We show that observations of solar $γ$-rays offer a novel probe of dark matter in scenarios where interactions with the visible sector proceed via a long-lived mediator. As a proof of principle, we demonstrate that there exists a class of models which yield solar $γ$-ray fluxes observable with the next generation of $γ$-ray telescopes, while being allowed by a variety of current experimental constraints. The parameter space allowed by big bang nucleosynthesis and beam dump experiments naturally leads to mediator lifetimes sufficient to produce observable solar $γ$-ray signals. The model allows for solar $γ$-ray fluxes up to orders of magnitude larger compared to dwarf spheroidal galaxies, without reaching equilibrium between dark matter annihilation and capture rate. Our results suggest that solar $γ$-ray observations are complementary, and in some cases superior, to existing and future dark matter detection efforts.
The solar modulation effect of Galactic cosmic rays is a time-dependent phenomenon that is caused by the transport of these particles through the magnetized plasma of the heliosphere. Using a data-driven model of cosmic-ray transport in the heliosphere, in combination with a large collection of data, we report the evidence for a eight-month time lag between observations of solar activity and measurements of cosmic-ray fluxes in space. As we will discuss, this result enables us to forecast the cosmic ray flux at Earth well in advance by monitoring solar activity. We also compare our predictions…
We show that solar $γ$-ray observations can provide a complementary probe of Dark Matter in scenarios where the interactions with the Standard Model proceed via long-lived mediators. For illustration we consider a simplified model which provides solar $γ$-ray fluxes observable with the next generation $γ$-ray telescopes, while complying with the existing experimental constraints. Our results suggest that solar $γ$-ray fluxes can be orders of magnitude larger than the ones from the Galactic center, while being subject to low backgrounds.
Secluded dark matter models, in which WIMPs annihilate first into metastable mediators, can present novel indirect detection signatures in the form of gamma rays and fluxes of charged particles arriving from directions correlated with the centers of large astrophysical bodies within the solar system, such as the Sun and larger planets. This naturally occurs if the mean free path of the mediator is in excess of the solar (or planetary) radius. We show that existing constraints from water Cerenkov detectors already provide a novel probe of the parameter space of these models, complementary to ot…
The solar disk is among the brightest gamma-ray sources in the sky. It is also among the most mysterious. No existing model fully explains the luminosity, spectrum, time variability, and morphology of its emission. We perform the first analysis of solar-disk gamma rays over a full 11-year solar cycle, utilizing a powerful new method to differentiate solar signals from astrophysical backgrounds. We produce: (i) a robustly measured spectrum from 100 MeV to 100 GeV, reaching a precision of several percent in the 1-10 GeV range, (ii) new results on the anti-correlation between solar activity and g…
The first evidence of the gamma-ray emission from the quiescent Sun was found in the archival EGRET data that was later confirmed by Fermi-LAT observations with high significance. This emission is produced by Galactic cosmic rays (CRs) penetrating the inner heliosphere and inter- acting with the solar atmosphere and optical photons. The solar emission is characterized by two spatially and spectrally distinct components: (i) disk emission due to the CR cascades in the solar atmosphere, and (ii) spatially extended inverse Compton (IC) emission due to the CR electrons scattering off of solar phot…
At the 33rd ICRC, we reported the possible detection of solar gamma rays by a ground level detector and later re-examined this event. On March 7, 2011, the solar neutron telescope (SNT) located at Mt. Sierra Negra, Mexico (4,600 m) observed enhancements of the counting rate from 19:49 to 20:02 UT and from 20:50 to 21:01 UT. The statistical significance was 9.7sigma and 8.5sigma, respectively. This paper discusses the possibility of using this mountain detector to detect solar gamma rays. In association with this event, the solar neutron detector SEDA-FIB onboard the International Space Station…
On March 7, 2011 from 19:48:00 to 20:03:00 UT, the solar neutron telescope located at Mt. Sierra Negra, Mexico (4,600m) observed a 8.8sigma enhancement. In this paper, we would like to try to explain this enhancement by a hypothesis that a few GeV gamma-rays arrived at the top of the mountain produced by the Sun. We postulate that protons were accelerated at the shock front. They precipitate at the solar surface and produced those gamma-rays. If hypothesis is confirmed, this enhancement is the first sample of GeV gamma-rays observed by a ground level detector.
