arXiv subjects

N. Gopalswamy

Publications and source records attributed to N. Gopalswamy.

At least 19 recordsLinked to original sources

arXiv2025

Solar Cycle Variation of Sustained Gamma-ray Emission Events from the Sun and Related Energetic Events

The sustained gamma ray emission (SGRE) from the Sun is one of the fascinating high energy phenomena closely related to the acceleration of protons to energies >300 MeV. Here we report on the solar cycle variation of SGRE events based on observations from Fermi's Large Area Telescope (LAT). This report covers solar cycles (SCs) 24 and 25 during which Fermi has been operating. Since SGRE events are closely related to solar energetic particle (SEP) events and interplanetary type II radio bursts caused by fast and wide coronal mass ejections (CMEs), we consider these phenomena as well. Many studies have shown that SC 25 is similar or slightly stronger than SC 24. The number of SEP events, GLE events, IP type II bursts, and fast and wide CMEs confirm this conclusion. However, the number of SGRE events observed by Fermi LAT has diminished significantly in SC 25 relative to SC 24. One of the issues has been the reduced coverage of the Sun since 2018 due to a mechanical problem with a solar array of the Fermi mission. By identifying the Fermi LAT gaps and the number of energetic events (fast and wide CMEs, interplanetary type II bursts) we conclude that about three times more SGRE events must have occurred than the 15 events observed by Fermi.

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arXiv2025

Imaging and Radio Signatures of Shock-Plasmoid Interaction

Understanding how shocks interact with coronal structures is crucial for understanding the mechanisms of particle acceleration in the solar corona and inner heliosphere. Using simultaneous radio and white-light observations, we investigate the interaction between a CME-driven shock and a plasmoid. LASCO and STEREO-A COR-2 white-light images are analyzed to track the evolution of the plasmoid, CME and its associated shock, while the Wind/WAVES and STEREO/WAVES dynamic spectra provide complementary radio signatures of the shock-plasmoid interaction at $\approx$7 R$_\odot$. An interplanetary Type II radio burst was detected as the shock propagated through the plasmoid. The merging of the plasmoid into the CME was accompanied by interplanetary Type III radio bursts, suggesting escaping electron beams during the reconnection process. These observations clearly demonstrate that shock-plasmoid interactions can enhance the efficiency of particle acceleration associated with CMEs, with implications for electron acceleration in flare and heliospheric current sheets as well.

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arXiv2025

CME activities on spotless days during descending phase of solar cycles 23 and 24

Spotless days (SLDs) as well as CMEs in the decay phase of the solar cycle are believed to be a good predictor of the forthcoming cycle. A sequential increase in SLDs is observed since cycle 21, and cycle 24 has the highest number of SLDs (since cycle 14), which offers a unique opportunity to probe the CME characteristics that occurred during SLDs (hereafter CME_SLD), in a statistical sense. Here, we investigate the CME_SLD during the descending phases of solar cycles 23 (2004-2008) and 24 (2015-2019). The fraction of CMEs that occurred on SLDs is found to be 14 and 11%, for cycles 23 and 24, respectively, compared to the total CMEs that occurred in the aforementioned durations. CME_SLD that occurred on the visible side of the solar disk are found to be slower, smaller in width, and carrying low Kinetic energy and mass compared to the entire population of CMEs. The distribution of annual evolution of speed, angular width and acceleration of the CMESLD with the CMEs that occurred on the non-SLD days (hereafter CME_Non-SLD) for the descending phases shows that the CME_SLD are different from the CME_non-SLD in terms of characteristics (such as speed, width and acceleration). A comparative analysis of CME_SLD kinematics of cycle 23 and 24 shows that the weakest cycle 24 has wider and more massive events. In contrast, other parameters of CME_SLD such as speed, acceleration and Kinetic energy do not have a disparate nature. CME_SLD in both SC23 and SC24 are statistically similar. Therefore, this investigation suggests that SLDs, and hence the sunspot number, may not be a sufficient candidate to predict the solar eruptive activities (e.g. CMEs). On the other hand, our analysis of the relation between the strength of the geomagnetic storm and probable candidate parameters revealed Dst index to be very well correlated with the product of V and Bz.

