Solar Eruptive Events Research Articles

High-energy insights from an escaping coronal mass ejection with Solar Orbiter/STIX observations

Context. Solar eruptive events, including solar flares and coronal mass ejections (CMEs), are typically characterised by energetically significant X-ray emissions from flare-accelerated electrons and hot thermal plasmas. However, the intense brightness of solar flares often overshadows high-coronal X-ray emissions from the associated eruptions due to the limited dynamic range of current instrumentation. Occulted events, where the main flare is blocked by the solar limb, provide an opportunity to observe and analyse the X-ray emissions specifically associated with CMEs. Aims. This study investigates the X-ray and extreme ultraviolet (EUV) emissions associated with a large filament eruption and CME that occurred on February 15, 2022. This event was highly occulted from the three vantage points of Solar Orbiter (∼45° behind the limb), Solar–TErrestrial RElations Observatory (STEREO-A), and Earth. Methods. We utilised X-ray observations from the Spectrometer/Telescope for Imaging X-rays (STIX) and EUV observations from the Full Sun Imager (FSI) of the Extreme Ultraviolet Imager (EUI) on board Solar Orbiter, supplemented by multi-viewpoint observations from STEREO-A/Extreme-UltraViolet Imager (EUVI). This enabled a comprehensive analysis of the X-ray emissions in relation to the filament structure observed in the EUV. We used STIX’s imaging and spectroscopy capabilities to characterise the X-ray source associated with the eruption. Results. Our analysis reveals that the X-ray emissions associated with the occulted eruption originate from an altitude exceeding 0.3 R⊙ above the main flare site. The X-ray time profile shows a sharp increase and exponential decay, and consists of both a hot thermal component at 17 ± 2 MK and non-thermal emissions (> 11.4 ± 0.2 keV) characterised by an electron spectral index of 3.9 ± 0.2. Imaging analysis shows an extended X-ray source that coincides with the EUV emission as observed from EUI, and was imaged until the source grew to a size exceeding the STIX imaging limit (180″). Conclusions. Filament eruptions and associated CMEs have hot and non-thermal components, and the associated X-ray emissions are energetically significant. Our findings demonstrate that STIX combined with EUI provides a unique and powerful tool for examining the energetic properties of the CME component of solar energetic eruptions. Multi-viewpoint and multi-instrument observations are crucial for revealing such energetically significant sources in solar eruptions that might otherwise remain obscured.

Solar Plasma Noise in TianQin Laser Propagation: An Extreme Case and Statistical Analysis

TianQin (TQ) proposes to detect gravitational-wave signals by using laser interferometry. However, the laser propagation effect introduces a potential noise factor for TQ. In this work, we used magnetohydrodynamic (MHD) simulations to obtain the space magnetic field and plasma distributions during an extremely strong solar eruption, and based on the MHD simulation result, we investigated laser propagation noise for TQ. For the extremely strong solar eruption event, we find that the laser propagation noise closely approaches 100% of TQ’s displacement noise requirement for the Michelson combination, while the laser propagation noise is still about 30% of TQ’s displacement noise requirement for time-delay interferometry (TDI)-X combination. In addition, we investigate the laser propagation noise for 12 cases with different solar wind conditions. Our finding reveals a linear correlation between the laser propagation noise and several space weather parameters, e.g., solar wind dynamic pressure, Sym-H, and Dst, where the correlation coefficients for solar wind dynamic pressure are strongest. Combining the cumulative distribution of solar wind dynamic pressure from 1999 to 2021 with the linear correlation between solar wind dynamic pressure and laser propagation noise, we have determined that the occurrence rate of the laser propagation noise to be greater than 30% of TQ’s displacement noise requirement for the Michelson combination over the entire solar activity week is about 15%. In addition, we find that TDI can suppress the laser propagation noise, and reduce the occurrence rate of the laser propagation noise exceeding 30% of TQ’s requirement to less than 1%.

