News

New Publication: “Bulk Properties of Pickup Ions Derived from the Ulysses Solar Wind Ion Composition Spectrometer Data” by William P. Smith et al.

Nonthermal, pickup ions (PUIs) represent an energetic component of the solar wind (SW). While a number of theoretical models have been proposed to describe the PUI flow, of major importance are in situ measurements providing us with the vital source of model validation. The Solar Wind Ion Composition Spectrometer (SWICS) instrument on board the Ulysses spacecraft was specifically designed for this purpose. Zhang et al. proposed a new, accurate method for the derivation of ion velocity distribution function in the SW frame on the basis of count rates collected by SWICS. We calculate the moments of these distribution functions for protons (H+) and He+ ions along the Ulysses trajectory for a period of 2 months including the Halloween 2003 solar storm. This gives us the time distributions of PUI density and temperature. We compare these with the results obtained earlier for the same interval of time, in which the ion spectra are converted to the SW frame using the narrow-beam approximation. Substantial differences are identified, which are of importance for the interpretation of PUI distributions in the 3D, time-dependent heliosphere. We also choose one of the shocks crossed by Ulysses during this time interval and analyze the distribution functions and PUI bulk properties in front of and behind it. The results are compared with the test-particle calculations and diffusive shock acceleration theory.

Distributions of pickup ion density and temperature along the Ulysses trajectory (blue and green lines are used for for pickup protons and pickup He+, respectively).

Full Article:
Smith, W. P., Renfroe, K., Pogorelov, N. V., Zhang, M., Gedalin, M. (SHARP) and Kim, T. K. (2022). Bulk Properties of Pickup Ions Derived from the Ulysses Solar Wind Ion Composition Spectrometer Data. The Astrophysical Journal, 933, doi: 10.3847/1538-4357/ac73f2

License: CC BY 4.0

New Publication: “Evidence for γ-ray emission from the remnant of Kepler’s supernova based on deep H.E.S.S. observations” by H.E.S.S. Collaboration

Observations with imaging atmospheric Cherenkov telescopes (IACTs) have enhanced our knowledge of nearby supernova (SN) remnants with ages younger than 500 yr by establishing Cassiopeia A and the remnant of Tycho’s SN as very-high-energy (VHE) γ-ray sources. The remnant of Kepler’s SN, which is the product of the most recent naked-eye SN in our Galaxy, is comparable in age to the other two, but is significantly more distant. If the γ-ray luminosities of the remnants of Tycho’s and Kepler’s SNe are similar, then the latter is expected to be one of the faintest γ-ray sources within reach of the current generation IACT arrays. Here we report evidence at a statistical level of 4.6σ for a VHE signal from the remnant of Kepler’s SN based on deep observations by the High Energy Stereoscopic System (H.E.S.S.) with an exposure of 152 h. The measured integral flux above an energy of 226 GeV is ∼0.3% of the flux of the Crab Nebula. The spectral energy distribution (SED) reveals a γ-ray emitting component connecting the VHE emission observed with H.E.S.S. to the emission observed at GeV energies with Fermi-LAT. The overall SED is similar to that of the remnant of Tycho’s SN, possibly indicating the same nonthermal emission processes acting in both these young remnants of thermonuclear SNe.

H.E.S.S. and Fermi-LAT maps. Left panel: H.E.S.S. γ-ray significance map of Kepler’s SNR. Right panel: Fermi-LAT TS map in the range of 4.75−300 GeV

Full Article:
H. E. S. S. Collaboration, Aharonian, F., Ait Benkhali, F., Angüner, O., et al. (2022). Evidence for γ-ray emission from the remnant of Kepler’s supernova based on deep H.E.S.S. observations. Astronomy and Astrophysics, 662, doi: 10.1051/0004-6361/202243096

License: CC BY 4.0

New Publication: “Additional Evidence for a Pulsar Wind Nebula in the Heart of SN 1987A from Multiepoch X-Ray Data and MHD Modeling” by Emanuele Greco et al.

