The Aditya-L1 is the first space-based solar observatory of the Indian Space Research Organization (ISRO). The spacecraft will carry seven payloads providing uninterrupted observations of the Sun from the first Lagrangian point. Aditya-L1 comprises four remote sensing instruments, viz. a coronagraph observing in visible and infrared, a full disk imager in Near Ultra-Violet (NUV), and two full-sun integrated spectrometers in soft X-ray and hard X-ray. In addition, there are three instruments for in-situ measurements, including a magnetometer, to study the magnetic field variations during energetic events. Aditya-L1 is truly a mission for multi-messenger solar astronomy from space that will provide comprehensive observations of the Sun across the electromagnetic spectrum and in-situ measurements in a broad range of energy, including magnetic field measurements at L1.

A characteristic pattern of solar hard X-ray emission, first identified in SOL1969-03-30 by Frost & Dennis (1971), turns out to have a close association with the prolonged high-energy gamma-ray emission originally observed by Forrest et al. (1985). This identification has become clear via the observations of long-duration γ-ray flares by the Fermi/LAT experiment, for example in the event SOL2014-09-01. The distinctive features of these events include flat hard X-ray spectra extending well above 100 keV, a characteristic pattern of time development, low-frequency gyrosynchrotron peaks, CME association, and gamma-rays identifiable with pion decay originating in GeV ions. The identification of these events with otherwise known solar structures nevertheless remains elusive, in spite of the wealth of EUV imagery available from SDO/AIA. The quandary is that these events have a clear association with SEPs in the high corona, and yet the gamma-ray production implicates the photosphere itself, despite the strong mirror force that should focus the particles away from the Sun We discuss the morphology of these phenomena and propose a solution to this problem.

Modelling of Solar Energetic Particles (SEPs) is usually carried out by means of the 1D focused transport equation and the same approach is adopted within several SEP Space Weather forecasting frameworks. We present an alternative approach, based on test particle simulations, which naturally describes 3D particle propagation. The SPARX forecasting system is an example of how test particle simulations can be used in real time in a Space Weather context. SPARX is currently operational within the COMESEP Alert System. The performance of the system, which is triggered by detection of a solar flare of class >M1.0 is evaluated by comparing forecasts for flare events between 1997 and 2017 with actual SEP data from the GOES spacecraft.

Forecasting Solar Energetic Particle (SEP) fluence, as integrated over an SEP event, is an important element when estimating the effect of solar eruptions on humans and technology in space. Current real-time estimates are based on SEP measurements at a single location in space. However, the interplanetary magnetic field corotates with the Sun approximately 13° each day with respect to Earth, thus in 4 days a near-Earth spacecraft will have changed their connection about 60° from the original SEP source. We estimate the effect of the corotation on particle fluence using a simple particle transport model, and show that ignoring corotation can cause up to an order of magnitude error in fluence estimations, depending on the interplanetary particle transport conditions. We compare the model predictions with STEREO observations of SEP events.

At present two concurrent paradigms of solar energetic particle (SEP) origin exist: acceleration directly in the flare site or by the shock wave of coronal mass ejection (CME). Active discussions on a relative role of flares and coronal mass ejections for SEP acceleration and propagation are continuous until now. In my opinion only future observations of solar high energy γ–emission with better spectral, spatial and temporal resolution may clarify this issue. In my report I discuss possible signatures of the flare and shock acceleration processes. What is a picture provided by the current instruments? What can we expect to observe with a perfect instrument in high energy gamma rays in one or another case on a time scale of impulsive and long decay flare phases?

We present the analysis of a large solar near-relativistic (>50 keV) electron event observed by the Wind spacecraft on 1998 April 20. In-situ data show variations of the local magnetic field direction accompanied by changes in the topology of the observed electron pitch-angle distributions. These suggest changes in the magnetic flux tubes scanned by Wind. Using simulations of the interplanetary particle transport, we model the early rising phase of the electron event, from 10:30 to 11:00 UT, and infer the propagation conditions and the injection history of the first arriving 50–82 keV solar electrons. The results reveal a prompt (≤1 min) release in coincidence with the soft X-ray and type III radio bursts, suggesting that the first arriving electrons were flare-accelerated.

