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Solar observation

Solar observation is the scientific practice of examining the Sun's physical properties, magnetic fields, atmosphere, and dynamic phenomena through diverse techniques, from ancient naked-eye records to advanced space-based imaging, enabling insights into solar variability and its broader impacts.^[1]^[2] This field encompasses the detection of features such as sunspots, solar flares, and coronal mass ejections (CMEs), which are critical for modeling the Sun's 11-year activity cycle and predicting space weather events.^[3]^[2] Historical records of solar observation date back over two millennia, with the earliest documented sunspot sightings appearing in Chinese, Korean, Japanese, Arabic, and Indian texts as early as 364 BC.^[4] These naked-eye observations provided initial evidence of solar variability, though limited by the absence of telescopes and the danger of direct viewing.^[5] The advent of telescopic observation in the early 17th century, pioneered by Galileo Galilei in 1610–1611, revolutionized the field by allowing detailed drawings of sunspots and sparking debates on their nature as planetary companions or solar surface features.^[1] Subsequent milestones include the identification of the solar cycle by Samuel Schwabe in 1843 through systematic sunspot counts and the establishment of the International Sunspot Number series from 1700 onward, which has tracked long-term patterns like the Maunder Minimum (1645–1715), a period of unusually low activity.^[1]^[5] Modern solar observation relies on sophisticated ground- and space-based methods to overcome atmospheric distortion and capture multi-wavelength data. Ground techniques include pinhole projectors for safe projection of the solar image and filtered telescopes, such as those using hydrogen-alpha (H-alpha) filters to reveal chromospheric structures like prominences and filaments.^[5] Space missions, unhindered by Earth's atmosphere, employ instruments like the Solar Dynamics Observatory (SDO), launched in 2010, which uses the Atmospheric Imaging Assembly (AIA) to image the corona in extreme ultraviolet (EUV) wavelengths every 10 seconds at 725 km resolution, the Extreme Ultraviolet Variability Experiment (EVE) to measure solar irradiance fluctuations, and the Helioseismic and Magnetic Imager (HMI) for mapping surface magnetic fields and probing the solar interior via helioseismology.^[2] These approaches have revealed the Sun's differential rotation and the origins of solar wind, enhancing our understanding of phenomena invisible from the ground.^[2] The importance of solar observation lies in its role in forecasting space weather, as solar eruptions can disrupt satellite operations, power grids, and communications on Earth, potentially causing economic losses comparable to major disasters.^[2] By monitoring solar irradiance and magnetic activity, observations contribute to climate studies, revealing how variations in solar output influence Earth's atmosphere and oceans over long timescales.^[6] Ongoing international efforts, including fleets of solar observatories, ensure continuous 24/7 monitoring to mitigate these risks and advance heliophysics.^[7]

Ancient and Pre-Telescopic Observations

Prehistoric Evidence

Prehistoric evidence for solar observation relies on indirect proxies preserved in geological and paleoclimatic archives, which reveal long-term variations in solar activity through their influence on Earth's environment. These records predate human written history and provide insights into solar variability over millions of years, demonstrating that the Sun's output has fluctuated in ways that affected terrestrial conditions. Such evidence is derived from natural sedimentation patterns, isotopic compositions in organic materials, and atmospheric deposition in polar ice, all modulated by solar-driven changes in cosmic ray flux and irradiance. One of the earliest indicators comes from Precambrian rock formations, where varved sediments exhibit cyclic layering attributable to solar influences. In the Elatina Formation of South Australia, dated to approximately 680 million years ago, varve thicknesses show rhythmic variations with periods resembling solar activity cycles, including an 11-year signal and longer harmonics, suggesting solar variability influenced seasonal sediment deposition during the late Precambrian era. These cycles, preserved in glacial varves, indicate that solar irradiance fluctuations impacted climate and sedimentation processes even in ancient geological times.^[8] Direct prehistoric evidence includes megalithic structures aligned with solar events, such as the solstice markers at sites like Stonehenge in England, constructed around 3000 BCE, which demonstrate early human tracking of solar cycles for ritual and calendrical purposes.^[9] Tree-ring records from dendrochronology offer a more recent but still prehistoric perspective, extending back over 11,400 years and linking solar magnetic activity to variations in atmospheric carbon-14 (¹⁴C). Annual growth rings in ancient bristlecone pines and oaks preserve ¹⁴C levels, which increase during periods of low solar activity due to enhanced cosmic ray penetration and subsequent production of cosmogenic isotopes in the atmosphere. Reconstructions from these tree rings reveal sunspot-like activity patterns, with high solar magnetic strength suppressing ¹⁴C production and correlating with grand solar maxima over the Holocene. For instance, the past 11,400 years show only about 10% of the time at levels of solar activity as high as recent decades, highlighting episodic solar variability.^[10] Ice core data from polar regions provide complementary evidence through beryllium-10 (¹⁰Be) isotopes, which track solar minima and maxima over millennia by recording cosmic ray-induced production rates. During low solar activity, weakened heliospheric magnetic fields allow more galactic cosmic rays to reach Earth, increasing ¹⁰Be deposition in ice layers; conversely, high activity reduces ¹⁰Be. Greenland and Antarctic ice cores document this for the past several thousand years prior to written records. These proxies thus capture solar irradiance changes indirectly, as variations in solar magnetic activity and total solar irradiance modulate cosmic ray flux, influencing isotope production without direct solar viewing.