Since the day of its explosion, supernova (SN) 1987A has been closely monitored to study its evolution and to detect its central compact relic. In fact, the formation of a neutron star is strongly supported by the detection of neutrinos from the SN. However, besides the detection in the Atacama Large Millimeter/submillimeter Array (ALMA) data of a feature that is compatible with the emission arising from a protopulsar wind nebula (PWN), the only hint of the existence of such an elusive compact object is provided by the detection of hard emission in NuSTAR data up to ∼20 keV. We report on the simultaneous analysis of multiepoch observations of SN 1987A performed with Chandra, XMM-Newton, and NuSTAR. We also compare the observations with a state-of-the-art three-dimensional magnetohydrodynamic simulation of SN 1987A. A heavily absorbed power law, consistent with the emission from a PWN embedded in the heart of SN 1987A, is needed to properly describe the high-energy part of the observed spectra. The spectral parameters of the best-fit power law are in agreement with the previous estimate, and exclude diffusive shock acceleration as a possible mechanism responsible for the observed nonthermal emission. The information extracted from our analysis is used to infer the physical characteristics of the pulsar and the broadband emission from its nebula, in agreement with the ALMA data. Analysis of the synthetic spectra also shows that, in the near future, the main contribution to the Fe K emission line will originate in the outermost shocked ejecta of SN 1987A.

Comparison between Fe K observed emission lines and the various contributions estimated from the B18.3 model.

Full Article:
Greco, E. (SHARP), Miceli, M., Orlando, S., Olmi, B., Bocchino, F., Nagataki, S., Sun, L., Vink, J. (SHARP), et al. (2022). Additional Evidence for a Pulsar Wind Nebula in the Heart of SN 1987A from Multiepoch X-Ray Data and MHD Modeling. The Astrophysical Journal, 931, doi: 10.3847/1538-4357/ac679d

License: CC BY 4.0

SHARP at the inauguration of the New Ångström Laboratory at the University of Uppsala, Sweden

As part of the inauguration of the New Ångström Laboratory at the University of Uppsala, a multidisciplinary theme week in science and technology was organised. Andreas Johlander gave a presentation with the title ‘Shock waves in space’ for students, researchers and the general public on May 17th (Theme day: The Universe and Mathematical Physics).

Andreas Johlander from IRF.

Link to the presentation.

New Publication: “Whistler Waves in the Foot of Quasi-Perpendicular Supercritical Shocks” by Ahmad Lalti et al.

Whistler waves are thought to play an essential role in the dynamics of collisionless shocks. We use the magnetospheric multiscale spacecraft to study whistler waves around the lower hybrid frequency, upstream of 11 quasi-perpendicular supercritical shocks. We apply the 4-spacecraft timing method to unambiguously determine the wave vector k of whistler waves. We find that the waves are oblique to the background magnetic field with a wave-normal angle between 20° and 42°, and a wavelength of around 100 km, which is close to the ion inertial length. We also find that k is predominantly in the same plane as the magnetic field and the normal to the shock. By combining this precise knowledge of k with high-resolution measurements of the 3D ion velocity distribution, we show that a reflected ion beam is in resonance with the waves, opening up the possibility for wave-particle interaction between the reflected ions and the observed whistlers. The linear stability analysis of a system mimicking the observed distribution suggests that such a system can produce the observed waves.

2D Ion VDFs reduced in the urn:x-wiley:21699380:media:jgra57197:jgra57197-math-0031 plane (top row) and 1D velocity distribution functions (VDFs) reduced in the k direction (bottom row) for three different events with θBn = 82° (a, d), 68° (b, e), and 55° (c, f). The black line shows the wave phase speed and the pink-shaded area shows its 2σ interval. The times indicate the center of the 150 ms acquisition interval of the ions VDFs.

Full Article:
Lalti, A. (SHARP), Khotyaintsev, Y. V. (SHARP), Graham, D. B. (SHARP), Vaivads, A., Steinvall, K. and Russell, C. T. (SHARP) (2022). Whistler waves in the foot of quasi-perpendicular supercritical shocks. Journal of Geophysical Research: Space Physics, 127, doi: 10.1029/2021JA029969

License: CC BY 4.0

New Publication “Implications of weak rippling of the shock ramp on the pattern of the electromagnetic field and ion distributions” by Michael Gedalin et al.

Collisionless shocks undergo structural changes with the increase of Mach number. Observations and numerical simulations indicate development of time-dependent rippling. It is not known at present what causes the rippling. However, effects of such rippling on the field pattern and ion motion and distributions can be studied without precise knowledge of the causes and detailed shape. It is shown that deviations of the normal component of the magnetic field from the constant value indicate certain spatial dependence of the rippling. Deviations of the motional electric field from the constant value indicate time dependence. It is argued that whistler waves should propagate towards upstream and downstream regions from the rippled ramp. It is shown that the downstream pattern of the fields and ion distributions should follow the rippling pattern, while collisionless relaxation should be faster than in the stationary planar case.