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arXiv2024

Origins of Very Low Helium Abundance Streams Detected in the Solar Wind Plasma

The abundance of helium ($A_{He}$) in the solar wind exhibits variations typically in the range from 2-5% with respect to solar cycle activity and solar wind velocity. However, there are instances where the observed $A_{He}$ is exceptionally low ($<$ 1%). These low-$A_{He}$ occurrences are detected both near the Sun and at 1 AU. The low $A_{He}$ events are generally observed near the heliospheric current sheet. We analyzed 28 low-$A_{He}$ events observed by the Wind spacecraft and 4 by Parker Solar Probe (PSP) to understand their origin. In this work, we make use of the ADAPT-WSA model to derive the sources of our events at the base of the solar corona. The modeling suggests that the low-$A_{He}$ events originated from the boundaries of coronal holes, primarily from large quiescent helmet streamers. We argue that the cusp above the core of the streamer can produce such very low helium abundance events. The streamer core serves as an ideal location for gravitational settling to occur as demonstrated by previous models, leading to the release of this plasma through reconnection near the cusp, resulting in low $A_{He}$ events. Furthermore, observations from Ulysses provide direct evidence that these events originated from coronal streamers.

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arXiv2024

Intense Geomagnetic Storms during Solar Cycles 23-25

Intense geomagnetic storms are characterized by a minimum value of the Dst index at or below -100 nT. It is well known that these storms are caused by the southward magnetic fields in coronal mass ejections (CMEs) and corotating interaction regions (CIRs). While CIR storms are confined to Dst values at or above -150 nT, CME storms can reach Dst -500 nT or lower. In this report, we illustrate the need to understand the storm evolution based on solar source and solar wind parameters using a recent storm (2023 April 24) by way of providing the motivation to catalog such events for a better understanding of the main phase time structure of geomagnetic storms

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arXiv2023

The Multiview Observatory for Solar Terrestrial Science (MOST)

We report on a study of the Multiview Observatory for Solar Terrestrial Science (MOST) mission that will provide comprehensive imagery and time series data needed to understand the magnetic connection between the solar interior and the solar atmosphere/inner heliosphere. MOST will build upon the successes of SOHO and STEREO missions with new views of the Sun and enhanced instrument capabilities. This article is based on a study conducted at NASA Goddard Space Flight Center that determined the required instrument refinement, spacecraft accommodation, launch configuration, and flight dynamics for mission success. MOST is envisioned as the next generation great observatory positioned to obtain three-dimensional information of large-scale heliospheric structures such as coronal mass ejections, stream interaction regions, and the solar wind itself. The MOST mission consists of 2 pairs of spacecraft located in the vicinity of Sun-Earth Lagrange points L4 (MOST1, MOST3) and L5 (MOST2 and MOST4). The spacecraft stationed at L4 (MOST1) and L5 (MOST2) will each carry seven remote-sensing and three in-situ instrument suites, including a novel radio package known as the Faraday Effect Tracker of Coronal and Heliospheric structures (FETCH). MOST3 and MOST4 will carry only the FETCH instruments and are positioned at variable locations along the Earth orbit up to 20° ahead of L4 and 20° behind L5, respectively. FETCH will have polarized radio transmitters and receivers on all four spacecraft to measure the magnetic content of solar wind structures propagating from the Sun to Earth using the Faraday rotation technique. The MOST mission will be able to sample the magnetized plasma throughout the Sun-Earth connected space during the mission lifetime over a solar cycle.