Ancient Civilizations

Ancient civilizations across the world made pioneering observations of solar phenomena, laying the groundwork for understanding celestial patterns through written records and monumental alignments. In Mesopotamia, Babylonian astronomers from the 8th century BC utilized clay tablets to document and predict solar eclipses by recognizing arithmetic progressions in solar-lunar cycles, such as the Saros cycle of approximately 18 years that allowed forecasting of eclipse occurrences.^[11]^[12] These predictions relied on meticulous tracking of lunar months and eclipse timings, enabling anticipatory calculations for events like the solar eclipse of 136 BC.^[13] In East Asia, Chinese observers recorded sunspots as early as 165 BC, with descriptions in historical texts noting dark spots on the sun's surface during periods of heightened solar activity.^[1] These accounts, preserved in later compilations such as the Book of Han, also linked auroral displays—termed "red qi" or atmospheric lights—to solar disturbances, providing early evidence of geomagnetic effects from solar flares.^[14] Such records highlighted recurring solar patterns, often interpreted through astrological lenses but grounded in direct visual observations. Egyptian and Mayan societies developed solar calendars attuned to solstices and equinoxes to regulate agriculture and rituals, with monumental structures facilitating precise tracking. The ancient Egyptian civil calendar, established around 3000 BC, divided the year into three seasons aligned with the Nile's flood cycle and solar events like the summer solstice, when the heliacal rising of Sirius marked the New Year.^[15] Similarly, early Mesoamerican orientations from 1100–750 BCE represent the earliest evidence of solar and calendrical alignments, while later Mayan sites like Chichen Itza integrated the Haab' (a 365-day solar year), where the El Castillo pyramid casts a serpent shadow during equinoxes, symbolizing seasonal transitions around 1000 CE.^[16] These civilizations employed stone megaliths and temple orientations—reminiscent of solstice-tracking structures from 2000 BC—for observing solar extremes, ensuring calendars reflected equinox balance and solstice shifts.^[9] In the Indian subcontinent, Vedic texts from around 1500 BC, particularly the Rigveda, described solar eclipses as mythical events where demons like Rahu obscured the sun, while correlating these phenomena with monsoon patterns essential for agriculture.^[17] Hymns in the Rigveda detailed eclipse timings and their perceived influence on seasonal rains, with ancient Sanskrit literature qualitatively noting year-to-year monsoon variations tied to solar observations.^[18] This integration of astronomical events with climatic cycles underscored early recognition of the sun's role in environmental rhythms.

Medieval Records

During the medieval period, Islamic astronomers advanced solar observation through precise measurements of eclipses and related phenomena. Al-Battani, working in the 9th century at Raqqa in present-day Syria, refined the length of the solar year to 365 days, 5 hours, 46 minutes, and 24 seconds by analyzing timings of solar and lunar eclipses alongside other observations, achieving an accuracy within about 2 minutes of modern values.^[19] His work, preserved in the Zij al-Sabi, built on Ptolemaic methods and influenced later European astronomy by improving predictions of solar positions. Solar eclipses were recognized as predictable events, extending ancient techniques with greater precision in timing and periodicity. In Europe, monastic scholars contributed to solar records through eclipse documentation and early astronomical tables. The 12th century saw the development of precursor tables to later works like those of Regiomontanus, including translations of Islamic Toledan tables into Latin around 1140 by scholars at the Cluny monastery and elsewhere, which facilitated eclipse predictions across the continent.^[20] These tables, often compiled in monastic scriptoria, correlated celestial events with earthly calamities; for instance, chroniclers like Gervase of Canterbury linked the annular solar eclipse of 1185 to subsequent famines and plagues, interpreting it as a divine omen signaling societal distress.^[21] A notable European record is the first known illustrated depiction of sunspots by the monk John of Worcester in his Chronicle, dated to December 8, 1128, showing two large dark spots on the solar disk observed from Worcester, England, likely with the naked eye due to their exceptional size.^[22] This observation coincided with heightened solar activity, as evidenced by a red auroral display recorded five days later on December 13 in Songdo, Korea, described as a crimson vapor filling the sky, possibly resulting from a geomagnetic storm triggered by the same solar event.^[23] In Asia, Chinese and Korean astronomers documented "dark spots" on the Sun during the 13th century, providing some of the earliest systematic naked-eye records of sunspots amid the Medieval Solar Maximum. The Song dynasty's official chronicle, Songshi, includes 38 such candidates between 960 and 1279 CE, with clusters in the 13th century noting black vapors or spots visible during daylight, reflecting periods of elevated solar activity.^[14] Korean records from the Goryeo dynasty similarly captured auroral phenomena linked to solar storms, enhancing the global picture of medieval solar variability.

