Variability in energetic particle observations at strong interplanetary shocks: Multi-spacecraft observations

Context. We studied energetic particle intensity profiles observed by Solar Orbiter during the time period from April 2020 to April 2023, associated with the passage of interplanetary (IP) shocks. For our study we considered 58 IP forward shocks and analysed the possible correlations between some IP shock parameters and the electron and proton responses to the passage of the IP shocks. We investigated which shock signatures are more likely related to the efficiency of the IP shocks with respect to particle acceleration. Aims. We introduced a variable that characterises the contamination induced by protons in the electron channels of the Electron Proton Telescope (EPT) part of the Energetic Particle Detector (EPD) suite of instruments on board Solar Orbiter, which allowed us to identify the cases in which the intensity time profiles of electrons at energies ≤240 keV showed a real response at the passage of IP shocks. In the case of protons, we searched for the response in seven energy ranges from 52 keV to 15 MeV, and based on the shape of the proton response at low energies (∼100 keV), we divided the profiles into weak responses, peaks (regular or irregular), plateaus, and unclear responses. For the regular peak and plateau types we constructed an average time profile by applying superposed epoch analysis. For the response in electrons and protons, and for the different types of proton responses at different energies, we analysed the corresponding IP shock parameters, aiming to understand which ones are important to form a certain type of time profile or to achieve a certain energy. We also included a comparison between the proton intensity time profiles in the upstream region and, assuming the predictions of the diffusive shock acceleration (DSA) theory, identified the values of the mean free path in several cases. Methods. We found that the IP shock efficiency in the energisation of both electrons and protons is strongly energy dependent. Cases of electron acceleration are rare. Only in about ∼8% of the events for energies ≤100 keV and in ∼2% for energies ≤250 keV did the electron intensities show an unambiguous response at the passage of IP shocks (with those accompanied by a response being mainly oblique or quasi-perpendicular). The shocks for which we identified a response in ∼100 keV proton intensity time profiles come to ∼83% of the IP shocks under study, and are parallel or quasi-parallel. The ability to accelerate protons to higher energies and to form a particular shape of the particle response to the IP shock passage mostly depends on the IP shock speed. Results. Based on the analysis of time profiles and the occurrence of unambiguous electron acceleration at shocks, the acceleration mechanism behind the electron energisation is unlikely to be DSA, but shock drift acceleration (SDA) remains a candidate for the acceleration mechanism. Proton time profiles of the plateau type around the IP shock front can be achieved with an IP shock speed above 800 km s−1 and an ambient mean free path ≤0.015 au, reproducing the asymptotic steady-state ion distribution reached in the classical DSA solution.

Interplanetary (IP) shocks are fundamental constituents of the heliosphere, where they form as a result of solar activity. We use previously unavailable measurements of IP shocks in the inner heliosphere provided by Solar Orbiter, and present a survey of the first 100 shocks observed in situ at different heliocentric distances during the rising phase of solar cycle 25. The fundamental shock parameters (shock normals, shock normal angles, shock speeds, compression ratios, Mach numbers) have been estimated and studied as a function of heliocentric distance, revealing a rich scenario of configurations. Comparison with large surveys of shocks at 1 au shows that shocks in the quasi-parallel regime and with high speed are more commonly observed in the inner heliosphere. The wave environment of the shocks has also been addressed, with about 50% of the events exhibiting clear shock-induced upstream fluctuations. We characterize energetic particle responses to the passage of IP shocks at different energies, often revealing complex features arising from the interaction between IP shocks and preexisting fluctuations, including solar wind structures being processed upon shock crossing. Finally, we give details and guidance on the access use of the present survey, available on the EU-project “Solar Energetic Particle Analysis Platform for the Inner Heliosphere” website. The algorithm used to identify shocks in large data sets, now publicly available, is also described.

The existing fleet of spacecraft at 1 AU represents an important opportunity for multi-mission, multi-spacecraft direct investigations of heliospheric plasmas.In this work, we focus on a strong interplanetary (IP) shock which crossed Wind, ACE, DSCOVR, THEMIS B and THEMIS C on 3 Nov 2021. Further, the shock was observed by well radially aligned Solar Orbiter at 0.8 AU. Such spacecraft configuration was used in a previous study to constrain the extent of the shock upstream populated by compressive structures (shocklets).Here, we study the acceleration of protons up to 5 MeV energies and focus on the variability of energetic particle production. By cross-correlating energetic particle fluxes for the different vantage points, we find that the production of low energy (up to 100 keV) protons is strongly influenced by the local shock conditions, while high energy ones (up to 1 MeV) respond to the average shock conditions. At higher energies, we find that the energetic particle fluxes are modulated by large-scale structuring in the shock surroundings.This study is relevant for IMAP, which will soon join such spacecraft fleet, and yield novel measurements of energetic particles at Lagrange point L1.

