P. Kajdič

Analysis of waves surrounding foreshock cavitons

Cavitons have been recently discovered in the Earth’s foreshock. Foreshock cavitons have first been predicted by global hybrid simulations and later structures with similar properties were discovered in observational data from the Cluster mission. Cavitons are observed as simultaneous depressions of the magnitude of the magnetic field and plasma density (δB/B, δn/n⩾0.2) that commonly last ⩽30 s and are surrounded by a rim of enhanced values of B and n. They are always immersed in a sea of compressive low frequency waves. Numerical simulations results suggest that the cavitons are produced by the interaction of foreshock sinusoidal plasma waves with perpendicularly propagating compressive fast mode waves. Here we present a study of the waves that were observed in the immediate neighborhood or inside two foreshock cavitons detected by the Cluster I spacecraft. We analyze the frequency, polarization, direction of propagation and compressibility of the waves.

Interplanetary coronal mass ejection observed at STEREO-A, Mars, comet 67P/Churyumov-Gerasimenko, Saturn, and New Horizons en route to Pluto: Comparison of its Forbush decreases at 1.4, 3.1, and 9.9 AU

We discuss observations of the journey throughout the Solar System of a large interplanetary coronal mass ejection (ICME) that was ejected at the Sun on 14 October 2014. The ICME hit Mars on 17 October, as observed by the Mars Express, MAVEN, Mars Odyssey and MSL missions, 44 hours before the encounter of the planet with the Siding-Spring comet, for which the space weather context is provided. It reached comet 67P/Churyumov-Gerasimenko, which was perfectly aligned with the Sun and Mars at 3.1 AU, as observed by Rosetta on 22 October. The ICME was also detected by STEREO-A on 16 October at 1 AU, and by Cassini in the solar wind around Saturn on the 12 November at 9.9 AU. Fortuitously, the New Horizons spacecraft was also aligned with the direction of the ICME at 31.6 AU. We investigate whether this ICME has a non-ambiguous signature at New Horizons. A potential detection of this ICME by Voyager-2 at 110-111 AU is also discussed. The multi-spacecraft observations allow the derivation of certain properties of the ICME, such as its large angular extension of at least 116°, its speed as a function of distance, and its magnetic field structure at four locations from 1 to 10 AU. Observations of the speed data allow two different solar wind propagation models to be validated. Finally, we compare the Forbush decreases (transient decreases followed by gradual recoveries in the galactic cosmic ray intensity) due to the passage of this ICME at Mars, comet 67P and Saturn.

Interplanetary shocks and foreshocks observed by STEREO during 2007–2010

Interplanetary shocks in the heliosphere modify the solar wind through which they pass. In particular, shocks play an important role in particle acceleration. During the extended solar minimum (2007–2010) STEREO observed 65 forward shocks driven by stream interactions (SI), with magnetosonic Mach numbers Mms ≈ 1.1–4.0 and shock normal angles θBN ~ 20–87°. We analyze the waves associated with these shocks and find that the region upstream can be permeated by whistler waves (f ~ 1 Hz) and/or ultra low frequency (ULF) waves (f ~ 10−2–10−1 Hz). While whistlers appear to be generated at the shock, the origin of ULF waves is most probably associated with local kinetic ion instabilities. We find that when the Mach number (Mms) is low and the shock is quasi-perpendicular ( θBN > 45°) whistler waves remain close to the shock. As Mms increases, the shock profile changes and can develop a foot and overshoot associated with ion reflection and gyration. Whistler precursors can be superposed on the foot region, so that some quasi-perpendicular shocks have characteristics of both subcritical and supercritical shocks. When the shock is quasi-parallel ( θBN < 45°) a large foreshock with suprathermal ions and waves can form. Upstream, there are whistler trains at higher frequencies whose characteristics can be slightly modified probably by reflected and/or leaked ions and by almost circularly polarized waves at lower frequencies that may be locally generated by ion instabilities. In contrast with planetary bow shocks, most of the upstream waves studied here are mainly transverse and no steepening occurs. Some quasi-perpendicular shocks (45° <  θBN < 60°) are preceded by ULF waves and ion foreshocks. Fluctuations downstream of quasi-parallel shocks tend to have larger amplitudes than waves in the sheath of quasi-perpendicular shocks. We compare SI-driven shock properties with those of shocks generated by interplanetary coronal mass ejections (ICMEs). During the same years, STEREO observed 20 ICME-driven shocks with Mms ≈ 1.2–4.0 and θBN ~ 38–85°. We find that shocks driven by ICMEs tend to have larger proton foreshocks (dr ~ 0.1 AU) than shocks driven by stream interactions (dr ≤ 0.05 AU). This difference of ion foreshock size should be linked to shock age: ICME-driven shocks form at shorter distances to the Sun and therefore can energize particles for longer times as they propagate to 1 AU, while stream interaction shocks form closer to Earths orbit and have been accelerating ions for a shorter interval of time.