Modelling non-radially propagating coronal mass ejections and forecasting the time of their arrival at Earth

We present the study of two solar eruptive events observed on December 7 2020 and October 28 2021. Both events were associated with full halo coronal mass ejections (CMEs) and flares. These events were chosen because they show a strong non-radial direction of propagation in the low corona and their main propagation direction observed in the inner heliosphere is not fully aligned with the Sun-Earth line. This characteristic makes them suitable for our study, which aims to inspect how the non-radial direction of propagation in the low corona affects the time of CMEs’ arrival at Earth. We reconstructed the CMEs using SOHO/LASCO and STEREO/COR observations and modelled them with the 3D MHD model EUHFORIA and the cone model for CMEs. In order to compare the accuracy of forecasting the CME and the CME-driven shock arrival time at Earth obtained from different methods, we also used so-called type II bursts, radio signatures of associated shocks, to find the velocities of the CME-driven shocks and forecast the time of their arrival at Earth. Additionally, we estimated the CME arrival time using the 2D CME velocity obtained from the white light images. Our results show that the lowest accuracy of estimated CME Earth arrival times is found when the 2D CME velocity is used (time difference between observed and modelled arrival time, Δt ≈ −29 h and −39 h, for the two studied events, respectively). The velocity of the type II radio bursts provides somewhat better – but still not very accurate – results (Δt ≈ +21 h and −29 h, for the two studied events, respectively). Employing, as an input to EUHFORIA, the CME parameters obtained from the graduated cylindrical shell (GCS) fittings at consequently increasing heights, results in a strongly improved accuracy of the modelled CME and shock arrival time; Δt changes from 20 h to 10 min in the case of the first event, and from 12 h to 30 min in the case of the second one. This improvement shows that when we increased the heights of the GCS reconstruction we accounted for the change in the propagation direction of the studied CMEs, which allowed us to accurately model the CME flank encounter at Earth. Our results show the great importance of the change in the direction of propagation of the CME in the low corona when modelling CMEs and estimating the time of their arrival at Earth.

Data-driven MHD Simulation of the Formation of a Magnetic Flux Rope and an Inclined Solar Eruption

Solar energetic events are caused by the release of magnetic energy accumulated in the solar atmosphere. To understand their initiating physical mechanisms, the dynamics of the coronal magnetic fields must be studied. Unfortunately, the dominant mechanisms are still unclear due to a lack of direct measurements. Numerical simulations based on magnetohydrodynamics (MHD) can reproduce the dynamical evolution of solar coronal magnetic field, providing a useful tool to explore flare initiation. Data-driven MHD simulations, in which the time-series observational data of the photospheric magnetic field is used as the simulation boundary condition, can explore different mechanisms. To investigate the accumulation of free magnetic energy through a solar eruption, we simulated the first of several large flares in NOAA active region 11283. We used a data-driven model that was governed by zero-beta MHD, focusing on the free magnetic energy accumulation prior to the M5.3 flare (2011 September 6 at 01:59 UT). We reproduced the flare-associated eruption following the formation of twisted magnetic fields, or a magnetic flux rope (MFR), formed by photospheric motions at its footpoints. We found that the eruption was first triggered by the growth of the torus instability. The erupting MFR caused magnetic reconnections with neighboring magnetic field lines located along the direction of the eruption. Using the simulation results and an axial-radial decay index centered on the MFR, we find a natural explanation for the inclination of the eruption and a possible approach to predict the direction of solar eruptive events.

Unraveling the Origins of an Extreme Solar Eruptive Event with Hard X-Ray Imaging Spectroscopy

Hard X-ray (HXR) observations are crucial for understanding the initiation and evolution of solar eruptive events, as they provide a key signature of flare-accelerated electrons and heated plasma. The potential of high-cadence HXR imaging for deciphering the erupting structure, however, has not received adequate attention in an era of extreme ultraviolet (EUV) imaging abundance. An extreme solar eruptive event on 2022 September 5 observed on the solar far side by both Parker Solar Probe and Solar Orbiter provides the opportunity to showcase the power of HXR imaging in the absence of high-cadence EUV imaging. We investigate the evolution of flare energy release through HXR timing, imaging, and spectral analyses using data from the Spectrometer/Telescope for Imaging X-rays (STIX) on board Solar Orbiter. STIX provides the highest cadence imaging of the energy release sites for this far-side event and offers crucial insight into the nature of energy release, timing of flare particle acceleration, and evolution of the acceleration efficiency. We find that this is a two-phase eruptive event, rather than two distinct eruptions, as has been previously suggested. The eruption begins with an initial peak in flare emission on one side of the active region (AR), marking the rise/destabilization of a loop system followed by notable episodes of energy release across the AR and an eruptive phase associated with a very fast coronal mass ejection, type III radio bursts, and solar energetic particles. We demonstrate that high-cadence HXR imaging spectroscopy is indispensable for understanding the formation of powerful, space-weather relevant eruptions.