The Sun is a bright source of GeV gamma rays, due to cosmic rays interacting with solar matter and photons. Key aspects of the underlying processes remain mysterious. The emission in the TeV range, for which there are neither observational nor theoretical studies, could provide crucial clues. The new experiments HAWC (running) and LHAASO (planned) can look at the Sun with unprecedented sensitivity. In this paper, we predict the very high-energy (up to 1000 TeV) gamma-ray flux from the solar disk and halo, due to cosmic-ray hadrons and electrons ($e^++e^-$), respectively. We neglect solar magne…
The possibility of estimating the mass composition of primary cosmic rays above the knee of its energy spectrum through the study of high energy gamma rays, muons and neutrinos produced in the interactions of cosmic rays with the solar ambient matter and radiation has been explored. It is found that the theoretical fluxes of TeV gamma rays, muons and neutrinos from a region around $15^{o}$ of the Sun are sensitive to mass composition of cosmic rays in the PeV energy range. The experimental prospects for detection of such TeV gamma rays/neutrinos by future experiments are discussed.
In a recent Letter (Rimmer et al. 2014), Jupiter is presented as an efficient detector for Ultra-High Energy Cosmic Rays (UHECRs), through measurement by an Earth-orbiting satellite of gamma rays from UHECRs showers produced in Jupiter's atmosphere. We show that this result is incorrect, due to erroneous assumptions on the angular distribution of shower particles. We evaluated other Solar System objects as potential targets for UHECRs detection, and found that the proposed technique is either not viable or not competitive with traditional ground-based UHECRs detectors.
The Alpha Magnetic Spectrometer (AMS) is a new generation high-energy physics experiment installed on the International Space Station in May 2011 and operating continuously since then. Using an unprecedently large collection of particles and antiparticles detected in space, AMS is performing precision measurements of cosmic ray energy spectra and composition. In this paper, we discuss the physics of solar modulation in Galactic cosmic rays that can be investigated with AMS my means of dedicated measurements on the time-dependence of cosmic-ray proton, helium, electron and positron fluxes.
An important factor limiting our ability to understand the production and propagation of cosmic rays pertains to the effects of heliospheric forces, commonly known as solar modulation. The solar wind is capable of generating time and charge-dependent effects on the spectrum and intensity of low energy ($\lsim$ 10 GeV) cosmic rays reaching Earth. Previous analytic treatments of solar modulation have utilized the force-field approximation, in which a simple potential is adopted whose amplitude is selected to best fit the cosmic-ray data taken over a given period of time. Making use of recently a…
First Observations of Solar Halo Gamma Rays Over a Full Solar Cycle
Stockholm University（斯德哥尔摩大学）；The Oskar Klein Centre for Cosmoparticle Physics（奥斯卡·克莱因宇宙粒子物理中心）；Friedrich-Alexander-Universität Erlangen-Nürnberg（埃尔朗根-纽伦堡弗里德里希·亚历山大大学）；Center for Cosmology and AstroParticle Physics, The Ohio State University（俄亥俄州立大学宇宙学与天体粒子物理中心）；Fermi National Accelerator Laboratory（费米国家加速器实验室）；Kavli Institute for Cosmological Physics, University of Chicago（芝加哥大学卡弗里宇宙物理学研究所）
The H.E.S.S. collaboration recently published its Galactic Plane Survey, which is a deep survey of half of the Milky Way in TeV gamma-rays, using almost 10 years of observations. It contains 78 sources, including 16 new sources. On the other hand, the HAWC collaboration also published its 2HWC catalogue based on 507 days of data. It is the result of the first source search performed with the complete HAWC detector, covering two thirds of the sky. In this search, 39 sources have been reported, amongst them 16 detected for the first time at TeV energies. The HAWC data set doubled since then, rev…
ESA's INTEGRAL space mission has achieved unique results for solar and terrestrial physics, although spacecraft operations nominally excluded the possibility to point at the Sun or the Earth. The Earth avoidance was, however, exceptionally relaxed for special occultation observations of the Cosmic X-ray Background (CXB), which on some occasions allowed the detection of strong X-ray auroral emission. In addition, the most intense solar flares can be bright enough to be detectable from outside the field of view of the main instruments. This article presents for the first time the auroral observa…