17th to 19th Centuries

Early Telescopic Observations

The introduction of the telescope revolutionized solar observation in the early 17th century, enabling detailed views of the Sun's surface that were previously impossible with the naked eye. English mathematician and astronomer Thomas Harriot conducted the earliest known telescopic observations of sunspots in December 1610, using a refracting telescope to produce sketches that captured dark patches on the solar disk. These drawings, numbering nearly 200 from 1610 to 1612, predated similar published work by several months and marked the first pictorial records of solar features through instrumentation.^[24]^[25] The first published account of telescopic sunspot observations appeared in June 1611, when German astronomer Johann Fabricius issued De Maculis in Sole observatis, describing spots as solar phenomena based on his and his father David Fabricius's sightings.^[26] Italian astronomer Galileo Galilei expanded on these initial sightings with systematic observations beginning in 1611, publishing his findings in the Letters on Sunspots in 1613. In these letters, Galileo described sunspots as transient phenomena occurring on or near the Sun's surface, using their daily motion across the disk to infer the Sun's rotation period of approximately one month—thus providing early evidence of solar rotation from surface markers. This interpretation directly challenged the Aristotelian doctrine of the heavens' immutable perfection, as sunspots revealed the Sun as a changeable body akin to Earth, sparking philosophical and scientific controversy.^[27] Jesuit astronomer Christoph Scheiner independently observed sunspots starting in 1611, publishing Tres Epistolae de Maculis Solaribus in 1612 under the pseudonym Apelles. To safely project the Sun's image without direct viewing, Scheiner employed a method using a telescope focused onto a screen behind the eyepiece, often with colored glass filters to reduce glare, allowing for precise tracing of spots over time. Initially, Scheiner argued that sunspots were not surface features but small, star-like satellites orbiting the Sun, a view intended to preserve celestial perfection; this sparked a heated debate with Galileo, who countered that the spots' irregular paths and disappearances proved they were atmospheric or surface irregularities rather than permanent bodies. Scheiner later conceded the surface-origin hypothesis in his comprehensive 1630 work Rosa Ursina sive Sol.^[27] Telescopic monitoring continued through the mid-17th century, revealing prolonged periods of anomalously low activity. The Maunder Minimum, spanning roughly 1645 to 1715, saw sunspot numbers plummet to near zero for decades, with observers like Polish astronomer Johannes Hevelius recording only sporadic groups—such as 19 between 1653 and 1679—despite diligent daily projections. This era of diminished solar activity coincided with the Little Ice Age, a time of cooler global temperatures in Europe and North America, though the causal link remains a subject of ongoing research.^[28]^[29]

Solar Cycle and Sunspots

In 1843, German apothecary and amateur astronomer Samuel Heinrich Schwabe announced the discovery of an approximately 11-year cycle in sunspot activity, based on his meticulous daily observations of sunspots from 1826 to 1843 using a small refracting telescope.^[30] Schwabe's work built upon earlier telescopic sightings of sunspots by Galileo and others in the 17th century, transforming qualitative descriptions into quantitative evidence of periodic solar variability.^[31] His findings, published in the Astronomische Nachrichten, revealed alternating periods of high and low sunspot numbers, laying the groundwork for understanding long-term solar behavior.^[30] Swiss astronomer Rudolf Wolf extended Schwabe's observations in 1852 by developing a standardized formula for the relative sunspot number, R_z = k(10g + f), where g represents the number of sunspot groups, f the number of individual spots, and k a correction factor accounting for observational conditions at different sites.^[32] This metric, derived from Wolf's analysis of historical records dating back to 1610, enabled consistent tracking of solar activity across observatories and facilitated the reconstruction of past cycles.^[33] Wolf's sunspot number series, maintained at the Zurich Observatory, became the primary tool for monitoring the solar cycle's approximately 11-year periodicity.^[32] British astronomer Richard Carrington advanced the study of sunspot dynamics through his systematic observations from 1853 to 1861, culminating in the 1863 publication Observations of the Spots on the Sun. By tracking the motion of individual sunspots over time, Carrington established the Sun's differential rotation: the equatorial regions complete a rotation in about 25 days, while higher latitudes near the poles take approximately 36 days.^[34] This discovery, confirmed through precise positional measurements, indicated that the solar photosphere behaves as a fluid rather than a rigid body, influencing models of solar convection and magnetic field generation.^[35] A pivotal event during this era occurred on September 1, 1859, when Carrington and independently Richard Hodgson observed the first white-light solar flare erupting from a large sunspot group, visible against the Sun's disk for about five minutes.^[36] This intense flare, part of what is now known as the Carrington Event, triggered a massive geomagnetic storm that disrupted telegraph systems worldwide, causing sparks, fires, and auroras visible as far south as the Caribbean.^[36] The event underscored the Sun's capacity for sudden, high-energy releases tied to sunspot activity, highlighting the need for coordinated solar monitoring.^[36]