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arXiv2023

Speed and Acceleration of CMEs Associated with Sustained Gamma-Ray Emission Events Observed by Fermi/LAT

The sustained gamma-ray emission (SGRE) from the Sun is a prolonged enhancement of >100 MeV gamma-ray emission that extends beyond the flare impulsive phase.The origin of the >300 MeV protons resulting in SGRE is debated, both flares and shocks driven by coronal mass ejections (CMEs) being the suggested sites of proton acceleration. We compared the near-Sun acceleration and space speed of CMEs with 'Prompt' and 'Delayed' (SGRE) gamma-ray components (Ajello et al. 2021). We found that 'Delayed'-component-associated CMEs have higher initial acceleration and space speed than 'Prompt-only'-component-associated CMEs. We selected halo CMEs (HCMEs) associated with type II radio bursts (shock-driving HCMEs) and compared the average acceleration and space speed between HCME populations with or without SGRE events, major solar energetic particle (SEP) events, metric, or decameter-hectometric (DH) type II radio bursts. We found that the SGRE-producing HCMEs associated with a DH type II radio burst and/or a major SEP event have higher space speeds and especially initial accelerations than those without an SGRE event. We estimated the radial distance and speed of the CME-driven shocks at the end time of the 2012 January 23 and March 07 SGRE events using white-light images of STEREO Heliospheric Imagers and radio dynamic spectra of Wind WAVES. The shocks were at the radial distances of 0.6-0.8 au and their speeds were high enough (~975 km s$^{-1}$ and ~750 km s$^{-1}$, respectively) for high-energy particle acceleration. Therefore, we conclude that our findings support the CME-driven shock as the source of >300 MeV protons.

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arXiv2022

What is Unusual about the Third Largest Geomagnetic Storm of Solar Cycle 24?

We report on the solar and interplanetary (IP) causes of the third largest geomagnetic storm (2018 August 26) in solar cycle 24. The underlying coronal mass ejection (CME) originating from a quiescent filament region becomes a 440 km/s magnetic cloud (MC) at 1 au after ~5 days. The prolonged CME acceleration (for about 24 hrs) coincides with the time profiles of the post-eruption arcade intensity and reconnected flux. Chen et al. (2019) obtain lower speed since they assumed that the CME does not accelerate after about 12 hrs. The presence of multiple coronal holes near the filament channel and the high-speed wind from them seem to have the combined effect of producing complex rotation in the corona and IP medium resulting in a high-inclination MC. The Dst time profile in the main phase steepens significantly (rapid increase in storm intensity) coincident with the density increase (prominence material) in the second half of the MC. Simulations using the Comprehensive Inner Magnetosphere-Ionosphere (CIMI) model shows that a higher ring current energy results from larger dynamic pressure in MCs. Furthermore, the Dst index is highly correlated with the main-phase time integral of the ring current injection that includes density, consistent with the simulations. A complex temporal structure develops in the storm main phase if the underlying MC has a complex density structure during intervals of southward interplanetary magnetic field. We conclude that the high intensity of the storm results from the prolonged CME acceleration, complex rotation, and the high density in the 1-au MC.

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arXiv2022

Eruption of EUV Hot-Channel near Solar Limb and Associated Moving Type-IV Radio Burst

Using the observations from Solar Dynamics Observatory, we study an eruption of a hot-channel flux rope (FR) near the solar-limb on February 9, 2015. The pre-eruptive structure is visible mainly in EUV 131 $\mathring{\mathrm{A}}$ images with two highly-sheared loop structures. They undergo slow rise motion and then reconnect to form an eruptive hot-channel as in the tether-cutting reconnection model. The J-shaped flare-ribbons trace the footpoint of the FR which is identified as the hot-channel. Initially, the hot channel is observed to rise slowly at 40 km s$^{-1}$, followed by an exponential rise from 22:55 UT at a coronal height of 87$\pm$2 Mm. Following the onset of the eruption at 23:00 UT, the flare-reconnection adds to the acceleration process of the CME within 3 R$_\odot$. Later on, the CME continues to accelerate at 8 m s$^{-2}$ during its propagation period. Further, the eruption launched type-II followed by III, IVm radio bursts. The start and end times of type-IVm correspond to the CME core height of 1.5 and 6.1 R$_\odot$, respectively. Also the spectral index is negative suggesting the non-thermal electrons trapped in the closed loop structure. Accompanied with type-IVm, this event is unique in the sense that the flare ribbons are very clearly observed along with the erupting hot channel, which strongly supports that the hooked-part of J-shaped flare ribbons outlines the boundary of the erupting FR.