Galactic cosmic rays and solar protons ionize the present terrestrial atmosphere, and the air showers are simulated by well-tested Monte-Carlo simulations, such as PHITS code. We use the latest version of PHITS to evaluate the possible ionization of protoplanetary disks by galactic cosmic rays (GCRs), solar protons, and by supernova remnants. The attenuation length of GCR ionization is updated as 118 g cm-2, which is approximately 20% larger than the popular value. Hard and soft possible spectra of solar protons give comparable and 20% smaller attenuation lengths compared with those from stand…
The impact of Solar magnetic field configurations on the production of gamma rays at the Solar disk
Ruhr-Universität Bochum（波鸿鲁尔大学）；Ruhr Astroparticle and Plasma Physics Center (RAPP Center)（鲁尔天体粒子与等离子体物理中心（RAPP中心））；Chalmers University of Technology（查尔姆斯理工大学）；Lockheed Martin Solar and Astrophysics Laboratory（洛克希德·马丁太阳与天体物理学实验室）；Bay Area Environmental Research Institute, NASA Research Park（美国宇航局研究公园湾区环境研究所）；Stanford University（斯坦福大学）
In the solar flare observed on June 3, 2012, high energy gamma-rays and neutrons were observed. The event includes a remarkable feature of a high neutron/gamma-ratio in the secondary particles. We have examined whether this high n/$γ$-ratio can be explained by simulation. As a result of simulations using the GEANT4 program, the high n/$γ$-ratio may be reproduced for the case that helium and other heavy ions were dominantly accelerated in the flare.
Measurements of the low energy spectrum of Galactic Cosmic Rays (GCR) by detectors at or near the Earth are affected by Solar modulation. To overcome this difficulty, we consider nearby molecular clouds as GCR detectors outside the Solar system. Using gamma-ray observations of the clouds by the Fermi telescope we derive the spectrum of GCRs in the clouds from the observed gamma-ray emission spectrum. We find that the GCR spectrum has a low energy break with the spectral slope hardening by 1.1+/-0.3 at an energy of 9+/-3 GeV. Detection of a low-energy break enables a measurement of GCR energy d…
We study the time-dependent modulation effect and derive the local interstellar spectra (LIS) for the cosmic ray (CR) proton, helium, boron and carbon. A two-dimensional modulation model including the variation of the interplanetary environment with time is adopted to describe modulation process. The propagation equation of CRs in the heliosphere is numerically solved by the package Solarprop. We derive the LIS by fitting the latest results of several experiments, including Voyager 1, PAMELA, BESS-POLARII and ACE, during low solar activity periods. We further study the modulation in the polari…
The solar disk is a bright gamma-ray source. Surprisingly, its flux is about one order of magnitude higher than predicted. As a first step toward understanding the physical origin of this discrepancy, we perform a new analysis in 1-100 GeV using 6 years of public Fermi-LAT data. Compared to the previous analysis by the Fermi Collaboration, who analyzed 1.5 years of data and detected the solar disk in 0.1-10 GeV, we find two new and significant results: 1. In the 1-10 GeV flux (detected at $>5σ$), we discover a significant time variation that anticorrelates with solar activity. 2. We detect gam…
Indian Centre for Space Physics is engaged in pioneering balloon borne experiments with typical payloads less than ~ 3.5kg. Low cost rubber balloons are used to fly them to a height of about 40km. In a double balloon system, the booster balloon lifts the orbiter balloon to its cruising altitude where data is taken for a longer period of time. In this Paper, we present our first scientific report on the variation of Cosmic Rays and muons with altitude and detection of several solar flares in X-rays between 20keV and 100keV. We found the altitude of the Pfotzer maximum at Tropic of Cancer for co…
The flux of cosmic rays in the heliosphere is subjected to variations that are related to the Sun's magnetic activity. To study this effect, updated time series of multichannel observations are needed. Here we present a web application that collects real-time data on solar activity proxies, interplanetary plasma parameters, and charged cosmic-ray data. The data are automatically retrieved on daily basis from several space missions or observatories. With this application, the data can be visualized and download into a common format. Along with observational data, the application aims to provide…
