growth phase. This type of glancing collision was for the first time directly observed with the STEREO Heliospheric Imagers. The magnetic cloud (MC) inside the ICME cannot be modeled with approaches assuming an invariant direction. These observations confirm the hypotheses that parts of ICMEs classified as (1) long-duration MCs or (2) magnetic-cloud-like (MCL) structures can be a consequence of a spacecraft trajectory through the ICME flank.

Anomalous magnetosheath properties during Earth passage of an interplanetary magnetic cloud

The aim of this paper is to model for the first time the variation of field and flow parameters in the magnetosheath during Earth passage of an interplanetary magnetic cloud. Under typical Solar wind conditions, magnetohydrodynamic (MHD) effects on the flow of plasma in the terrestrial magnetosheath are important only in a layer adjacent to the magnetopause which is a few thousand kilometers thick (“depletion layer” or “magnetic barrier”). During the passage of an interplanetary magnetic cloud, however, conditions upstream of the bow shock depart strongly from the norm. In this case, interplanetary parameters vary slowly over a wide range of values. Values of the upstream Alfven Mach number are much lower than those otherwise sampled (∼3 versus 8–10). Together with the magnetic shear across the magnetopause, this parameter plays a central role in determining the structure of the magnetosheath close to the magnetopause. As a consequence of sustained low values of the upstream Alfven Mach number, the magnetic field exerts a strong influence on the flow over a very substantial fraction of the magnetosheath throughout the duration of cloud passage, i.e., for a time period of the order of 1–2 days. We apply an algorithm to integrate the ideal MHD equations, using a boundary layer technique, and compute the variations of field and flow parameters along the stagnation streamline. We choose as our example the magnetic cloud which passed Earth on January 14–15, 1988. The interaction of this cloud with the magnetosphere, as regards the resulting ionospheric flow patterns and the substorm activity, has been the subject of various investigations. Using information from these studies, we obtain results on the magnetosheath when the magnetopause is modeled, first as a tangential discontinuity and then as a rotational discontinuity. Our results are in good general agreement with recent observations on the behavior of field and flow quantities in the magnetosheath region adjacent to the magnetopause. In addition, we predict the existence of a magnetic barrier when the upstream Alfven Mach number is low, irrespective of the magnetic shear across the magnetopause.

Little or no solar wind enters Venus' atmosphere at solar minimum.

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere2,3. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

A reconnection layer associated with a magnetic cloud

Abstract We examine a 3-hour long interval on December 24, 1996, containing a magnetic hole associated with an interplanetary magnetic cloud. Two sets of perturbations are observed by the Wind spacecraft at 1 AU. In the first, the field and flow rotate at constant field strength, and the plasma is accelerated to the local Alfven speed. We show this to be a rotational discontinuity. In the second, observed 25 min later, the plasma is heated and the field decreases. We show this to be a slow shock. The whole structure is in pressure balance. We interpret the observations as MHD discontinuities arriving with varying delays from a reconnection site closer to the Sun. Energetic particle observations suggest further that ejecta material is present for many hours prior to the magnetic cloud observation and separated from it by the layer. This suggests that reconnection took place between field lines of a CME of which the magnetic cloud formed a part.

LINKING REMOTE IMAGERY OF A CORONAL MASS EJECTION TO ITS IN SITU SIGNATURES AT 1 AU

In a case study (2008 June 6-7) we report on how the internal structure of a coronal mass ejection (CME) at 1 AU can be anticipated from remote observations of white-light images of the heliosphere. Favorable circumstances are the absence of fast equatorial solar wind streams and a low CME velocity which allow us to relate the imaging and in situ data in a straightforward way. The STEREO-B spacecraft encountered typical signatures of a magnetic flux rope inside an interplanetary CME (ICME) whose axis was inclined at 45° to the solar equatorial plane. Various CME direction-finding techniques yield consistent results to within 15°. Further, remote images from STEREO-A show that (1) the CME is unambiguously connected to the ICME and can be tracked all the way to 1 AU, (2) the particular arc-like morphology of the CME points to an inclined axis, and (3) the three-part structure of the CME may be plausibly related to the in situ data. This is a first step in predicting both the direction of travel and the internal structure of CMEs from complete remote observations between the Sun and 1 AU, which is one of the main requirements for forecasting the geo-effectiveness of CMEs.

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

[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.

A comparison and review of steady-state and time-varying reconnection

Abstract Reconnection is a ubiquitous energy conversion process operating in current sheets. It appears in a variety of applications and is, for example, the dominant coupling process at the Earths magnetopause, the current sheet which separates the solar wind and the terrestrial magnetosphere. Reconnection at the magnetopause is investigated theoretically and experimentally, and in both areas a division exists between so-called steady-state and time-dependent reconnection. In theoretical research the former is associated with the time-invariant analysis of Petschek and coworkers, and this is often put into contrast with an intrinsically time-varying process such as tearing. In experimental research, manifestations of reconnection have been classified either as large scale and (quasi) steady-state, or time dependent, with the former corresponding to accelerated plasma flows along the magnetopause, and the latter to the flux transfer event signature. This division is a source of confusion, in particular since it is unlikely that a true steady-state is ever achieved in nature. To clarify the relationship between steady-state and time-dependent reconnection we discuss here an extension of Petscheks analysis to include time variations in the reconnection rate. In this generalized analysis the reconnection electric field is imposed as an initial-boundary condition which can be specified as an arbitrary function of space and time. Different types of reconnection behaviour can therefore be investigated and we take advantage of this to compare steady-state and time-varying reconnection. We show that the former is just a special case of the latter and that there are no jumps in conceptual understanding required from one to the other. Furthermore, the time-dependent analysis is easily understood and gives a framework which unifies the interpretation of reconnection phenomena observed at the magnetopause. In particular, the theoretical results indicate that the same reconnection rate can give rise to both accelerated plasma flows and the flux transfer event signature; thus there is no physical reason to make a distinction in the underlying process giving rise to different reconnection phenomena.