We study the temporal and spatial evolution of magnetic field parameters, and multi-wavelength observations of the AR 11226 and AR 11261, observed during 02-09 June 2011 and 31 July to 06 August 2011 respectively, to probe the origin of the associated solar energetic particles (SEPs). The magnetic-field toplogy of both ARs shows complex magnetic structure of β and βγ configuration. The X-ray observations by GOES and RHESSI reveal that the AR 11226 as well as AR 11261 has been most turbulent to produce flares accompanying CMEs and SEPs. We restrict our study of magnetic field evolution of ARs to helio-longitudes E20-W70. The AR 11226 produced 28 flares (⩾ GOES C-class) and 8 CMEs as well as 5 SEPs. On the other hand, the AR 11261 produced 29 flares (⩾ GOES C class), 8 CMEs as well as 5 SEPs. We study the temporal evolution of magnetic field parameters viz. gradient, shear, rotation to probe the origin of the energy release in the flare and associated CME and SEP. We do not find convincing relationship in view of predicting SEPs.

The total spectral irradiance of the Sun is seen to vary on many time scales. Three timescales are more prominent: (1) the longest one of about 11 years; (2) an intermediate timescale of the order of a few weeks; and (3) the shortest variation from hours to seconds. Every 11 years, the total solar irradiance periodically shows intervals of great activity and periods of almost no activity. The peak to peak variability, however, is less than 0.1%. This periodic variation of 11 years is called the solar cycle, which main tracer are sunspots. During times of maximum activity, there are many sunspots on the surface of the Sun, whereas during minimum there may be none. Sunspots are dark, and therefore cool, regions of enhanced magnetic fields of about a few hundred Gauss, that usually appear in groups on the solar photosphere. Basically, the solar cycle is regulated by the magnetic dynamo acting below the solar surface. Right now, the Sun is going through a time of minimum activity. The sunspot lifetime is of the order of one to two weeks, and are thus responsible for the intermediate variability timescale. The magnetic loops seen in ultraviolet and X-ray images have their footpoints anchored on sunspots. The most energetic phenomena of solar activity are flares and coronal mass ejections. Flares are large explosions that occur on the solar atmosphere and may last from a few seconds to hours. A solar flare is caused by a sudden, and yet unpredicted, energy release high above the magnetic loops. This magnetic energy is then used into particle acceleration and heating of the surrounding atmosphere. Both the energetic particles and the hot gas produce emission throughout the whole electromagnetic spectrum, from the very energetic gamma-rays all the way to long radio waves. From the observation of the emission produced during flares it is possible to infer the energetic particles spectra and thus have a clue on the acceleration mechanism that produced these particles. The recent findings of flare observations at gamma-rays by the RHESSI satellite and at high radio frequencies by the Solar Submillimeter Telescope are presented and discussed.

Proper characterization of the host star to a planet is a key element to the understanding of its overall properties. The star has a direct impact through the modification of the structure and evolution of the planet atmosphere by being the overwhelmingly larger source of energy. The star plays a central role in shaping the structure, evolution, and even determining the mere existence of planetary atmospheres. The vast majority of the stellar flux is well understood thanks to the impressive progress made in the modeling of stellar atmospheres. At short wavelengths (X-rays to UV), however, the information is scarcer since the stellar emission does not originate in the photosphere but in the chromospheric and coronal regions, which are much less understood. The same can be said about particle emissions, with a strong impact on planetary atmospheres, because a detailed description of the time-evolution of stellar wind is still lacking. Here we review our current understanding of the flux and particle emissions of the Sun and low-mass stars and briefly address their impact in the context of planetary atmospheres.

We present a study of solar energetic particles (SEPs) in association with coronal mass ejections (CMEs) and type II radio bursts. The particle and CME observations cover the years 1996–2007. We find that heavy-ion events in association with type II bursts and proton events are produced in more western and most energetic CMEs. In addition, the source distribution of type II associated proton events with heavy ions reminds the source distribution expected for events with flare particles. Therefore, the estimation of relative contributions by flares and shocks in SEP events and separation of suggested different particle acceleration models is complicated.