The solar disk is a bright source of multi-GeV gamma rays, due to the interactions of hadronic cosmic rays with the solar atmosphere. However, the underlying production mechanism is not understood, except that its efficiency must be greatly enhanced by magnetic fields that redirect some cosmic rays from ingoing to outgoing before they interact. To elucidate the nature of this emission, we perform a new analysis of solar atmospheric gamma rays with 9 years of Fermi-LAT data, which spans nearly the full 11-year solar cycle. We detect significant gamma-ray emission from the solar disk from 1 GeV …
The Spectrometer/Telescope for Imaging X-rays (STIX) will look at solar flares across the hard X-ray window provided by the Solar Orbiter cluster. Similarly to the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), STIX is a visibility-based imaging instrument, which will ask for Fourier-based image reconstruction methods. However, in this paper we show that, as for RHESSI, also for STIX count-based imaging is possible. Specifically, here we introduce and illustrate a mathematical model that mimics the STIX data formation process as a projection from the incoming photon flux into a…
Three dimensional (3D) galactic cosmic ray (GCR) anisotropy has been studied for 2006- 2012. The GCR anisotropy, both in the ecliptic plane and in polar direction, were obtained based on the neutron monitors (NMs) and Nagoya muon telescopes (MT) data. We analyze two dimensional (2D) GCR anisotropy in the ecliptic plane and north-south anisotropy normal to the ecliptic plane. We reveal quasi-periodicities - the annual and 27-days waves in the GCR anisotropy in 2006-2012. We investigate the relationship of the 27-day variation of the GCR anisotropy in the ecliptic plane and in the polar directio…
Aims. We study the 27-day variations of galactic cosmic rays (GCRs) based on neutron monitor (NM), ACE/CRIS, STEREO and SOHO/EPHIN measurements, in solar minima 23/24 and 24/25 characterized by the opposite polarities of solar magnetic cycle. Now there is an opportunity to reanalyze the polarity dependence of the amplitudes of the recurrent GCR variations in 2007-2009 for negative A < 0 solar magnetic polarity and to compare it with the clear periodic variations related to solar rotation in 2017-2019 for positive A > 0. Methods. We use the Fourier analysis method to study the periodicity in th…
The solar modulation effect of cosmic rays in the heliosphere is an energy-, time-, and particle-dependent phenomenon which arises from a combination of basic particle transport processes such as diffusion, convection, adiabatic cooling, and drift motion. Making use of a large collection of time-resolved cosmic-ray data from recent space missions, we construct a simple predictive model of solar modulation which depends on direct solar-physics inputs: the number of solar sunspots and the tilt angle of the heliospheric current sheet. Under this framework, we present calculations of cosmic-ray pr…
We report the first detection of >100 MeV gamma rays associated with a behind-the-limb solar flare, which presents a unique opportunity to probe the underlying physics of high-energy flare emission and particle acceleration. On 2013 October 11 a GOES M1.5 class solar flare occurred ~ 9.9 degrees behind the solar limb as observed by STEREO-B. RHESSI observed hard X-ray emission above the limb, most likely from the flare loop-top, as the footpoints were occulted. Surprisingly, the Fermi Large Area Telescope (LAT) detected >100 MeV gamma-rays for ~30 minutes with energies up to GeV. The LAT emiss…
Dark matter capture and annihilation in the Sun can produce detectable high-energy neutrinos, providing a probe of the dark matter-proton scattering cross section. We consider the case when annihilation proceeds via long-lived dark mediators, which allows gamma rays to escape the Sun and reduces the attenuation of neutrinos. For gamma rays, there are exciting new opportunities, due to detailed measurements of GeV solar gamma rays with Fermi, and unprecedented sensitivities in the TeV range with HAWC and LHAASO. For neutrinos, the enhanced flux, particularly at higher energies ($\sim$TeV), allo…