Spectroscopy and Photography

In the early 19th century, solar spectroscopy emerged as a pivotal tool for analyzing the Sun's composition, beginning with the work of Joseph von Fraunhofer. In 1814, Fraunhofer constructed a spectroscope and meticulously mapped 574 dark absorption lines in the solar spectrum, which appeared as gaps in the otherwise continuous rainbow of colors produced by dispersing sunlight through a prism.^[37] These lines, now known as Fraunhofer lines, remained unexplained for decades, as they defied contemporary understanding of light and matter. The mystery of these absorption lines was resolved in 1859 by Gustav Kirchhoff, who developed a theoretical framework linking them to gaseous absorption in the Sun's atmosphere. Kirchhoff's gas theory posited that cooler gases surrounding the hotter solar interior absorb specific wavelengths of light emitted from below, creating the dark lines observed on Earth.^[38] Collaborating with Robert Bunsen, Kirchhoff used prism spectroscopes to compare laboratory emission spectra of heated elements with the solar spectrum, successfully identifying hydrogen and sodium as key constituents in the solar atmosphere through matching absorption patterns, such as the prominent yellow D-lines for sodium.^[39] This breakthrough not only explained Fraunhofer's observations but also established spectroscopy as a method for remote chemical analysis of celestial bodies. Parallel to these spectroscopic advances, early solar photography captured the Sun's visible features, enabling permanent records beyond fleeting visual observations. In 1840, John William Draper produced the first daguerreotype solar image using a camera obscura setup with a polished metal plate sensitized to light, marking a foundational step in astrophotography despite the era's technical challenges like long exposures.^[40] Building on this, Warren de la Rue designed the photoheliograph in 1857, a specialized telescope-camera hybrid equipped with wet collodion plates that facilitated daily imaging of the solar disk and sunspots at observatories like Kew, providing systematic documentation of solar surface dynamics.^[41] Spectroscopy and photography converged dramatically during the 1868 total solar eclipse observed by Jules Janssen in India. Using a spectroscope attached to his telescope, Janssen examined the emission lines from solar prominences—fiery gaseous extensions beyond the Sun's edge—confirming their chromospheric origin as hydrogen-dominated structures through bright-line spectra visible only during totality, while also detecting an unidentified yellow line (later known as the D3 line of helium). This observation, independently corroborated by Norman Lockyer, not only validated the gaseous nature of prominences but also led to the discovery of helium as a new element in 1868 and paved the way for routine non-eclipse studies by demonstrating how spectral isolation could reveal solar atmospheric features.^[42]^[43]

20th Century Advances

Ground-Based Instruments

In the 20th century, ground-based solar observation advanced significantly through the development of specialized instruments that overcame atmospheric limitations and enabled detailed imaging of the Sun's dynamic layers. Building upon the spectroscopic techniques pioneered in the 19th century, astronomers created devices capable of isolating specific wavelengths and suppressing overwhelming disk brightness to study phenomena like sunspots and the corona. These innovations, often mounted at high-altitude observatories, provided foundational data on solar activity despite challenges from Earth's atmosphere.^[44] A pivotal instrument was the spectroheliograph, invented by George Ellery Hale and first deployed at Mount Wilson Observatory in 1908 using the 60-foot solar tower telescope. This device employed a spectrograph and moving slit to capture monochromatic images of the Sun by isolating narrow wavelength bands, such as the hydrogen-alpha (Hα) line at 656.3 nm, allowing visualization of the chromosphere's filaments, prominences, and spicules that are invisible in broadband light. Hale's initial Hα spectroheliogram on March 28, 1908, revealed intricate solar structures, revolutionizing the study of the Sun's outer atmosphere.^[45]^[46] Using the same spectroheliograph setup, Hale discovered solar magnetic fields in sunspots later in 1908 by observing the Zeeman effect, where spectral lines split in the presence of a magnetic field. In observations of sunspot spectra, he detected polarized line splitting proportional to field strength, quantified by the formula for longitudinal Zeeman displacement:

\Delta \lambda = 4.67 \times 10^{-13} g \lambda^2 B

where \Delta \lambda is the wavelength shift in angstroms, g is the Landé factor, \lambda is the central wavelength in angstroms, and B is the magnetic field strength in gauss. This breakthrough, detailed in Hale's analysis of calcium and hydrogen lines, confirmed fields up to several thousand gauss in sunspots and established magnetism as central to solar dynamics.^[47]^[48] Another landmark was the coronagraph, invented by Bernard Lyot and first successfully operated at Pic du Midi Observatory in 1931. By using an occulting disk to artificially eclipse the solar disk, a Lyot stop to block diffracted light, and high-quality optics to minimize scattering, the instrument enabled routine observation of the faint solar corona without waiting for a total eclipse. Lyot's first photograph of the corona on July 12, 1931, captured its pearly structure and polarized light, opening the field to studies of coronal mass ejections and streamer evolution from ground sites.^[49] By the late 20th century, facilities like the Big Bear Solar Observatory, established in 1969 by the California Institute of Technology on Big Bear Lake, enhanced full-disk monitoring with vacuum towers to reduce air turbulence. Its patrol telescopes provided continuous Hα and white-light full-disk images for tracking solar flares and eruptions, while the vector magnetograph measured both magnitude and direction of photospheric fields using Zeeman splitting in Stokes parameters, yielding insights into magnetic shear and energy buildup in active regions.^[50]