Interplanetary (IP) shocks are important sites of particle acceleration in the Heliosphere and can be observed in-situ utilizing spacecraft measurements. Such observations, crucial to address important aspects of energy conversion for a variety of astrophysical systems. From this point of view, Solar Orbiter provides observations of interplanetary shocks at different locations in the inner heliosphere with unprecedented time and energy resolution in the suprathermal (usually above 50 keV) energy range. The general trends observed by Solar Orbiter and other spacecraft in the near-Earth environment for such shocks, highlighting their typical parameters, will be presented first. Then, the presence shock-induced wave activity in association with such shocks and their association with the presence of energetic particles will be discussed, summarizing some of the work performed in the framework of the the Solar EneRgetic ParticlE aNalysis plaTform for the INner hEliosphere (SERPENTINE) Project funded by EU H2020. Finally, particular emphasis will be devoted on the role of space/time irregularities at IP shocks and their effect on suprathermal particle production, focusing on some of the most interesting shocks observed by Solar Orbiter. 

The solar wind is a supersonic and super-Alfvenic plasma flow of solar origin filling the whole heliosphere. It has a strong turbulent character with several energy cascades. Interplanetary (IP) shocks are characterized by abrupt changes of plasma parameters and the magnetic field strength. Primary drivers of IP shocks are transient phenomena such as coronal mass ejections and corotating interaction regions. We discuss an occurrence of large-amplitude low-frequency fluctuations of the magnetic field in upstream and downstream of IP shocks. We have used a newly developed automated algorithm for detection of IP shocks which is planned to be implemented onboard the future Solar Orbiter spacecraft. We used the fluxgate magnetometer onboard Wind with a sampling frequency of 10 Hz which allows us to analyze both inertial ranges and the beginning of the kinetic scale. For a time series analysis the Morlet wavelet transform was performed. We have identified 971 IP shocks in the Wind measurements between 1995 and 2014 with using the detection algorithm. We have analyzed four types of shocks: 488 Fast Forward (FF, panel a), 101 Fast Reverse (FR, panel b), 212 Slow Forward (SF, panel c), and 170 Slow Reverse (SR, panel d). Blue and red data points show statistical results of the density of magnetic field fluctuations for five minutes before and after the shock passage, respectively. We can identify the break of the power law spectra between the inertial and dissipation scales around 0.4 Hz. Generally, we observe larger fluctuations in the downstream when compared to the upstream as it can be expected.

The acceleration of thermal solar wind (SW) protons at spherical interplanetary shocks driven by coronal mass ejections is investigated. The SW velocity distribution is represented using κ-functions, which are transformed in response to simulated shock transitions in the fixed-frame flow speed, plasma number density, and temperature. These heated SW distributions are specified as source spectra at the shock from which particles with sufficient energy can be injected into the diffusive shock acceleration process. It is shown that for shock-accelerated spectra to display the classically expected power-law indices associated with the compression ratio, diffusion length scales must exceed the width of the compression region. The maximum attainable energies of shock-accelerated spectra are found to be limited by the transit times of interplanetary shocks, while spectra may be accelerated to higher energies in the presence of higher levels of magnetic turbulence or at faster-moving shocks. Indeed, simulations suggest that fast-moving shocks are more likely to produce very high energy particles, while strong shocks, associated with harder shock-accelerated spectra, are linked to higher intensities of energetic particles. The prior heating of the SW distribution is found to complement shock acceleration in reproducing the intensities of typical energetic storm particle (ESP) events, especially where injection energies are high. Moreover, simulations of ∼0.2–1 MeV proton intensities are presented that naturally reproduce the observed flat energy spectra prior to shock passages. Energetic particles accelerated from the SW, aided by its prior heating, are shown to contribute substantially to intensities during ESP events.