Plasma turbulence at various length scales affects practically all mechanisms proposed to be responsible for particle acceleration in the heliosphere. In this paper, we concentrate on providing a synthesis of some recent efforts to understand particle acceleration in the solar corona and inner heliosphere. Acceleration at coronal and interplanetary shock waves driven by coronal mass ejections (CMEs) is the most viable mechanism for producing large gradual solar energetic particle (SEP) events, whereas particle acceleration in impulsive flares is assumed to be responsible for the generation of smaller impulsive SEP events. Impulsive events show enhanced abundances of 3He and heavy ions over the gradual SEP events. Gradual events often show charge states consistent with acceleration of ions in a dilute plasma at 1–2 MK temperature, while impulsive events have higher charge states. The division of SEP events to gradual and impulsive has been challenged by the discovery of events, which show intensity-vs.-time profiles typical for gradual events but, especially at the highest energies (above 10 MeV/nucl), abundances and charge states more typical of impulsive events. Although a direct flare component cannot be ruled out, we find that particle acceleration at quasi-perpendicular shocks in the low corona also offer a plausible explanation for the hybrid events. By carefully modeling shock acceleration and coronal turbulence and its modification by the accelerated particles, a consistent picture of gradual events thus emerges from the shock acceleration hypothesis.

The current contribution investigates the solar flare of 16th August 2004 with the multi-wavelength observations with high temporal resolution from RHESSI, Large Solar Vacuum Telescope (LSVT), Hiraiso Solar observatory, Nobeyama Radioheliograph (NoRH, 17 and 34 GHz) and Siberian Solar Radio Telescope (SSRT, 5.7 GHz), TRACE. The main flare was preceded by a pre-flare event with a very short energy release time. The observations of the main flare reveal a close temporal correlation between the Hα intensity observed with LSVT and those in hard and soft X-ray emissions observed with RHESSI, and in microwave fluxes observed with NoRH and SSRT. This close temporal correlation can be only associated with high-energy particles. The role of energetic particles in energy transport and non-thermal excitation and ionisation on Hα emission during the pre-flare and pre-flare event is investigated with full non-LTE approach and possible agents and scenarios of energy transport are discussed.

Properties of near-relativistic (E ≳ 30 keV, NR) and relativistic (E ≳ 0.3 MeV) electron events produced near the Sun and observed within 1 AU are reviewed. Observations suggest the CME-driven shocks are the sources of many events, but flares are often sources for NR events.

Major properties of the solar atmosphere and wind are not understood. The energy distribution of the solar atmosphere perturbations, over nearly ten orders of magnitude in energy, is close (within one order of magnitude) to a power-law of index -2, even though there is no agreement on the detailed shape. There is no agreed explanation of the origin of the solar wind, either, nor of the fact that the total wind energy flux is independent of speed, latitude, and phase of solar cycle, with an average base flux of 70 W/m$^2$ – a figure that is similar for a number of other cool stellar winds, and is close to the total observed energy flux of solar atmosphere perturbations. A major theoretical difficulty is that both coronal heating and wind production depend fundamentally on the heat flux, and the solar atmosphere is not collisional enough for classical transport theory to hold. Heat transport thus behaves non classically and the particle velocity distributions may have significant high energy tails, which should dramatically affect coronal heating and solar wind acceleration, in a way that is outside the scope of standard MHD.

From all the transient events identified in interplanetary space by in-situ measurements, Magnetic Clouds (MCs) are among the most intriguing ones. They are a special kind of Interplanetary Coronal Mass Ejections (ICMEs), characterized by a well-defined magnetic field configuration. We use a list of 40 MCs detected by Ulysses to study bidirectional flows of protons in the $\sim$0.5 MeV energy range. Solar wind ions are also analysed in order to compare cloud to non-cloud ICMEs.

