arXiv subjects

Raúl Gómez-Herrero

Publications and source records attributed to Raúl Gómez-Herrero.

6 recordsLinked to original sources

arXiv2026

Solar Orbiter observations of solar energetic electron events associated with hard microflares

Generally, large solar flares accelerate electrons to high energies more efficiently than microflares. However, some microflares, known as hard microflares (HMFs), also produce high-energy electrons, as indicated by their flat hard X-ray (HXR) spectra. These events are typically associated with footpoints located in or at the edge of sunspots. The mechanisms behind this efficient acceleration, and their connection to solar energetic electrons (SEEs), remain unclear. We compare, for the first time, HXR spectra of HMFs with in-situ electron spectra of associated SEEs using Solar Orbiter STIX and EPD observations. This provides insight into acceleration processes and the transport of high-energy electrons into interplanetary space. We identify eight HMFs observed jointly by Solar Orbiter and Earth-based instruments that are associated with SEEs, confirmed through timing and magnetic connectivity analysis. Each event is studied using HXR spectroscopy, SEE velocity-dispersion analysis, and in-situ electron spectral analysis. Seven of eight events show consistent timing between flare HXR emission and inferred electron injection, as well as good agreement with magnetic connectivity estimates. The known correlation between HXR photon and in-situ electron spectral indices extends to HMFs, which occupy the hard end of the distribution, even compared to larger flares. We conclude that HMFs produce prompt SEEs with hard spectra, demonstrating efficient electron acceleration without requiring large flare energy release. Their magnetic configuration, involving open field lines from the sunspot, suggests they may be an important contributor to filling the heliosphere with energetic particles.

astro-ph.SR↗

arXiv2025

A Tale of Two Shocks

Energetic particles in interplanetary space are normally measured at time scales that are long compared to the ion gyroperiod. Such observations by necessity average out the microphysics associated with the acceleration and transport of 10s - 100s keV particles. We investigate previously unseen non-equilibrium features that only become observable at very high time resolution, and discuss possible explanations of these features. We use unprecedentedly high-time-resolution data that were acquired by the in situ instruments on Solar Orbiter in the vicinity of two interplanetary shocks observed on 2023-11-29 07:51:17 UTC and 2023-11-30 10:47:26 UTC at $\sim 0.83$ astronomical units from the Sun. The solar-wind proton beam population follows the magnetic field instantaneously, on time scales which are significantly shorter than a gyro-period. Energetic particles, despite sampling large volumes of space, vary on remarkably short time scales, typically on the order of the convection time of their gyro-radius. Non-equilibrium features such as bump-on-tail distributions of energetic particles are formed by small-scale magnetic structures in the IMF. High-time-resolution observations show previously unobserved microphysics in the vicinity of two traveling interplanetary shocks, including ion reflection at a current sheet, which may explain where ions are reflected in shock acceleration.

astro-ph.SR↗

arXiv2020

The Electron Proton Helium INstrument as an Example for a Space Weather Radiation Instrument

The near-Earth energetic particle environment has been monitored since the 1970's. With the increasing importance of quantifying the radiation risk for, e.g. for the human exploration of the Moon and Mars, it is essential to continue and further improve these measurements. The Electron Proton Helium INstrument (EPHIN) on-board SOHO continually provides these data sets to the solar science and space weather communities since 1995. Here, we introduce the numerous data products developed over the years and present space weather related applications. Important design features that have led to EPHINs success as well as lessons learned and possible improvements to the instrument are also discussed with respect to the next generation of particle detectors.