The Large Imaging Spectrometer for Solar Accelerated Nuclei (LISSAN): A next-generation solar gamma-ray spectroscopic imaging instrument concept

We present the Large Imaging Spectrometer for Solar Accelerated Nuclei (LISSAN), a new solar dedicated satellite instrument concept. LISSAN relies on an indirect Fourier imaging technique valid over an energy range from 40 keV up to 100 MeV. Spatial information is encoded into 15 moiré patterns by 15 pairs of slightly offset grids (bigrids) separated by a fixed distance enabling a predicted spatial resolution of 10”. The time, location, and energy of each incoming photon is recorded via a pixelated gadolinium aluminium gallium garnet (GAGG) crystal scintillator detector placed in alignment with each bigrid, therefore providing simultaneous imaging and spectroscopy from the same imaging system. X-ray and gamma-ray emission are key diagnostics of electron and ion acceleration, respectively. However, despite being a fundamental process that occurs throughout the Universe, particularly in the solar atmosphere, only one resolved gamma-ray image of ion acceleration in a solar flare has ever been achieved. LISSAN will shed new light on this process by providing spectral resolution better than 1.5% FWHM at 6.1 MeV, an imaging effective area at 2.2 MeV of 100 cm2 (more than 25 times greater than past missions such as RHESSI) and a 10 s cadence. Thanks to these significant advances over the previous satellite-based solar detectors, LISSAN will provide reliable imaging and the spectral characterization of both electron and ion acceleration in solar eruptive events simultaneously for the first time, enabling to it to answer several important open questions regarding solar particle acceleration and the initiation of space weather events.

The Solar Particle Acceleration Radiation and Kinetics (SPARK) Mission Concept

Particle acceleration is a fundamental process arising in many astrophysical objects, including active galactic nuclei, black holes, neutron stars, gamma-ray bursts, accretion disks, solar and stellar coronae, and planetary magnetospheres. Its ubiquity means energetic particles permeate the Universe and influence the conditions for the emergence and continuation of life. In our solar system, the Sun is the most energetic particle accelerator, and its proximity makes it a unique laboratory in which to explore astrophysical particle acceleration. However, despite its importance, the physics underlying solar particle acceleration remain poorly understood. The SPARK mission will reveal new discoveries about particle acceleration through a uniquely powerful and complete combination of γ-ray, X-ray, and EUV imaging and spectroscopy at high spectral, spatial, and temporal resolutions. SPARK’s instruments will provide a step change in observational capability, enabling fundamental breakthroughs in our understanding of solar particle acceleration and the phenomena associated with it, such as the evolution of solar eruptive events. By providing essential diagnostics of the processes that drive the onset and evolution of solar flares and coronal mass ejections, SPARK will elucidate the underlying physics of space weather events that can damage satellites and power grids, disrupt telecommunications and GPS navigation, and endanger astronauts in space. The prediction of such events and the mitigation of their potential impacts are crucial in protecting our terrestrial and space-based infrastructure.

The Large Imaging Spectrometer for Solar Accelerated Nuclei (LISSAN): A Next-Generation Solar γ-ray Spectroscopic Imaging Instrument Concept

Models of particle acceleration in solar eruptive events suggest that roughly equal energy may go into accelerating electrons and ions. However, while previous solar X-ray spectroscopic imagers have transformed our understanding of electron acceleration, only one resolved image of γ-ray emission from solar accelerated ions has ever been produced. This paper outlines a new satellite instrument concept—the large imaging spectrometer for solar accelerated nuclei (LISSAN)—with the capability not only to observe hundreds of events over its lifetime, but also to capture multiple images per event, thereby imaging the dynamics of solar accelerated ions for the first time. LISSAN provides spectroscopic imaging at photon energies of 40 keV–100 MeV on timescales of ≲10 s with greater sensitivity and imaging capability than its predecessors. This is achieved by deploying high-resolution scintillator detectors and indirect Fourier imaging techniques. LISSAN is suitable for inclusion in a multi-instrument platform such as an ESA M-class mission or as a smaller standalone mission. Without the observations that LISSAN can provide, our understanding of solar particle acceleration, and hence the space weather events with which it is often associated, cannot be complete.