Observationally, it is difficult to establish at what rigidity the modulation of galactic cosmic rays (CRs) actually begins in the heliosphere. It should be possible to do if the relevant local interstellar CR spectra were known and reliable measurements were made between 10 GV and about 200 GV, inside and outside the heliosphere. Numerical models for solar modulation studies are based on simply assuming that CR modulation begins at a given spatial boundary and at rigidities between 30 to 50 GV, usually handled as an initial condition. The Stochastic Differential Equations approach to numerica…
When traveling in the heliosphere, Galactic cosmic rays (GCRs) are subjected to the solar modulation effect, a quasiperiodical change of their intensity caused by the 11-year cycle of solar activity. Here we investigate the association of solar activity and cosmic radiation over five solar cycles, from 1965 to 2020, using a collection of multichannel data from neutron monitors, space missions, and solar observatories. In particular, we focus on the time lag between the monthly sunspot number and the GCR flux variations. We show that the modulation lag is subjected to a 22-year periodical varia…
Galactic cosmic rays (CRs) inside the heliosphere are affected by solar modulation. To investigate this phenomenon and its underlying physical mechanisms, we have performed a data-driven analysis of the temporal dependence of the CR proton flux over the solar cycle. The modulation effect was modeled by means of stochastic simulations of cosmic particles in the heliosphere. The model were constrained using measurements of CR protons made by AMS-02 and PAMELA experiments on monthly basis from 2006 to 2017. With a global statistical analysis of these data, we have determined the key model paramet…
A key goal of heliophysics is to understand how cosmic rays propagate in the solar system's complex, dynamic environment. One observable is solar modulation, i.e., how the flux and spectrum of cosmic rays changes as they propagate inward. We construct an improved force-field model, taking advantage of new measurements of magnetic power spectral density by Parker Solar Probe to predict solar modulation within the Earth's orbit. We find that modulation of cosmic rays between the Earth and Sun is modest, at least at solar minimum and in the ecliptic plane. Our results agree much better with the l…
During the last few years our knowledge about the X-ray emission from bodies within the solar system has significantly improved. Several new solar system objects are now known to shine in X-rays at energies below 2 keV. Apart from the Sun, the known X-ray emitters now include planets (Venus, Earth, Mars, Jupiter, and Saturn), planetary satellites (Moon, Io, Europa, and Ganymede), all active comets, the Io plasma torus (IPT), the rings of Saturn, the coronae (exospheres) of Earth and Mars, and the heliosphere. The advent of higher-resolution X-ray spectroscopy with the Chandra and XMM-Newton X-…
Neutrinos generated during solar flares remain elusive. However, after $50$ years of discussion and search, the potential knowledge unleashed by their discovery keeps the search crucial. Neutrinos associated with solar flares provide information on otherwise poorly known particle acceleration mechanisms during solar flare. For neutrino detectors, the separation between atmospheric neutrinos and solar flare neutrinos is technically encumbered by an energy band overlap. To improve differentiation from background neutrinos, we developed a method to determine the temporal search window for neutrin…
The maximum cosmic ray energy achievable by acceleration by a relativistic blast wave is derived. It is shown that forward shocks from long GRB in the interstellar medium are powerful enough to produce the Galactic cosmic-ray component up to the ankle at $4\times 10^{18}$eV, as per an earlier suggestion (Levinson and Eichler, 1993). It is further argued that, were extragalactic long GRB responsible for the component {\it above} the ankle as well, the contribution from an occasional Galactic GRB within the solar circle would yield more than the observational limits on the outward flux from the …
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 gamma-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 …