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arXiv2021

The Total Solar Irradiance variability in the Evolutionary Timescale and its Impact on the Mean Earth's Surface Temperature

The Sun is the primary source of energy for the Earth. The small changes in total solar irradiance (TSI) can affect our climate in the longer timescale. In the evolutionary timescale, the TSI varies by a large amount and hence its influence on the Earth's mean surface temperature (T$_{s}$) also increases significantly. We develop a mass-loss dependent analytical model of TSI in the evolutionary timescale and evaluated its influence on the T$_{s}$. We determined the numerical solution of TSI for the next 8.23 Gyrs to be used as an input to evaluate the T$_{s}$ which formulated based on a zero-dimensional energy balance model. We used the present-day albedo and bulk atmospheric emissivity of the Earth and Mars as initial and final boundary conditions, respectively. We found that the TSI increases by 10\% in 1.42 Gyr, by 40\% in about 3.4 Gyrs, and by 120\% in about 5.229 Gyrs from now, while the T$_{s}$ shows an insignificant change in 1.644 Gyrs and increases to 298.86 K in about 3.4 Gyrs. The T$_{s}$ attains the peak value of 2319.2 K as the Sun evolves to the red giant and emits the enormous TSI of 7.93$\times10^{6} Wm^{-2}$ in 7.676 Gys. At this temperature Earth likely evolves to be a liquid planet. In our finding, the absorbed and emitted flux equally increases and approaches the surface flux in the main sequence, and they are nearly equal beyond the main sequence, while the flux absorbed by the cloud shows opposite trend.

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arXiv2021

Spotless days and geomagnetic index as the predictors of solar cycle 25

We study the sunspot activity in relation to spotless days (SLDs) during the descending phase of solar cycle $11$--$24$ to predict the amplitude of sunspot cycle $25$. For this purpose, in addition to SLD, we also use the geomagnetic activity (aa index) during the descending phase of a given cycle. A very strong correlation of the SLD (R=$0.68$) and aa index (R=$0.86$) during the descending phase of a given cycle with the maximum amplitude of next solar cycle has been estimated.The empirical relationship led us to deduce the amplitude of cycle $25$ to be 99.13$\pm$ 14.97 and 104.23$\pm$ 17.35 using SLD and aa index, respectively as predictors.Both the predictors provide comparable amplitude for solar cycle $25$ and reveal that the solar cycle $25$ will be weaker than cycle $24$. Further we derive that the maximum of cycle $25$ is likely to occur between February and March 2024. While the aa index has been used extensively in the past, this work establishes SLDs as another potential candidate for predicting the characteristics of the next cycle.

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arXiv2021

A Weak Fermi Gamma-ray Event Associated with a Halo CME and a Type II Radio Burst

We report on the 2015 June 25 sustained gamma-ray emission (SGRE) event associated with a halo coronal mass ejection and a type II radio burst in the decameter-hectometric (DH) wavelengths. The duration and ending frequency of the type II burst are linearly related to the SGRE duration as found in previous works involving intense gamma-ray events. This study confirms that the SGRE event is due to protons accelerated in the shock that produced the DH type II burst.

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arXiv2021

Diffuse Interplanetary Radio Emission from a Polar Coronal Mass Ejection

We report on the first detection of nonthermal radio emission associated with a polar coronal mass ejection. We call the radio emission as diffuse interplanetary radio emission (DIRE), which occurs in the decameter-hectometric wavelengths. The radio emission originates from the shock flanks that interact with nearby streamers.