Sequential Remote Brightenings and Co-spatial Fast Downflows during Two Successive Flares

Remote brightenings often appear at the outskirts of the active regions of solar eruptive events. Nevertheless, their origin remains to be ascertained. In this study, we report imaging and spectroscopic observations of sequential remote brightenings with a combination of observations from the Hα Imaging Spectrograph on board the Chinese Hα Solar Explorer, which is the first space-based solar telescope of China, and from the Solar Dynamics Observatory. It is found that during two successive M-class flares occurring on 2022 August 17 multiple ribbon-like brightenings appeared in sequence away from the flaring active region. Meanwhile, abundant cool filament materials moved downward to the sequential remote brightenings, which were visible at the Hα red wing with a line-of-sight speed of up to 70 km s−1. The extrapolated three-dimensional magnetic field configuration shows that the sequential remote brightenings correspond to the footpoints of closed ambient field lines whose conjugate footpoints are rooted in the main flare site. We suggest that the sequential remote brightenings are most likely to be caused by the heating of the interchange reconnection between the erupting flux rope and the closed ambient field, during which the rope-hosting filament materials are transferred to the periphery of the flaring active region along the closed ambient field rather than to the interplanetary space, such as in the scenario of slow solar wind formation.

Ion Acceleration in Solar Eruptive Events

Ion Acceleration in Solar Eruptive Events

Quantifying Energy Release in Solar Flares and Solar Eruptive Events: New Frontiers with a Next-Generation Solar Radio Facility

Quantifying Energy Release in Solar Flares and Solar Eruptive Events: New Frontiers with a Next-Generation Solar Radio Facility

Next-Generation Comprehensive Data-Driven Models of Solar Eruptive Events

Next-Generation Comprehensive Data-Driven Models of Solar Eruptive Events

Strategic Study for the Development of Solar Physics in Space

Solar physics specializes the study of the fundamental processes occurred in the Sun and their influences on the interplanetary space. It deals with detailed measurements that are possible only for our closest star which is a natural plasma laboratory with multi-scale processes taking place. In addition, solar activities have a high impact on the space weather and habitability of the Earth, major solar eruptive events such as solar flare and coronal mass ejection have harmful effects on communication systems, satellites, power grid, and so forth. The study of the Sun not only helps us to understand the universe, the solar-terrestrial relationship, and the planetary habitability, but also serves national strategies in space and aerospace security. In the 21st century, there has been considerable progress in solar physics due to the development of detecting the Sun from space. In this paper, recent space exploration development trends of solar physics are examined both at home and abroad, key fields of the future space exploration development of solar physics in China are summarized, the disciplinary layout is optimized, and high-quality development of solar physics is promoted.

Causality in heliophysics: Magnetic fields as a bridge between the Sun’s interior and the Earth’s space environment

Our host star, the Sun, is a middle-aged main sequence G type star whose activity varies. These variations are primarily governed by solar magnetic fields which are produced in the Sun’s interior via a magnetohydrodynamic dynamo mechanism. Solar activity manifests across different timescales, spanning transient phenomena such as flares, energetic particle events and coronal mass ejections to short to long-term modulation of solar irradiance, plasma winds, open flux and cosmic ray flux in the heliosphere. Collectively, these phenomena define space weather and space climate, which impact the state of the near-Earth space environment, the Earth’s magnetosphere, atmosphere and our space-reliant technologies. Understanding physical processes that are at the heart of solar variability and which causally connect the Sun–Earth system is therefore of immense importance to humanity. Such understanding leads to predictions of the impact of solar activity on our planet and provides a window to explore the plasma universe and other star–planet systems, including assessing the habitability of (exo)planets. In this review, based on our research on the solar–terrestrial system and extant scientific literature, we illuminate processes related to the genesis of solar magnetic fields in the Sun’s interior, their emergence and evolution, their manifestation as solar eruptive events, and their eventual impact on the geospace environment mediated via solar winds and storms. We focus on few phenomena that establish causal connections and demonstrate how our current understanding can lead to development of predictive capabilities encompassing the domain of heliophysics.