In this paper, we study the galactic cosmic ray (GCR) variations over the solar cycles 23 and 24, with measurements from the NASA's ACE/CRIS instrument and the ground-based neutron monitors (NMs). The results show that the maximum GCR intensities of heavy nuclei (nuclear charge 5-28, 50-500 MeV/nuc) at 1 AU during the solar minimum in 2019-2020 break their previous records, exceeding those recorded in 1997 and 2009 by ~25% and ~6%, respectively, and are at the highest levels since the space age. However, the peak NM count rates are lower than those in late 2009. The difference between GCR inte…
The possible effect of solar variability, accordingly cosmic rays variation on atmospheric physics and chemistry is highly debated over the last years. According to several recent models the induced by cosmic rays atmospheric ionization plays a key role in several different processes. At recent, an apparent effect on minor constituents and aerosols over polar regions during major solar proton events was observed. The ion production rate during ground level enhancements is a superposition of the contribution of cosmic rays with galactic and solar origin. The solar cycle 23 provided several stro…
X-ray observatories contribute fundamental advances in Solar System studies by probing Sun-object interactions, developing planet and satellite surface composition maps, probing global magnetospheric dynamics, and tracking astrochemical reactions. Despite these crucial results, the technological limitations of current X-ray instruments hinder the overall scope and impact for broader scientific application of X-ray observations both now and in the coming decade. Implementation of modern advances in X-ray optics will provide improvements in effective area, spatial resolution, and spectral resolu…
It is suggested that essentially all of the UHECRs we detect, including those at the highest energy, originate in our Galaxy. It is shown that even if the density of sources decreases with Galactic radius, then the anisotropy and composition can be understood. Inward anisotropy, as recently reported by the Auger collaboration can be understood as drift along the current sheet of UHECRs originating outside the solar circle, as predicted in Kumar and Eichler (2014), while those originating within the solar circle exit the Galaxy at high latitudes.
Mars was observed in X-rays during April 3-5 2008 for 82 ksec with the Japanese Suzaku observatory. Mars has been known to emit X-rays via the scattering of solar X-rays and via the charge exchange between neutral atoms in the exosphere and solar wind ions. Past theoretical studies suggest that the exospheric neutral density may vary by a factor of up to 10 over the solar cycle. To investigate a potential change of the exospheric charge exchange emission, Mars was observed with Suzaku at solar minimum. Significant signals were not detected at the position of Mars in the energy band of 0.2-5 ke…
Cosmic rays in the inner solar system are subject to deflection by both the geomagnetic and interplanetary magnetic fields, and simultaneously interact with the Sun's photosphere resulting in the production of gamma rays. This phenomenon can be studied by observing the deficit ("shadow") in the cosmic ray flux from the direction of the Sun and searching for an excess photon signal above the isotropic background. The High Altitude Water Cherenkov (HAWC) Observatory in Mexico has been taking data on the solar disk at TeV energies since the end of 2014. We present our first efforts to estimate th…
We investigate accelerated electron energy spectra for different sources in a large flare using simultaneous observations obtained with two instruments, the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz, and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) at hard X-rays. This flare is one of the few in which emission up to energies exceeding 200 keV can be imaged in hard X-rays. Furthermore, we can investigate the spectra of individual sources up to this energy. We discuss and compare the HXR and microwave spectra and morphology. Although the event overall appears to corr…
The heliospheric modulation model HelMod solve the transport-equation for the Galactic Cosmic Rays propagation through the heliosphere down to Earth. It is based on a 2-D Monte Carlo approach, that includes a general description of the symmetric and antisymmetric parts of the diffusion tensor, thus, properly treating the particle drift effects as well as convection within the solar wind and adiabatic energy loss. The model was tuned in order to fit 1) the data observed outside the ecliptic plane at several distances from the Earth and 2) the spectra observed near the Earth for both, high and l…