The digisonde Global Ionospheric Radio Observatory (GIRO) has recorded strong ionospheric responses to two powerful interplanetary shock passages on 7 November 7 2004 and 21 January 2005. Both events provide excellent opportunities to study geospace response processes to strong interplanetary forward shocks in great detail. The angle between the normal of the 7 November 2004, shock front and the Sun‐Earth line was ∼3.0 degrees, indicating that the shock hit the equatorial magnetosphere at ∼1200 LT (local noon). The subsequent dayside shock‐induced ionospheric phenomena were found to have marked longitudinal and latitudinal distributions. Comparative studies of the intense interplanetary shocks (including the sheath) associated with northward and southward interplanetary magnetic fields (IMF) revealed their different geospace effects on the dayside ionospheric region. The equatorial ionosphere responds to the interplanetary shock rather quickly, and if the shock is associated with southward IMF the plasma electrons in the equatorial ionosphere are rapidly uplifted. During the 7 November 2004 event the averaged uplift velocity was close to ∼67 m/s, and the ionospheric total electron content (TEC) increased from the original 16 TEC units (TECU) to ∼38 TECU (1 TECU = 1016 m−2) which may be due to the shock effect. When the interplanetary shock (including the magnetosheath) is associated with northward IMF, the plasma in the equatorial ionosphere moves downward, causing a sudden drop in the total electron content. During the 21 January 2005 event, the averaged downward velocity was ∼120 m/s, and the TEC dropped from 75 TECU to 22 TECU at the time of maximum solar wind dynamic pressure, and then recovered to ∼68 TECU in about 2 h after the shock passage. The middle and high latitude ionospheric TEC enhancements may be due to particle (ion and electron) precipitation ionization losses caused by the strong impacts of the interplanetary shocks.

The interaction between interplanetary (IP) shocks and the Earth's magnetosphere would generate/excite various types of geomagnetic phenomena. In order to analyze what kind of IP shock is more likely to trigger intense substorms (SME/AE > 1,000 nT) and how the energetic electrons response to intense substorms at geosynchronous orbit, we perform a systematic survey of 246 IP shock events using SuperMag and LANL observations between 2001 and 2013. The statistical analysis shows that intense substorms (SME > 1,000 nT) triggered by IP shocks are most likely to occur under the southward interplanetary magnetic field (IMF) and fast solar wind preconditions. Besides, intense substorms after the IP shock arrival are much more likely to occur when IMF points toward (away from) the Sun around spring (autumn) equinox, which can be ascribed to the Russell‐McPherron effect. Thus, the IMF Bs precondition of an IP shock and the Russell‐McPherron effect can be considered as precursors of an intense substorm. Furthermore, after the shock arrival associated with intense substorms, low‐energy (<200 keV) electron fluxes increase significantly at geosynchronous orbit, and high‐energy (>200 keV) electron fluxes decrease. The spectral index becomes softer with intense substorms and remains almost unchanged with moderate substorms no matter whether the IMF precondition is southward or northward.

Using magnetic field, plasma, and energetic particle data from Wind and ACE, we analyze interplanetary features associated with the first strongly geoeffective interval in the rising phase of solar cycle 23, which affected Earth on 1–4 May 1998. As shown by Skoug et al. [1999], the configuration consisted of a compound stream made up of an interplanetary coronal mass ejection (ICME) containing a magnetic cloud and being trailed by a hot, faster flow. In addition, we find that the front boundary of the ICME is a rotational discontinuity and the leading edge of the fast stream has a zero normal magnetic field component and is followed by a magnetic field which is strongly enhanced (by a factor of ∼4) and whose fluctuations lie in a plane approximately parallel to the leading edge. Energetic particle and composition observations confirm that the field lines of the magnetic cloud were connected to at least two different flare sites in the same active region. We infer a lower limit for the size of the solar footprint of the connected flux tube of 0.02 Rs2, i.e., ∼1010 km2. A dramatic weakening of the halo electron distribution occurred during 3 May at the time when other experimenters have documented the presence of prominence material. We hypothesize that the solar wind halo population was scattered by enhanced frequency of Coulomb collisions in the dense and very cold plasma. We discuss our energetic particle observations in terms of local acceleration at interplanetary shocks and field discontinuities as well as in terms of acceleration in flares and CME‐driven shocks. We also compare, in a specific formulation, the power and energy input of the May 1998 configuration to the magnetosphere with other much studied geoeffective events. We find that the power input far exceeded that in all previous geoeffective events in our sample and attribute this to the fact that the 1–4 May 1998 event consisted of a compound stream structure with an unprecedented power input during a ∼3‐hour burst of high‐speed flow on 4 May. A statistical survey using the OMNI database for the 6‐year period 1995–2000 confirms these inferences and indicates further approximate saturation levels for energy and power input of 10 J m−2 and 0.3 mW m−2, only exceeded in exceptional events such as May 1998 and July 2000. The solar energetic particle event at the leading edge of the fast stream might be the only advance warning the Earth would receive of the approach of a configuration of such a concentrated geoeffective potential.