Evolution of Solar Eruptive Events: Investigating the Relationships among Magnetic Reconnection, Flare Energy Release, and Coronal Mass Ejections

We study the evolution of solar eruptive events by investigating the temporal relationships among magnetic reconnection, flare energy release, and the acceleration of coronal mass ejections (CMEs). Leveraging the optimal viewing geometry of the Solar TErrestrial RElations Observatory (STEREO) relative to the Solar Dynamics Observatory (SDO) and the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) during 2010–2013, we identify 12 events with sufficient spatial and temporal coverage for a detailed examination. STEREO and SDO data are used to measure the CME kinematics and the reconnection rate, respectively, and hard X-ray (HXR) measurements from RHESSI provide a signature of the flare energy release. This analysis expands upon previous solar eruptive event timing studies by examining the fast-varying features, or “bursts,” in the HXR and reconnection rate profiles, which represent episodes of energy release. Through a time lag correlation analysis, we find that HXR bursts occur throughout the main CME acceleration phase for most events, with the HXR bursts lagging the acceleration by 2 ± 9 minutes for fast CMEs. Additionally, we identify a nearly one-to-one correspondence between bursts in the HXR and reconnection rate profiles, with HXRs lagging the reconnection rate by 1.4 ± 2.8 minutes. The studied events fall into two categories: events with a single dominant HXR burst and events with a train of multiple HXR bursts. Events with multiple HXR bursts, indicative of intermittent reconnection and/or particle acceleration, are found to correspond with faster CMEs.

Robust construction of differential emission measure profiles using a regularized maximum likelihood method

Context.Extreme-ultraviolet (EUV) observations provide considerable insight into evolving physical conditions in the active solar atmosphere. For a prescribed density and temperature structure, it is straightforward to construct the corresponding differential emission measure profileξ(Τ), such thatξ(Τ) dTis proportional to the emissivity from plasma in the temperature range [T,T+ dT]. Here we study the inverse problem of obtaining a validξ(T) profile from a set of EUV spectral line intensities observed at a pixel within a solar image.Aims.Our goal is to introduce and develop a regularized maximum likelihood (RML) algorithm designed to address the mathematically ill-posed problem of constructing differential emission measure profiles from a discrete set of EUV intensities in specified wavelength bands, specifically those observed by the Atmospheric Imaging Assembly (AIA) on the NASA Solar Dynamics Observatory.Methods.The RML method combines features of maximum likelihood and regularized approaches used by other authors. It is also guaranteed to produce a positive definite differential emission measure profile.Results.We evaluate and compare the effectiveness of the method against other published algorithms, using both simulated data generated from parametric differential emission profile forms, and AIA data from a solar eruptive event on 2010 November 3. Similarities and differences between the differential emission measure profiles and maps reconstructed by the various algorithms are discussed.Conclusions.The RML inversion method is mathematically rigorous, computationally efficient, and produces acceptable measures of performance in the following three key areas: fidelity to the data, accuracy in the reconstruction, and robustness in the presence of data noise. As such, it shows considerable promise for computing differential emission measure profiles from datasets of discrete spectral lines.

High Energy Solar Particle Events and Their Relationship to Associated Flare, CME and GLE Parameters

AbstractLarge solar eruptive events, including solar flares and coronal mass ejections (CMEs), can lead to solar energetic particle (SEP) events. During these events, protons are accelerated up to several GeV and pose numerous space weather risks. These risks include, but are not limited to, radiation hazards to astronauts and disruption to satellites and electronics. The highest energy SEPs are capable of reaching Earth on timescales of minutes and can be detected in ground level enhancements (GLEs). Understanding and analyzing these events is critical to future forecasting models. However, the availability of high energy SEP data sets is limited, especially that which covers multiple solar cycles. The majority of analysis of SEP events considers data at energies <100 MeV. In this work, we use a newly calibrated data set using data from Geostationary Operational Environmental Satellite‐high energy proton and alpha detector between 1984 and 2017. Analysis of the SEP events in this time period over three high energy channels is performed, and SEP properties are compared to flare and CME parameters. In addition, neutron monitor (NM) observations are examined for the relevant GLE events. We find that correlations between SEP peak intensity and the CME speed are much weaker than for lower SEP energies. Correlations with flare intensity are broadly similar or weaker. Strong correlations are seen between >300 MeV data and GLE properties from NM data. The results of our work can be utilized in future forecasting models for both high energy SEP and GLE events.