Full article:
Gedalin, M. (SHARP) and Ganushkina, N. (SHARP) (2022). Implications of weak rippling of the shock ramp on the pattern of the electromagnetic field and ion distributions. Journal of Plasma Physics, 88(3), doi: 10.1017/S0022377822000356

License: CC BY 4.0

New Publication: “Analysis of multiscale structures at the quasi-perpendicular Venus bow shock – Results from Solar Orbiter’s first Venus flyby” by Andrew Dimmock et al.

This study aims to investigate the outbound Venus bow shock crossing measured by Solar Orbiter during the first flyby. We study the complex features of the bow shock traversal in which multiple large amplitude magnetic field and density structures were observed as well as higher frequency waves. Our aim is to understand the physical mechanisms responsible for these high amplitude structures, characterize the higher frequency waves, determine the source of the waves, and put these results into context with terrestrial bow shock observations.

The Venus bow shock at a moderately high Mach number (∼5) in the quasi-perpendicular regime exhibits complex features similar to the Earth’s bow shock at comparable Mach numbers. The study highlights the need to be able to distinguish between large amplitude waves and spatial structures such as shock rippling. The simultaneous high frequency observations also demonstrate the complex nature of energy dissipation at the shock and the important question of understanding cross-scale coupling in these complex regions. These observations will be important to interpreting future planetary missions and additional gravity assist maneuvers.

Shock substructure in density and magnetic field. Panels a and b: electron density determined from the spacecraft potential and the magnetic field modulus. A wavelet coherency spectrum is shown in panel c, which demonstrates that panels a and b share common variations around 0.5–1 Hz during the shock front, which are in phase.

Full Article:
Dimmock, A. P. (SHARP), Khotyaintsev, Yu. V. (SHARP), Lalti, A. (SHARP), Yordanova, E., Edberg, N. J. T., Steinvall, K., Graham, D. B. (SHARP), et al. (2022). Analysis of multiscale structures at the quasi-perpendicular Venus bow shock – Results from Solar Orbiter’s first Venus flyby. Astronomy and Astrophysics, 660, doi: 10.1051/0004-6361/202140954

License: CC BY 4.0

New Publication: “The Forward and Reverse Shock Dynamics of Cassiopeia A” by Jacco Vink et al.

We report on proper motion measurements of the forward- and reverse shock regions of the supernova remnant Cassiopeia A (Cas A), including deceleration/acceleration measurements of the forward shock. The measurements combine 19 yr of observations with the Chandra X-ray Observatory, using the 4.2–6 keV continuum band, preferentially targeting X-ray synchrotron radiation. The average expansion rate is 0.218 ± 0.029% yr−1 for the forward shock, corresponding to a velocity of ≈5800 km s−1. The time derivative of the proper motions indicates deceleration in the east, and an acceleration up to 1.1 × 10−4 yr−2 in the western part. The reverse shock moves outward in the east, but in the west it moves toward the center with an expansion rate of −0.0225 ± 0.0007 % yr−1, corresponding to −1884 ± 17 km s−1. In the west, the reverse shock velocity in the ejecta frame is ≳3000 km s−1, peaking at ∼8000 km s−1, explaining the presence of X-ray synchrotron emitting filaments there. The backward motion of the reverse shock can be explained by either a scenario in which the forward shock encountered a partial, dense, wind shell, or one in which the shock transgressed initially through a lopsided cavity, created during a brief Wolf–Rayet star phase. Both scenarios are consistent with the local acceleration of the forward shock. Finally we report on the proper motion of the northeastern jet, using both the X-ray continuum band, and the Si xiii K-line emission band. We find expansion rates of, respectively, 0.21% and 0.24% yr−1, corresponding to velocities at the tip of the X-ray jet of 7830–9200 km s−1.

Chandra VLP (year 2004) continuum image (4.2–6 keV) of Cassiopeia A with the annuli depicted that were used for selecting the forward (cyan) and reverse shock regions (red)

Full Article:
Vink, J. (SHARP), Patnaude, D. J. and Castro, D. (2022). The Forward and Reverse Shock Dynamics of Cassiopeia A. The Astrophysical Journal, 929, doi: 10.3847/1538-4357/ac590f

License: CC BY 4.0