The transport of energetic particles is intimately related to the properties of plasma turbulence, a ubiquitous dynamic process that transfers energy across a broad range of spatial and temporal scales. However, the mechanisms governing the interactions between plasma turbulence and energetic particles are not completely understood. Here we present comprehensive observations from the upstream region of a quasi-perpendicular interplanetary (IP) shock on 2004 January 22, using data from four Cluster spacecraft to investigate the interplay between turbulence dynamics and energetic particle transport. Our observations reveal a transition in energetic proton fluxes from exponential to power-law decay with increasing distance from the IP shock. This result provides possible observational evidence of a shift in transport behavior from normal diffusion to superdiffusion. This transition correlates with an increase in the time ratio from τ s /τ c < 1 to τ s /τ c ≫ 1, where τ s is the proton isotropization time, and τ c is the turbulence correlation time. Additionally, the frequency–wavenumber distributions of magnetic energy in the power-law decay zone indicate that energetic particles excite linear Alfvén-like harmonic waves through gyroresonance, thereby modulating the original turbulence structure. These findings provide valuable insights for future studies on the propagation and acceleration of energetic particles in turbulent astrophysical and space plasma systems.

Interplanetary (IP) shocks are important sites of particle acceleration in the Heliosphere and can be observed in-situ utilizing spacecraft measurements. Such observations are crucial to address important aspects of energy conversion for a variety of astrophysical systems.Under this point of view, Solar Orbiter provides observations of interplanetary shocks at different locations in the inner heliosphere with unprecedented time and energy resolution in the suprathermal (usually above 50 keV) energy range. We present a comprehensive identification of such shocks, highlighting their typical parameters.We then study a strong shock showing novel dispersive signals in the suprathermal particle fluxes observed by the Solar Orbiter SupraThermal Electron and Proton sensor. These are probably due to irregular injection of particles to suprathermal energies along the shock front, as inferred using the Solar Orbiter in-situ observations and self-consistent, kinetic modelling of the shock transition.

The energetic particle event associated with the quasi‐perpendicular interplanetary shock which passed ISEE 3 on June 6, 1979, is characterized by persistent beamlike antisunward particle fluxes on both sides of the shock. We found that the shock has no significant nonadiabatic effects on the energetic particles near 1 AU. The lack of particles with pitch angles larger than 60° accounts for the absence of signatures of shock drift acceleration. The adiabatic behavior of the bulk of the particles at the shock offers a unique opportunity to understand the role of the postshock magnetic regime. A recently formed magnetic discontinuity just downstream from the shock forms an effective obstacle for particles, particularly those with a small gyroradius. From the spatial dependence of the particle population in front of the magnetic discontinuity we derived the escape probability for particles to cross the discontinuity. Strong anisotropic particle bursts are observed as intensity spikes (duration less than 1 min) both upstream and downstream from the shock. Velocity dispersion in some of these spikes is consistent with impulsive release at the magnetic discontinuity. We propose that the spikes are accomplished by a disturbance propagating along the discontinuity which develops a normal component perhaps accompanied by induced electric fields, thus enabling, in particular, low‐energy particles with their small gyroradii to cross and stream into the upstream region. The energetic particle fluxes in this event are among the highest we have observed at ISEE 3. These fluxes are not produced by shock acceleration near 1 AU, but originate somewhere else far downstream from the magnetic discontinuity. From what we have learned from the unambiguous particle population signatures in this event, we believe that most shock‐associated particle events cannot be understood on the basis of shock interactions alone; rather, the intensity history of energetic particles below 1 MeV is determined by the complete ensemble of magnetic structures embedded in the compressed plasma well behind the shock.