Space weather effects on the bow shock, the magnetic barrier, and the ion composition boundary at Venus

We present a statistical study on the interaction between interplanetary coronal mass ejections (ICMEs) and the induced magnetosphere of Venus when the peak magnetic field of the magnetic barrier was anomalously large (>65 nT). Based on the entire available Venus Express data set from April 2006 to October 2014, we selected 42 events and analyzed the solar wind parameters, the position of the bow shock, the size and plasma properties of the magnetic barrier, and the position of the ion composition boundary (ICB). It was found that the investigated ICMEs can be characterized with interplanetary shocks and unusually large tangential magnetic fields with respect to the Venus-Sun line. In most of the cases the position of the bow shock was not affected by the ICME. In a few cases the interaction between magnetic clouds and the induced magnetosphere of Venus was observed. During these events the small magnetosonic Mach numbers inside magnetic clouds caused the bow shock to appear at anomalously large distances from the planet. The positions of the upper and lower boundaries of the magnetic barrier were not affected by the ICMEs. The position of the ICB on the nightside was found closer to the planet during ICME passages which is attributed to the increased solar wind dynamic pressure.

STIS optical spectroscopy of the lobes of CRL 618

Context. Many proto-planetary nebulae show collimated structures sometimes showing multiple outflows. Aims. We present the results of new optical spectroscopic observations (both medium (with a dispersion of 0.56 A pixel −1 )a nd low (2.73 and 4.92 A pixel −1 for the G430L and G750L gratings) spectral resolution) of the lobes of the proto-planetary nebula CRL 618 obtained with the Space Telescope Imaging Spectrograph on board of the Hubble Space Telescope. Methods. We analyse the density structure and the excitation conditions of the lobes of CRL 618. The spectra obtained at medium spectral resolution (∼50 km s −1 ) allow us to quote the fraction of unscattered (intrinsic) Hα emission. We have also obtained dereddened emission line ratios of several features from the low spectral resolution spectra. Results. We find that most of the analysed line ratios are reproduced by planar shocks moving through a dense medium (with preshock densities ∼10 4 cm −3 ) with shock velocities from 30 to 40 km s −1 (except the [O III]/Hβ line ratios which require shock velocities of 80 to 90 km s −1 ). We find that the [S II]-weighted ionization fraction ranges from 0.015 to 0.06. The total densities derived from the electron density and the ionization fraction are ∼10 5 to 10 6 cm −3 . Conclusions. We conclude that the spectra of the lobes of CRL 618 can be margially reproduced by steady plane–parallel shock models for shock velocities which are significantly lower than the velocities at which the jet moves outwards (∼200 km s −1 ). These results are consistent with the predictions of a jet with a variable ejection velocity. The mirror-symmetry, the luminosity asymmetry between both lobes and the ejection velocity variability suggest that its central source may host a binary system.

Low‐frequency waves within isolated magnetic clouds and complex structures: STEREO observations

Complex Structures (CSs) formed by the interaction of magnetic cloud (MC)-like structures with other transients (e.g., another MC, a stream interaction region, or a fast stream of solar wind) were frequently observed in the interplanetary space by STEREO spacecraft during the solar minimum 23 and the rising phase of the solar cycle 24. Here we report the presence of low-frequency waves (LFWs) inside some isolated MCs (IMCs) and inside the CSs observed by STEREO during such period (2007–2011). It is important to study in detail the properties of waves in space plasmas since particle distribution functions can be modified by wave-particle interactions. We compare wave characteristics within IMCs with those waves observed inside CSs. Both left-handed (LH) and right-handed (RH), near-circularly polarized, transverse and almost parallel-propagating LFWs (around the proton cyclotron frequency) were sporadically observed inside both IMCs and CSs. In contrast, compressive mirror-mode waves (MMs) were observed only within CSs. We studied local plasma conditions inside the IMCs and CSs to gain insight about wave origin: most of the MMs within CSs were observed in regions with enhanced plasma beta (β>1); the majority of the LH waves were found in low beta plasmas (β<1), and the RH waves were predominantly observed at moderate betas (0.4<β≤2). These observations are in agreement with linear kinetic theory predictions for the growth of the mirror, the LH ion cyclotron, and the RH ion firehose instability, respectively. It is possible that the waves were generated locally inside the IMCs and CSs via temperature anisotropies. The plasma beta enhancements that were frequently observed inside the CSs may be the result of compressions and heating taking place inside the interacting structures.