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arXiv2021

The Common Origin of High-energy Protons in Solar Energetic Particle Events and Sustained Gamma-ray Emission from the Sun

We report that the number of > 500 MeV protons (Ng) inferred from sustained gamma ray emission (SGRE) from the Sun is significantly correlated with that of protons propagating into space (NSEP) as solar energetic particles (SEPs). Under the shock paradigm for SGRE, shocks driven by coronal mass ejections (CMEs) accelerate high-energy protons sending them toward the Sun to produce SGRE by interacting with the atmospheric particles. Particles also escape into the space away from the Sun to be detected as SEP events. Therefore, the significant NSEP vs. Ng correlation (correlation coefficient 0.77) is consistent with the common shock origin for the two proton populations. Furthermore, the underlying CMEs have properties akin to those involved in ground level enhancement (GLE) events indicating the presence of high-energy (up to GeV) particles required for SGRE. We show that the observed gamma-ray flux is an underestimate in limb events (central meridian distance > 60 degrees) because SGRE sources are partially occulted when the emission is spatially extended. With the assumption that the SEP spectrum at the shock nose is hard and that the 100 MeV particles are accelerated throughout the shock surface (half width in the range 60 to 120 degrees) we find that the latitudinal widths of SEP distributions are energy dependent with the smallest width at the highest energies. Not using the energy-dependent width results in an underestimate of NSEP in SGRE events occurring at relatively higher latitudes. Taking these two effects into account removes the apparent lack of NSEP - Ng correlation reported in previous studies.

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arXiv2021

Investigating width distribution of slow and fast CMEs in solar cycles 23 and 24

Coronal Mass Ejections (CMEs) are highly dynamic events originating in the solar atmosphere, that show a wide range of kinematic properties and are the major drivers of the space weather. The angular width of the CMEs is a crucial parameter in the study of their kinematics. The fact that whether slow and fast CMEs (as based on their relative speed to the average solar wind speed) are associated with different processes at the location of their ejection is still debatable. Thus, in this study, we investigate their angular width to understand the differences between the slow and fast CMEs. We study the width distribution of slow and fast CMEs and find that they follow different power law distributions, with a power law indices ($α$) of -1.1 and -3.7 for fast and slow CMEs respectively. To reduce the projection effects, we further restrict our analysis to only limb events as derived from manual catalog and we find similar results. We then associate the slow and fast CMEs to their source regions, and classified the sources as Active Regions (ARs) and Prominence Eruptions (PEs). We find that slow and fast CMEs coming from ARs and PEs, also follow different power laws in their width distributions. This clearly hints towards a possibility that different mechanisms might be involved in the width expansion of slow and fast CMEs coming from different sources.These results are also crucial from the space weather perspective since the width of the CME is an important factor in that aspect.

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arXiv2020

The Balloon-borne Investigation of Temperature and Speed of Electrons in the corona (BITSE): Mission Description and Preliminary Results