Space-Based Observations

Space-based observations of the Sun began in the 20th century with satellites that circumvented Earth's atmospheric absorption, enabling unprecedented views in ultraviolet and X-ray wavelengths to study the solar corona and flares. These missions provided continuous monitoring without weather disruptions or seeing effects, revealing dynamic processes invisible from ground-based telescopes.^[51] The Orbiting Solar Observatory (OSO-1), launched in 1962, marked the first satellite dedicated to solar observations from orbit and included instruments for X-ray detection.^[51] It detected the first satellite-based X-ray emission from the Sun, demonstrating that the corona reaches temperatures exceeding 1 million K, far hotter than the photosphere. These findings confirmed theoretical models of coronal heating and highlighted the role of magnetic fields in maintaining such extreme conditions.^[52] In 1973, NASA's Skylab mission featured the Apollo Telescope Mount (ATM), a suite of solar instruments that produced extreme ultraviolet (XUV) spectroheliograms of solar flares and prominences.^[53] The XUV spectroheliograph (experiment S082A) resolved fine structures in these events, such as looping prominences and flare loops, with spatial resolution of approximately 1 arcminute, offering insights into plasma dynamics during eruptions.^[54] These observations, conducted over multiple manned missions, amassed a vast dataset on coronal mass ejections and energy release mechanisms.^[55] The Solar Maximum Mission (SMM), launched in 1980 near the peak of solar cycle 21, included the Active Cavity Radiometer Irradiance Monitor (ACRIM) to measure total solar irradiance (TSI). It established the mean TSI value at TSI = 1366 \pm 0.5 W/m², with variations of about 0.1% over the 11-year sunspot cycle, linking irradiance changes directly to solar activity levels. SMM's gamma-ray and X-ray spectrometers further correlated flare emissions with these irradiance fluctuations, advancing understanding of solar output's impact on Earth's climate.^[58] Launched in 1991 by Japan (with international collaboration), the Yohkoh satellite provided the first high-resolution imaging in both soft and hard X-rays, focusing on solar flares. Its Hard X-ray Telescope (HXT) and Soft X-ray Telescope (SXT) revealed compact sources at flare loop tops, supporting models of magnetic reconnection as the primary energy release mechanism.^[59] Observations of events like the 1992 Masuda flare showed hard X-ray emission above soft X-ray loops, indicating reconnection sites in the low corona.^[60] The Solar and Heliospheric Observatory (SOHO), a joint NASA-ESA mission launched in December 1995, further advanced space-based solar monitoring with instruments such as the Extreme-ultraviolet Imaging Telescope (EIT) for full-disk EUV imaging of the corona, the Large Angle and Spectrometric Coronagraph (LASCO) for observing CMEs from 1.1 to 30 solar radii, and the Michelson Doppler Imager (MDI) for helioseismic and magnetic field measurements. SOHO provided near-continuous observations, enabling the discovery of hundreds of comets and detailed studies of solar wind origins and interior dynamics.^[61]

Helioseismology and Proxies

Helioseismology, a technique developed in the late 20th century, enables indirect probing of the Sun's interior by studying global oscillations on its surface, analogous to seismology on Earth. These oscillations arise from convective motions in the solar interior, generating standing acoustic waves that reveal properties such as density, temperature, and rotation profiles otherwise inaccessible to direct observation. The Global Oscillation Network Group (GONG), initiated in 1995, marked a significant advancement by deploying a worldwide network of six instruments to acquire nearly continuous Doppler velocity measurements of the solar surface, minimizing gaps due to Earth's rotation and weather.^[62] Central to helioseismology are p-mode oscillations, pressure-driven acoustic waves with periods ranging from 3 to 15 minutes that propagate through the solar interior and are trapped by the surface boundary. By analyzing the frequencies and splitting of these modes, researchers map the Sun's internal rotation, revealing differential rotation where the equator rotates faster (about 25 days) than the poles (about 35 days), with a transition to rigid rotation in the radiative interior below the convection zone. GONG data have been instrumental in refining these maps, providing high-resolution insights into angular momentum distribution and its implications for solar dynamo processes.^[63]^[64] A key analytical method in helioseismology is ray tomography inversion, which reconstructs the radial sound speed profile c(r) and density from observed p-mode frequencies. This involves solving an inverse problem where travel times of acoustic rays between surface points are modeled using the eikonal equation, approximating wave propagation along curved paths in a spherically symmetric model; the resulting inversions yield localized averages of c(r), typically showing an increase from about 500 km/s in the core to about 7 km/s near the surface, enabling precise density profiles that match standard solar models within a few percent. Such techniques, applied to GONG observations, have constrained the depth of the convection zone to approximately 0.71 solar radii.^[65] Complementing helioseismology, cosmogenic proxies offer reconstructions of long-term solar activity and total solar irradiance (TSI) variations extending back over 10,000 years, far beyond direct measurements. Beryllium-10 (^10Be), a radionuclide produced in Earth's atmosphere by cosmic ray spallation and deposited in polar ice cores, serves as a primary proxy; its concentration inversely correlates with solar magnetic modulation of cosmic rays, allowing inference of past solar cycles. Analyses of Greenland Ice Core Project (GRIP) samples spanning the Holocene reveal TSI fluctuations of up to 0.4% (about 6 W/m²), with grand minima like the Maunder Minimum showing elevated ^10Be levels indicative of reduced solar output. These reconstructions, combining ^10Be with carbon-14 (^14C) tree-ring data, highlight periodicities around 200 and 2,300 years in solar variability.^[66]^[67] Solar radio bursts, first systematically observed in the 1940s using early radar systems during World War II, provided another indirect probe of solar activity through metric-wavelength emissions from the corona. These bursts were classified into Types I through V based on their dynamic spectra and durations: Type I as short, narrowband noise storms associated with active regions; Type II as drifting, harmonic emissions from shock waves; Type III as fast-drifting bursts from electron streams; Type IV as long-duration continua from flare ejecta; and Type V as post-Type III continua. Pioneering Australian observations from 1945–1947 identified these patterns, linking them to optical flares and later to coronal mass ejections (CMEs) via shock-driven Type II and particle-accelerated Type IV emissions, with about 70% of such bursts accompanying CMEs in cataloged events.^[68]^[69]