Mars plasma system response to solar wind disturbances during solar minimum

This paper is a phenomenological description of the ionospheric plasma and induced magnetospheric boundary (IMB) response to two different types of upstream solar wind events impacting Mars in March 2008, at the solar minimum. A total of 16 Mars Express orbits corresponding to five consecutive days is evaluated. Solar TErrestrial RElations Observatory-B (STEREO-B) at 1 AU and Mars Express and Mars Odyssey at 1.644 AU detected the arrival of a small transient interplanetary coronal mass ejection (ICME-like) on the 6 and 7 of March, respectively. This is the first time that this kind of small solar structure is reported at Marss distance. In both cases, it was followed by a large increase in solar wind velocity that lasted for ~10 days. This scenario is simulated with the Wang-Sheeley-Arge (WSA) - ENLIL + Cone solar solar wind model. At Mars, the ICME-like event caused a strong compression of the magnetosheath and ionosphere, and the recovery lasted for ~3 orbits (~20 h). After that, the fast stream affected the upper ionosphere and the IMB, which radial and tangential motions in regions close to the subsolar point are analyzed. Moreover, a compression in the Martian plasma system is also observed, although weaker than after the ICME-like impact, and several magnetosheath plasma blobs in the upper ionosphere are detected by Mars Express. We conclude that, during solar minimum and at aphelion, small solar wind structures can create larger perturbations than previously expected in the Martian system.

SUPRATHERMAL ELECTRON STRAHL WIDTHS IN THE PRESENCE OF NARROW-BAND WHISTLER WAVES IN THE SOLAR WIND

We perform the first statistical study of the effects of the interaction of suprathermal electrons with narrow-band whistler mode waves in the solar wind. We show that this interaction does occur and that it is associated with enhanced widths of the so called strahl component. The latter is directed along the inter- planetary magnetic field away from the Sun. We do the study by comparing the strahl pitch angle widths in the solar wind at 1AU in the absence of large scale discontinuities and transient structures, such as interplanetary shocks, interplanetary coronal mass ejections, stream interaction regions, etc. during times when the whistler mode waves were present and when they were absent. This is done by using the data from two Cluster instruments: STAFF data in frequency range between ~0.1 Hz and ~200 Hz were used for determining the wave properties and PEACE datasets at twelve central energies between ~57 eV (equivalent to ~10 typical electron thermal energies in the solar wind, E_T ) and ~676 eV (~113 E_T ) for pitch angle measurements. Statistical analysis shows that during the inter- vals with the whistler waves the strahl component on average exhibits pitch angle widths between 2 and 12 degrees larger than during the intervals when these waves are not present. The largest difference is obtained for the electron central energy of ~344 eV (~57 E_T ).

Variability of the Magnetic Field Power Spectrum in the Solar Wind at Electron Scales

At the electron scales the power spectrum of solar-wind magnetic fluctuations can be highly variable and the dissipation mechanisms of the magnetic energy into the various particle species is under debate. In this paper we investigate data from the Cluster missions STAFF Search Coil magnetometer when the level of turbulence is sufficiently high that the morphology of the power spectrum at electron scales can be investigated. The Cluster spacecraft sample a disturbed interval of plasma where two streams of solar wind interact. Meanwhile, several discontinuities (coherent structures) are seen in the large scale magnetic field, while at small scales several intermittent bursts of wave activity (whistler waves) are present. Several different morphologies of the power spectrum can be identified: (1) two power laws separated by a break (2) an exponential cutoff near the Taylor shifted electron scales and (3) strong spectral knees at the Taylor shifted electron scales. These different morphologies are investigated by using wavelet coherence, showing that in this interval a clear break and strong spectral knees are features which are associated with sporadic quasi parallel propagating whistler waves, even for short times. On the other hand, when no signatures of whistler waves at 0.1 - 0.2fce are present, a clear break is difficult to find and the spectrum is often more characteristic of a power law with an exponential cutoff.

Adaptive grid simulations of ionized flows

This paper presents a discussion of our “yguazu‐a” code, which integrates systems of differential equations (generally, the gasdynamic or MHD equations together with and atomic/chemical network) on a hierarchical, binary adaptive grid. Examples of recent calculations are shown, and a general discussion of the capabilities of the code and the published results is presented.