2026 Spring Schedule

Abstract:  Pulsars and Magnetars [i.e., Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs)] are believed to be isolated magnetic neutron stars. Pulsars have dipole magnetic fields of the order of $10^{12}$ G, while magnetars are thought to have dipole magnetic fields three orders of magnitude larger and even larger internal toroidal magnetic fields. Pulsars are relatively old, isolated neutron stars and it is therefore natural for them to rotate in vacuum. Magnetars, on the other hand, are relatively young isolated neutron stars and might be surrounded by matter left over from their formation. If this is the case, then their spin down may not be due to magnetic dipole emission, but rather due to the interaction between the magnetosphere of the neutron star and the surrounding matter. As a consequence, matter may fall along dipole magnetic-field lines onto the neutron star, producing X-ray emission. We have proposed that the quiescent and transient X-ray luminosity of AXPs and SGRs is the result of accretion from a fallback disk onto neutron stars with dipole magnetic fields in the range $10^{12} – 10^{13}$ G. Our picture is similar to that of normal X-ray pulsars; only the accretion rate is significantly smaller in AXPs and SGRs. Within the very stringent model of accreting pulsars, we have been able to explain quantitatively a) the comparable soft and hard X-ray luminosities, b) the X-ray spectra (soft and hard), c) the energy-dependent pulse profiles and d) the period derivative resulting from the accretion torque. Our model makes the prediction that no AXP/SGR will ever be observed with a hard X-ray power-law spectrum extending beyond 400 keV. The magnetar model, on the other hand, allows the power law to extend to 1 MeV or more. For the outbursts with super-Eddington luminosities, our model proposes that they are produced by magnetic field decay in localized, super-strong ($10^{14} – 10^{15}$ G) multipole fields. The extent of the X-ray power law and the existence or non-existence of fallback disks around magnetars will decide whether the accretion picture or the classical magnetar picture is the correct one. 

Speaker info: link

Title:  "Collisionless Shocks in the Heliosphere: Transient Processes and Particle Energization" 

Abstract: Collisionless shocks are the primary sites for particle acceleration across the Universe, yet the specific mechanisms that bridge the gap from thermal seeds to relativistic cosmic rays remain a subject of active research. This seminar explores the evolving paradigm of shock acceleration, at planetary bow shocks, moving beyond steady-state models to highlight the critical role of upstream transient processes. 

Focusing primarily on Earth's bow shock using high-resolution data from NASA’s MMS and THEMIS, alongside ESA’s Cluster missions, we discuss how the dynamic region upstream of a shock (the foreshock) is not merely a precursor zone but an extended environment of elevated particle acceleration. By examining the formation and evolution of foreshock transient structures, we demonstrate how these local disturbances fundamentally alter the broader shock environment. These transients facilitate a reinforced shock acceleration process, injecting suprathermal particles and providing the confinement necessary for energization through a multiscale process that includes adiabatic and non-adiabatic acceleration processes as they transition downstream. Finally, we scale these physical insights to larger systems, using recent observations from Jupiter’s bow shock via NASA’s Juno mission, to demonstrate that these processes are a universal feature of planetary environments. By bridging these in-situ observations with broader scaling laws, we show how localized transient dynamics can be potentially used to constrain the maximum limits of particle acceleration at shocks.

Abstract: TBA

Abstract: Out of the several billions of G-type stars in the Milky Way and the trillions of other galaxies in the Universe, only the Sun, due to its proximity to Earth, is accessible to detailed observations of its atmosphere, something that affects via space weather our planet.  The density of the coronal plasma is extremely low, such that the solar corona scatters approximately one-millionth of the   photospheric photons that pass through it, with the result that the corona is overshined by the photosphere, and can only be seen during total solar eclipses.   Those impressive coronal images can be observed once in a year, since Thales predicted the first solar eclipse in Asia Minor, the 28th of May 585 B.C. An eclipse occurs because of the great cosmic coincidence, or Divine Providence that the angular diameter of the Sun and the Moon as seen from Earth are equal.  It should be mention also that Aristarchos calculated from the lunar and solar eclipses that the size of the Sun is much greater than that of the Earth and therefore, he argued, it is impossible such a large body like the Sun to revolve around a much smaller body like Earth, proposing thus the Heliocentric system, more than 2000 years before Copernicus. 

The launching of the twin spacecrafts of Proba 3 in Dec 2024, in a formation flight around the Earth started the new era of long duration, detailed observations of the corona in the range of the radial distances r of 1.1 R⊙ < r < 3 R⊙.  We shall discuss some first results from observations of the solar corona by the ASPIICS coronagraph aboard the Proba-3 mission. For example, ASPIICS  observes quasi-stationary structures, such as coronal loops, streamers, quiescent prominences and a variety of dynamic phenomena, such as erupting prominences, coronal mass ejections, jets, solar wind outflows and coronal inflows. Also, ubiquitous, weak and persistent small-scale dynamic features (blobs, outflows, jets), between 1.3 and 3 R⊙ observed at a high spatial (5.6 arcsec) and temporal (30 s) resolution for the first time, with speeds ranging  from 15 km/s to 500 km/s, expanding thus the range of scales at which the variable slow solar wind is observed to form. All these features appear to originate from streamers and pseudostreamers, thus providing important constraints on the role of small-scale dynamics in the cradle of the solar wind. 

Abstract: Protoplanetary (PP) discs comprise the swirling flows of gas and dust around new-born stars. Not only are they essential to stellar accretion, they control the assembly of planets and, subsequently, set the conditions in which these planets evolve. This talk presents theoretical and numerical results concerning the evolution of the inner regions of PP discs (< a few AU), focussing on their particularly intricate (non-ideal) magnetohydrodynamic properties and its imprint on turbulence, outflows, observed variability, and planet formation.