The enhancement in freezing-in temperatures inside the clouds, obtained with data from the SWICS instrument, provides insights into processes occurring early during the ejection of the material and represents a complementary tool to differentiate cloud from non-cloud ICMEs. At higher energies, directional information for protons obtained with the EPAC instrument allows a comparison with previous results concerning bidirectional suprathermal electrons. The findings are qualitatively comparable. Apparently, the portion of bidirectional flows inside magnetic clouds is neither heavily dependent on distance from the Sun nor on parameters obtained from a flux rope model.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

On acceleration of relativistic electrons by any means, the coherent pinch effect brings the relativistic electrons together. The inclusion of a suitable number of positive ions, preferentially heavy ions would stabilize the beam. The heavy trapped nuclei would have to go with the electrons, thus acquire relativistic speeds. The electron beam may be divided into small bunches. This mechanism can accelerate positive nuclei to GeV and TeV energies during solar flares, stellar and galactic high energy events. During the acceleration of 1000 GeV electron bunches, the energy gained by trapped positive nuclei is estimated to reach 10$^{15}$ eV per nucleon. Higher energies can be achieved by head on collision as the energy gained by the target nucleus is proportional to $\gamma^{2},$ however fission reactions and very high energy gamma rays may occur. The principle of this mechanism is applied in electron-ring accelerators.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

The solar sources of near-relativistic $(E > $30 keV) electron events observed at 1 AU are poorly understood. In general, the solar injection times deduced from the observed 1 AU onset times and assumed 1.2 AU travel distances yield injection times about 10 minutes after the associated flare impulsive phases and type III radio burst times. One interpretation is that the apparent delays occur in the interplanetary medium, probably due to scattering of the electrons. If the injection times are delayed from the impulsive phases, the electron acceleration might take place in CME-driven shocks. Here a large number of electron events observed with the UC/Berkeley 3DP detector on the Wind spacecraft are compared with CMEs observed by the Lasco coronagraph on SOHO and with type II bursts observed by the 40 to 800 MHz radio receiver at the Astrophysikalisches Institut Potsdam (AIP) and by the 20 kHz to 14 MHz WAVES instrument on the Wind spacecraft. The acceleration of at least some of the electron events is not consistent with the shock hypothesis.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Very large solar energetic particle (SEP) events occur at the rate of about 10 per year during solar maximum. The primary accelerators in these events are shocks driven by fast coronal mass ejections, in which speeds are greater than about 1000 km/s. Solar Cycle 23 probably produced the largest fluence of > 10 MeV/nucleon SEPs seen since the start of the Space Age. New instruments on Wind, ACE, SOHO, and other satellites have provided unprecedented detail on the energy spectra, elemental and isotopic composition, ionic charge states, and temporal evolution of these SEP events, as well as their associated CMEs and flares. In this talk, I will review some of the new insights provided by these data. A particular challenge in SEP studies has been the very large event-to-event variability in composition and spectral characteristics, particularly at energies above a few tens of MeV per nucleon. I will discuss recent efforts to understand this variability in terms of seed populations and shock geometry. I will also review recent studies of the time at which SEPs first appear on the Sun-Earth magnetic field line and the implications of these studies for the conditions under which SEP production was initiated.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

Large Solar Energetic Particles (SEPs) are closely associated with coronal mass ejections (CMEs). The significant correlation observed between SEP intensity and CME speed has been considered as the evidence for such a close connection. The recent finding that SEP events with preceding wide CMEs are likely to have higher intensities compared to those without was attributed to the interaction of the CME-driven shocks with the preceding CMEs or with their aftermath. It is also possible that the intensity of SEPs may also be affected by the properties of the solar source region. In this study, we found that the active region area has no relation with the SEP intensity and CME speed, thus supporting the importance of CME interaction. However, there is a significant correlation between flare size and the active region area, which probably reflects the spatial scale of the flare phenomenon as compared to that of the CME-driven shock.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html

We describe a set of millimeter interferometric observations of solar flares carried out in conjunction with GRO experiments during the 1991 June Campaign of the Max'91 Program. We show evidence that millimeter emission probes the most energetic (MeV) electrons in solar flares; we also find that in the same flare there can be both impulsive nonthermal and gradual thermal millimeter emission. Millimeter emission usually occurs at the steep rise phase of the hard X-ray emitting electrons (25-100 KeV). There appears to exist some delay between BIMA mm-emission onset and GRO-BATSE 25-100 KeV X-ray emission. Both results have implications for the particle acceleration process.