We report on the Balloonborne Investigation of Temperature and Speed of Electrons in the corona (BITSE) mission launched recently to observe the solar corona from about 3 Rs to 15 Rs at four wavelengths (393.5, 405.0, 398.7, and 423.4 nm). The BITSE instrument is an externally occulted single stage coronagraph developed at NASA's Goddard Space Flight Center in collaboration with the Korea Astronomy and Space Science Institute (KASI). BITSE used a polarization camera that provided polarization and total brightness images of size 1024 x 1024 pixels. The Wallops Arc Second Pointing (WASP) system developed at NASA's Wallops Flight Facility (WFF) was used for Sun-pointing. The coronagraph and WASP were mounted on a gondola provided by WFF and launched from the Fort Sumner, New Mexico station of Columbia Scientific Balloon Facility (CSBF) on September 18, 2019. BITSE obtained 17,060 coronal images at a float altitude of about 128,000 feet (39 km) over a period of about 4 hrs. BITSE flight software was based on NASA's core Flight System, which was designed to help develop flight quality software. We used EVTM (Ethernet Via Telemetry) to download science data during operations; all images were stored onboard using flash storage. At the end of the mission, all data were recovered and analyzed. Preliminary analysis shows that BITSE imaged the solar minimum corona with the equatorial streamers on the east and west limbs. The narrow streamers observed by BITSE are in good agreement with the geometric properties obtained by SOHO coronagraphs in the overlapping physical domain. In spite of the small signal-to-noise ratio (about 14) we were able to obtain the temperature and flow speed of the western steamer region in the range 4 to 7 Rs as: For the equatorial streamer on the west limb, we obtained a temperature of 1.0 +/- 0.3 MK and a flow speed of about 260 km/s with a large uncertainty interval.

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arXiv2020

Effect of the Weakened Heliosphere in Solar Cycle 24 on the Properties of Coronal Mass Ejections

Solar cycle (SC) 24 has come to an end by the end of 2019, providing information on two full cycles to understand the manifestations of SC 24, the smallest cycle in the Space Age that has resulted in a weak heliospheric state indicated by the reduced pressure. The backreaction of the heliospheric state is to make the coronal mass ejections (CMEs) appear physically bigger than in SC 23, but their magnetic content has been diluted resulting in a lower geoeffectiveness. The heliospheric magnetic field is also lower in SC 24, leading to the dearth of high-energy solar energetic particle (SEP) events. These space weather events closely follow fast and wide (FW) CMEs. All but FW CMEs are higher in number in SC 24. The CME rate vs. sunspot number (SSN) correlation is high in both cycles but the rate increases faster in SC 24. We revisit the study of limb CMEs (those with source regions within 30 degrees from the limb) previously studied over partial cycles. We find that limb CMEs are slower in SC 24 as in the general population but wider. Limb halo CMEs follow the same trend of slower SC-24 CMEs. However, the SC-24 CMEs become halos at a shorter distance from the Sun. Thus, slower CMEs becoming halos sooner is a clear indication of the backreaction of the weaker heliospheric state on CMEs. We can further pin down the heliospheric state as the reason for the altered CME properties because the associated flares have similar distributions in the two cycles, unaffected by the heliospheric state.

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arXiv2020

A Catalog of Prominence Eruptions Detected Automatically in the SDO/AIA 304 Å Images

We report on a statistical study of prominence eruptions (PEs) using a catalog of these events routinely imaged by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) in the 304 Å pass band. Using an algorithm developed as part of an LWS project, we have detected PEs in 304 Å synoptic images with 2-min cadence since May 2010. A catalog of these PEs is made available online (https://cdaw.gsfc.nasa.gov/CME_list/autope/). The 304 Å images are polar-transformed and divided by a background map (pixels with minimum intensity during one day) to get the ratio maps above the limb. The prominence regions are defined as pixels with a ratio $\ge$2. Two prominence regions with more than 50% of pixels overlapping are considered the same prominence. If the height of a prominence increases monotonically in 5 successive images, it is considered eruptive. All the PEs seen above the limb are detected by the routine, but only PEs with width $\ge$15° are included in the catalog to eliminate polar jets and other small-scale mass motions. The identifications are also cross-checked with the PEs identified in Nobeyama Radioheliograph images (http://solar.nro.nao.ac.jp/norh/html/prominence/). The catalog gives the date, time, central position angle, latitude, and width of the eruptive prominence. The catalog also provides links to JavaScript movies that combine SDO/AIA images with GOES soft X-ray data to identify the associated flares, and with SOHO/LASCO C2 images to identify the associated coronal mass ejections. We examined the statistical properties of the PEs and found that the high-latitude PE speed decreased with the decreasing of the average polar magnetic field strength of the previous cycle.

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