Observations of energetic particles at interplanetary shocks are important to study acceleration mechanisms and their connection with magnetohydrodynamic turbulence. Energetic storm particle (ESP) events are increases in proton fluxes that occur locally at the passage time of interplanetary shocks. These events are more dangerous when they are superimposed on the solar energetic particles (SEPs) produced by the eruption of flares and/or CME-driven shocks propagating from the corona to the interplanetary space. We considered ESP events occurring in association with SEPs on 3 November 2021. We used proton fluxes provided by Solar Orbiter (located at 0.85 AU) in the energy range of 30 keV–82 MeV, by Wind at energies from 70 keV to 72 MeV, and ACE in the range from 40 keV to 5 MeV (both located at the Lagrangian point L1, close to 1 AU along the Sun-Earth direction). In order to broaden the range of analyzed energies (40 keV - 72 MeV), we combine these data with the proton fluxes from the SOHO spacecraft, also located at L1. We analyzed the ESP event and fitted the proton energy spectra at both locations with several distributions to shed light on the mechanisms leading to the acceleration of energetic particles. We also investigated the turbulent magnetic field fluctuations around the shock. The obtained ESP spectra, best reproduced by the so-called double power law function, the spectral differences at the two locations, and the shock features (quasi-parallel geometry, enhanced downstream turbulence) suggest that diffusive shock acceleration is responsible for acceleration of low energy particles, whereas stochastic acceleration contributes to the (re) acceleration of high energies ones.

We report observations of an interplanetary (IP) shock observed by Parker Solar Probe (PSP) on 2024 September 29 at ∼07:50:29 UTC. PSP was only ∼17.07 R s from the Sun, making this one of the closest observed IP shocks to date. The IP shock was a weak (M f ∼ 1.2), quasi-perpendicular (θ Bn ∼ 50°), and of moderate speed (V shn ∼ 465 km s−1). The standard shock acceleration mechanisms (e.g., Fermi acceleration) predict that such an unremarkable shock cannot generate energetic particles (i.e., over 4 orders of magnitude above thermal energies), which is supported by decades of IP shock observations near 1 au. However, ∼MeV energy protons with an inverse velocity arrival and synchrotron radiation (due to ∼MeV energy electrons) were observed upstream. This raised the question of what was different about this shock. One observation was that of a fast/magnetosonic-whistler precursor with peak-to-peak magnetic field amplitudes >700 nT, electric fields >2000 mV m−1, and Poynting fluxes >230 mW m−2. These are 2 orders of magnitude larger than any previously observed whistler precursor. To put the amplitudes in context, the lower bound Poynting flux estimates are >200 times what is necessary to drive the terrestrial aurora. Note that the normalized wave parameters (e.g., frequency) were found to be consistent with previous studies near 1 au. Thus, the precursors cannot likely generate a larger fraction of energetic particles than similar precursors near 1 au. However, the much larger amplitudes would allow for higher maximum energies. This raises important questions about inaccessible shocks in more extreme astrophysical environments and what potential energization they may have in light of these observations.

The passage of an interplanetary (IP) shock was detected by Wind, ACE, Geotail, and THEMIS‐B in the solar wind on 24 November 2008. From the propagation time of the IP shock at the spacecraft, it is expected that the IP shock front is aligned with the Parker spiral and strikes the postnoon dayside magnetopause first. Using multipoint observations of the sudden commencement (SC) at THEMIS probes, GOES 11, and ETS in the dayside magnetosphere, we confirmed that the magnetospheric response to the IP shock starts earlier in the postnoon sector than in the prenoon sector. We found that the estimated normal direction of the SC front is nearly aligned with the estimated IP shock normal. We also found that the SC front normal speed is much slower than the fast mode speed and is about 22–56% of the IP shock speed traveling in the solar wind. Thus, we suggest that the major field changes of the SC in the dayside magnetosphere are not due to the magnetic flux carried by hydromagnetic waves but to the increased solar wind dynamic pressure behind the shock front sweeping the magnetopause. The SC event appears as a step‐like increase in theHcomponent at the low‐latitude Bohyun station and a negative‐then‐positive variation in theHcomponent at the high‐latitude Chokurdakh (CHD) station in the morning sector. During the negative perturbation at CHD, the SuperDARN Hokkaido radar detected a downward motion in the ionosphere, implying westward electric field enhancement. Using the THEMIS electric field data, it is confirmed that the westward electric field corresponds to the inward plasma motion in the dayside magnetosphere due to the magnetospheric compression.