physics.space-ph↗

arXiv2018

Long-lasting injection of solar energetic electrons into the heliosphere

The main sources of solar energetic particle (SEP) events are solar flares and shocks driven by coronal mass ejections (CMEs). While it is generally accepted that energetic protons can be accelerated by shocks, whether or not these shocks can also efficiently accelerate solar energetic electrons is still debated. In this study we present observations of the extremely widespread SEP event of 26 Dec 2013. To the knowledge of the authors, this is the widest longitudinal SEP distribution ever observed together with unusually long-lasting energetic electron anisotropies at all observer positions. Further striking features of the event are long-lasting SEP intensity increases, two distinct SEP components with the second component mainly consisting of high-energy particles, a complex associated coronal activity including a pronounced signature of a shock in radio type-II observations, and the interaction of two CMEs early in the event. The observations require a prolonged injection scenario not only for protons but also for electrons. We therefore analyze the data comprehensively to characterize the possible role of the shock for the electron event. Remote-sensing observations of the complex solar activity are combined with in-situ measurements of the particle event. We also apply a Graduated Cylindrical Shell (GCS) model to the coronagraph observations of the two associated CMEs to analyze their interaction. We find that the shock alone is likely not responsible for this extremely wide SEP event. Therefore we propose a scenario of trapped energetic particles inside the CME-CME interaction region which undergo further acceleration due to the shock propagating through this region, stochastic acceleration, or ongoing reconnection processes inside the interaction region. The origin of the second component of the SEP event is likely caused by a sudden opening of the particle trap.

physics.space-ph↗

arXiv2016

Injection of solar energetic particles into both loop legs of a magnetic cloud

Each of the two STEREO spacecraft carries a SEPT Instrument which measures electrons and protons. Anisotropy observations are provided in four viewing directions. The SEP event on 7 Nov 2013 was observed by both STEREO spacecraft, which were longitudinally separated by 68° at that time. While STEREO A observed the expected characteristics of an SEP event at a well-connected position, STEREO B detected a very anisotropic bi-directional distribution of near-relativistic electrons and was situated inside a magnetic-cloud-like structure during the early phase of the event. We examine the source of the bi-directional SEP distribution at STEREO B. On the one hand this distribution could be caused by a double injection into both loop legs of the MC. On the other hand, a mirroring scenario where the incident beam is reflected in the opposite loop leg could be the reason. Furthermore, the energetic electron observations are used to probe the magnetic structure inside the magnetic cloud. We show that STEREO B was embedded in an MC-like structure ejected three days earlier. We apply a GCS model to the coronagraph observations from three viewpoints as well as the Global Magnetic Cloud model to the in situ measurements at STEREO B to determine the orientation and topology of the MC close to the Sun and at 1 AU. We also estimate the path lengths of the electrons propagating through the MC to estimate the amount of magnetic field line winding inside the structure. The relative intensity and timing of the energetic electron increases in the different SEPT telescopes at STEREO B strongly suggest that the bi-directional electron distribution is formed by SEP injections in both loop legs of the MC separately instead of by mirroring farther away beyond the STEREO orbit. Observations by the Nancay Radioheliograph of two distinct radio sources during the SEP injection further support the above scenario.

physics.space-ph↗

arXiv2012

The large longitudinal spread of solar energetic particles during the January 17, 2010 solar event

We investigate multi-spacecraft observations of the January 17, 2010 solar energetic particle event. Energetic electrons and protons have been observed over a remarkable large longitudinal range at the two STEREO spacecraft and SOHO suggesting a longitudinal spread of nearly 360 degrees at 1AU. The flaring active region, which was on the backside of the Sun as seen from Earth, was separated by more than 100 degrees in longitude from the magnetic footpoints of each of the three spacecraft. The event is characterized by strongly delayed energetic particle onsets with respect to the flare and only small or no anisotropies in the intensity measurements at all three locations. The presence of a coronal shock is evidenced by the observation of a type II radio burst from the Earth and STEREO B. In order to describe the observations in terms of particle transport in the interplanetary medium, including perpendicular diffusion, a 1D model describing the propagation along a magnetic field line (model 1) (Dröge, 2003) and the 3D propagation model (model 2) by (Dröge et al., 2010) including perpendicular diffusion in the interplanetary medium have been applied, respectively. While both models are capable of reproducing the observations, model 1 requires injection functions at the Sun of several hours. Model 2, which includes lateral transport in the solar wind, reveals high values for the ratio of perpendicular to parallel diffusion. Because we do not find evidence for unusual long injection functions at the Sun we favor a scenario with strong perpendicular transport in the interplanetary medium as explanation for the observations.

astro-ph.SR↗