21st Century Developments

Modern Ground Observatories

Modern ground observatories in the 21st century have leveraged larger apertures, advanced adaptive optics, and computational enhancements to achieve unprecedented resolutions in solar imaging, surpassing the limitations of earlier 20th-century instruments like spectroheliographs by providing dynamic, real-time views of solar features. These facilities focus on high-resolution studies of the solar atmosphere, particularly magnetic structures and dynamic processes, using visible and near-infrared wavelengths to probe phenomena such as sunspots and chromospheric activity. Key examples include telescopes equipped with multi-conjugate adaptive optics systems that correct for atmospheric turbulence, enabling diffraction-limited observations over wider fields of view.^[70] The Daniel K. Inouye Solar Telescope (DKIST), located on Haleakalā in Hawaii, represents the pinnacle of modern ground-based solar observation with its 4-meter off-axis Gregorian aperture, which achieved first light in 2020.^[71] This design delivers a theoretical angular resolution better than 0.03 arcseconds at visible wavelengths, corresponding to scales of about 20 kilometers on the solar surface, allowing detailed mapping of magnetic fields in the photosphere and their role in energy transfer to the upper atmosphere.^[72] Equipped with a suite of instruments including the Visible Spectro-Polarimeter (ViSP) and the Visible Tunable Filter (VTF), DKIST captures spectropolarimetric data to infer vector magnetic fields with high sensitivity, revealing fine-scale structures in sunspots and faculae that drive solar eruptions.^[73] At Big Bear Solar Observatory in California, the Goode Solar Telescope (GST), a 1.6-meter off-axis telescope upgraded in the late 2000s, excels in near-infrared polarimetry for studying sunspot magnetism. Its adaptive optics system and the Near-Infrared Imaging Spectropolarimeter (NIRIS) provide diffraction-limited imaging at wavelengths around 1.56 micrometers, where the Zeeman effect is stronger, enabling precise measurements of umbral magnetic fields and penumbral fibril dynamics with resolutions approaching 0.1 arcseconds.^[74] These observations have illuminated how twisted magnetic fields in sunspots contribute to flux emergence and torsional motions, building on earlier infrared capabilities but with enhanced stability and throughput.^[75] The Swedish 1-meter Solar Telescope (SST) on La Palma, Canary Islands, operational since 2003, specializes in chromospheric dynamics using groundbreaking adaptive optics.^[76] Its 85-electrode deformable mirror corrects wavefront distortions in real time, achieving resolutions of 0.1 arcseconds in the near-ultraviolet, which reveals wave propagation and magnetic reconnection in spicules and Ellerman bombs.^[77] Instruments like the CRisp Imaging SPectropolarimeter (CRISP) facilitate high-cadence imaging in H-alpha and Ca II lines, capturing transient events such as chromospheric swirls with temporal resolutions down to seconds, thus advancing understanding of energy dissipation in the solar transition region.^[78] Integration of artificial intelligence has further revolutionized operations at these observatories, enabling real-time flare detection and automated data processing. For instance, at the National Solar Observatory's Dunn Solar Telescope in New Mexico, which ceased NSO operations in the late 2010s before transfer to a consortium, AI algorithms analyzed high-cadence Ca II K-line images to identify pre-flare signatures, improving alert times for space weather events.^[79] Similar machine learning techniques applied to ground-based networks like the Global Oscillation Network Group (GONG) now process vast datasets for predictive modeling of solar activity, enhancing the overall efficacy of modern facilities.^[80]

Recent Space Missions

The Solar Dynamics Observatory (SDO), launched in 2010 by NASA, has provided continuous high-resolution observations of the Sun's magnetic field and atmosphere throughout Solar Cycle 24, capturing its maximum activity phase around 2014. Its Helioseismic and Magnetic Imager (HMI) produces magnetograms that map the Sun's photospheric magnetic fields, revealing the evolution of sunspots and active regions, while the Atmospheric Imaging Assembly (AIA) delivers extreme ultraviolet (EUV) images across multiple wavelengths to track coronal dynamics and plasma heating. These instruments have been instrumental in studying the corona's response to magnetic reconnection events, contributing to our understanding of solar variability during the cycle's peak.^[81]^[82]^[83] Building on earlier space-based efforts like the Solar and Heliospheric Observatory (SOHO) from the 1990s, the Parker Solar Probe, launched in 2018, represents a leap in in-situ measurements of the solar corona and wind. By 2024, the probe achieved its closest approaches to the Sun at approximately 8.5 solar radii from the surface, enduring extreme conditions to sample the young solar wind directly. Observations have revealed switchbacks—sudden reversals in the magnetic field—within solar wind streams with velocities ranging from approximately 300 to 800 km/s, providing insights into the mechanisms accelerating the wind and heating the corona during the ascent of Solar Cycle 25.^[84]^[85] Launched in 2020 as an ESA mission with NASA contributions, Solar Orbiter combines remote sensing and in-situ instruments to probe the Sun from distances as close as 0.28 AU, enabling unprecedented views of the solar poles. For the first time, it has imaged the polar regions, revealing weaker magnetic activity and fewer sunspots compared to equatorial zones, which informs models of the Sun's global dynamo during Cycle 25. Its suite of sensors measures interplanetary magnetic fields, particles, and waves alongside coronal imagery, linking surface processes to heliospheric structures inaccessible from Earth-orbiting observatories.^[86]^[87] During Solar Cycle 25, which began in 2019 and is projected to peak in 2025, X-class flares have intensified coronal studies, with an example being the X1.0 event on May 8, 2024, from active region NOAA 13663. These powerful eruptions, observed by missions like SDO and Parker, highlight increased solar activity, including larger sunspot coverage that causes total solar irradiance (TSI) dips of about 0.1% at cycle peaks due to reduced radiative output from dark umbrae. Such variations underscore the missions' role in monitoring space weather impacts from heightened coronal mass ejections and wind turbulence.^[88]^[6]^[89]