Imaging-spectroscopy of a band-split type II solar radio burst with the Murchison Widefield Array

Type II solar radio bursts are caused by magnetohydrodynamic (MHD) shocks driven by solar eruptive events such as coronal mass ejections (CMEs). Often, both fundamental and harmonic bands of type II bursts are split into sub-bands, which are generally believed to be coming from upstream and downstream regions of the shock; however, this explanation remains unconfirmed. Here, we present combined results from imaging analyses of type II radio burst band splitting and other fine structures observed by the Murchison Widefield Array (MWA) and extreme ultraviolet observations from Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) on 28 September 2014. The MWA provides imaging-spectroscopy in the range 80−300 MHz with a time resolution of 0.5 s and frequency resolution of 40 kHz. Our analysis shows that the burst was caused by a piston-driven shock with a driver speed of ∼112 km s−1 and shock speed of ∼580 km s−1. We provide rare evidence that band splitting is caused by emission from multiple parts of the shock (as opposed to the upstream–downstream hypothesis). We also examine the small-scale motion of type II fine structure radio sources in MWA images, and suggest that this motion may arise because of radio propagation effects from coronal turbulence, and is not due to the physical motion of the shock location. We present a novel technique that uses imaging spectroscopy to directly determine the effective length scale of turbulent density perturbations, which is found to be 1−2 Mm. The study of the systematic and small-scale motion of fine structures may therefore provide a measure of turbulence in different regions of the shock and corona.

Tracking magnetic flux and helicity from the Sun to Earth

Aims.We analyze the complete chain of effects – from the Sun to Earth – caused by a solar eruptive event in order to better understand the dynamic evolution of magnetic-field-related quantities in interplanetary space, in particular that of magnetic flux and helicity.Methods.We study a series of connected events – a confined C4.5 flare, a flare-less filament eruption, and a double-peak M-class flare – that originated in NOAA active region (AR) 12891 on late 2021 November 1 and early 2021 November 2. We deduce the magnetic structure of AR 12891 using stereoscopy and nonlinear force-free (NLFF) magnetic field modeling, allowing us to identify a coronal flux rope and to estimate its axial flux and helicity. Additionally, we compute reconnection fluxes based on flare ribbon and coronal dimming signatures from remote sensing imagery. Comparison to corresponding quantities for the associated magnetic cloud (MC) deduced from in situ measurements from Solar Orbiter and near-Earth spacecraft allows us to draw conclusions on the evolution of the associated interplanetary coronal mass ejection (CME). The latter analysis is aided by the application of geometric fitting techniques (graduated cylindrical shell modeling; GCS) and interplanetary propagation models (drag-based ensemble modeling; DBEM) to the interplanetary CME.Results.NLFF modeling suggests the magnetic structure of the host AR was in the form of a left-handed (negative-helicity) flux rope reaching altitudes of 8−10 Mm above photospheric levels, which is in close agreement with the corresponding stereoscopic estimate. GCS and DBEM modeling suggest that the ejected flux rope propagated in a self-similar expanding manner through interplanetary space. Comparison of magnetic fluxes and helicities processed by magnetic reconnection in the solar source region and the respective budgets of the MC indicate a considerable contribution from the eruptive process, though the pre-eruptive budgets also appear to be relevant.

The time profile of relativistic solar particle events as observed by neutron monitors

The most energetic particles accelerated in solar eruptive events are protons and nuclei with energies that may reach a few tens of GeV. They can be detected on the Earth through the secondaries they produce when interacting with the atmosphere. Solar energetic particle events where this happens are called Ground-Level Enhancements (GLEs). Their study is relevant on the one hand because the high particle energies pose particularly strong challenges to the understanding of the acceleration processes. On the other hand, the secondary particles constitute a source of radiation in the atmosphere that may temporarily exceed the permanent dose rate from galactic cosmic rays. This makes the monitoring of radiation doses received by aircrew from GLEs one issue of space weather services for civil aviation. This study addresses the time profiles of GLEs, in the search for commonalities that can be used to constrain models of acceleration and propagation and to forecast the evolution of an ongoing event. We investigate historical GLEs (1971–2012) with the worldwide network of neutron monitors, comparing the rise and the decay as observed by the neutron monitor with the strongest response. The sample comprises 23 events. We evaluate statistical correlations between rise time and decay time inferred from fits to the time profiles and compute a normalised median GLE time profile. An empirical correlation reported in earlier work between the observed rise times and decay times of the neutron monitor count rate profiles is confirmed. We find indications of a statistical relationship between the rise times and the parent eruptive activity. We discuss ideas on the mechanisms behind the correlation of rise and decay times and on its usefulness for space weather services.