Current Challenges and Future Prospects

One major challenge in solar observation remains the distortion caused by Earth's atmosphere, which limits the angular resolution of ground-based telescopes to approximately 1 arcsecond, preventing the capture of fine-scale solar features despite advances in adaptive optics.^[90] Space-based missions like the Solar Dynamics Observatory (SDO) generate vast data volumes, approximately 1.5 terabytes per day, necessitating advanced machine learning techniques to process and analyze imagery for patterns such as sunspot evolution and flare precursors.^[91]^[92] Accurate space weather forecasting is hindered by uncertainties in coronal mass ejection (CME) propagation, where models like WSA-ENLIL provide 1- to 4-day advance warnings of solar wind disturbances reaching Earth, but prediction errors can still reach several hours due to variable solar wind speeds and magnetic field interactions.^[93] These limitations underscore the need for enhanced observational vantage points to improve lead times for geomagnetic storm alerts. Future prospects include missions like NASA's Tandem Reconnection and Cusp Electrodynamics Reconnaissance Satellites (TRACERS), launched in July 2025, which study magnetic reconnection processes in Earth's magnetosphere to better understand solar-driven energy transfers.^[94] Similarly, the European Space Agency's Vigil mission, scheduled for launch around 2031 and positioned at the Sun-Earth L5 Lagrange point, will enable early detection of CMEs up to five days in advance by viewing the Sun from a 60-degree offset relative to Earth.^[95] These efforts align with observations during the maximum phase of Solar Cycle 25, predicted to peak around July 2025 with continued high activity into late 2025, including significant solar storms in November 2025, which heightened the urgency for refined predictive capabilities.^[96]^[97] Emerging multi-messenger approaches promise deeper insights by integrating solar neutrino detections, such as those from the Borexino experiment measuring pp-chain fluxes, with electromagnetic observations to probe the Sun's interior dynamics and activity cycles beyond surface phenomena. This synergy could refine models of solar variability, linking core fusion processes to observable flares and CMEs for more holistic heliophysics understanding.

References

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[PDF] Historical Sunspot Observations: A Review - arXiv
In these documents, we can find direct observations of the Sun such as measurements by early astronomers, observations of naked-eye sunspots, and eclipses.<|control11|><|separator|>
[2]
[PDF] SDO - Solar Dynamics Observatory - NASA
SDO will improve our understanding of the physics behind the activity displayed by the Sun's atmosphere, which drives space weather in the heliosphere, ...
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The Solar Cycle - NASA SVS
Oct 27, 2011 · The number of sunspots increases and decreases over time in a regular, approximately 11-year cycle, called the sunspot cycle.
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Observing the Sun for Yourself - Stanford Solar Center
Mt Wilson Historical Sunspot Drawing Resource Page. Sunspot observations were first recorded in China during the Shang Dynasty (~1700 BC to ~1027 BC). In the I ...
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The high-quality solar irradiance data from the space era provide the key record to study the Sun-Climate connection and its underlying coupling processes.
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NASA and other international space agencies monitor the Sun 24/7 with a fleet of solar observatories, studying everything from the Sun's atmosphere to its ...The Heliosphere · Magnetosphere · Solar Physics
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How Did the Ancients Predict Eclipses? The Saros Cycle
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How Ancient Humans Studied—And Predicted—Solar Eclipses
Apr 5, 2024 · Humans have been calculating the recurrence of solar eclipses for thousands of years. Many ancient cultures predicted these events mathematically.
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Ice Core Records - The Historical Sunspot Record
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Records of sunspot and aurora during CE 960–1279 in the Chinese ...
May 29, 2015 · In this paper, we present the results of a comprehensive survey of records of sunspots and auroras in the Sòngshǐ, a Chinese formal chronicle ...
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The Egyptian Civil Calendar: a Masterpiece to Organize the Cosmos
Creating a Calendar in ancient Egypt: the Nile-solar Paradigm The civil calendar ... the sun at the solstices and equinoxes, respectively. The lines for Sirius ...
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Origins of Mesoamerican astronomy and calendar: Evidence from ...
Jan 6, 2023 · The orientations of complexes built between 1100 and 750 BCE, in particular, represent the earliest evidence of the use of the 260-day calendar.
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6 Ancient Sites Aligned with the Solstice & Equinox
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Monsoon rainfall cycles as depicted in ancient Sanskrit texts - jstor
Year to year variation of Indian monsoon rainfall is described qualitatively in some ancient Sanskrit texts. the rainfall. frogs who sound in chorus at the ...
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Solar and Lunar Eclipse Measurements by Medieval Muslim ...
Four eclipses witnessed by al-Battãni and his colleagues are reported in his Solar ... Owing to tides and other influences, the length of the mean solar day ...
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Solar Physics Historical Timeline (0 - 1599) | High Altitude Observatory
1128: The first sunspot drawing ... Sunspot drawing in the Chronicles of John of Worcester, twelfth century. Notice the depiction of the penumbra around each spot ...
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[PDF] Solar and auroral evidence for an intense recurrent geomagnetic ...
Description of an observation of the aurora borealis on the night of AD 1128 December 13 from Songdo, Korea (Kaesong), as recorded in the Koryo-sa (the official ...
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Thomas Harriot (1560–1621) | High Altitude Observatory
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Thomas Harriot - The Galileo Project | Science
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Galileo wrote his first letter to Welser on sunspots, in which he argued that spots were, in fact, on the surface of the Sun or in its atmosphere, and although ...Missing: describing | Show results with:describing
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Sunspot Characteristics at the Onset of the Maunder Minimum ...
Nov 14, 2019 · An analysis of the sunspot observations made by Hevelius during 1642–1645 is presented. These records are the only systematic sunspot ...
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The Maunder minimum (1645–1715) was indeed a grand minimum
We conclude that solar activity was indeed at an exceptionally low level during the Maunder minimum. Although the exact level is still unclear, it was ...
[28]
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Nov 25, 2001 · Excerpts from the 1843 article announcing discovery of the sunspot cycle, linked to the educational exposition 'The Exploration of the ...
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Samuel Heinrich Schwabe (1789–1875) | High Altitude Observatory
In 1843 Schwabe still had not discovered any new planets, but instead, his 17 years of nearly continuous sunspot observations revealed a 10-year periodicity in ...
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Solar activity – past, present, future
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Sunspot Numbers | NCEI - NOAA
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Richard Christopher Carrington (1826–1875)
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[PDF] The Rotation of the Sun
Thereby, Carrington deduced that the visible surface of the Sun rotates differentially and thus is not solid. This discovery helped revolutionize ideas about.
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Around 1814, Fraunhofer started to investigate this phenomenon using a spectrometer of his own invention and in the process discovered 574 dark lines among the ...
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Spectral analysis 1859 - Kurpfälzer Mile of Innovations
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Solar spectrum,1842. Daguerreotype photograph by John Draper (1811-1882). Draper was a chemist and pioneer of scientific photography.
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[PDF] THE KEW PHOTOHELIOGRAPH
The Photoheliograph was designed by Warren De la Rue (1815-1889) (Figure 2) for the London Royal. Society in 1857 and made by Andrew Ross (1798-1859). It was ...
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Jules Janssen (1824–1907) | High Altitude Observatory
He made lasting contributions in solar spectroscopy, in particular in the observation of solar prominences. Following his observations of the 1868 solar eclipse ...
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George Ellery Hale - Mount Wilson Observatory |
George Ellery Hale using the spectrograph in the 60 foot solar telescope, where, in 1908, he discovered magnetic fields on the Sun, the key to understanding ...
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Discovering Mount Wilson Chapter 16: The Hale-Nicholson Law
Sep 28, 2021 · On March 28, 1908, George Ellery Hale took this revolutionary photograph of the sun. The very first H-alpha spectroheliograph, taken with the ...
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[PDF] Magnetic Fields in the Atmospheres of the Sun and Stars
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The Study of the Solar Corona without an Eclipse (with Plate V)
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The image shows the Sun at X-ray wavelengths and highlights prominences and gaseous activity in the Sun's hot atmosphere, or corona.
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Skylab Missions - NASA/Marshall Solar Physics
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Extreme ultraviolet spectroheliograph ATM experiment S082A
The XUV spectroheliograph, Apollo Telescope Mount experiment S082A, is described. The instrument was a slitless Wadsworth grating spectrograph that employed ...
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A Preliminary Study of the Extreme Ultraviolet Spectroheliograms ...
PRELIMTNARY STUDY OF EXTREME ULTRAVIOLET SPECTROHELIOGRAMS FROM SKYLAB 267 These spectroheliograms were obtained during the flare of June 15, 1973. The flare ...
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... maximum of the sunspot cycle when more spots are observed on the Sun's surface. Although the sunspots themselves are dark and produce dimming, they are ...Missing: 1979 TSI 1366 W/ m²
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The Solar Maximum Mission - NASA ADS
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Coronal Hard X-Ray Sources in Solar Flares Observed with Yohkoh ...
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[56]
Discovery of a loop-top hard X-ray source in impulsive solar flares
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The Global Oscillation Network Group (GONG) Project
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[PDF] Solar Interior Rotation and its Variation - Sirrah
Jan 30, 2009 · Abstract. This article surveys the development of observational understanding of the interior rotation of the Sun and its temporal variation ...
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[PDF] arXiv:1510.07058v1 [astro-ph.SR] 23 Oct 2015
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Inversion methods in helioseismology and solar tomography
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9,400 years of cosmic radiation and solar activity from ice cores and ...
Here we combine different 10 Be ice core records from Greenland and Antarctica with the global 14 C tree ring record using principal component analysis.
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A Retrospective View of Australian Solar Radio Astronomy 1945–1960
... types of solar bursts first identified by Australian and French radio astronomers ... Type IV solar bursts of continuum radiation. on 340 and 87 MHz. Astrophysic ...
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Solar type II and type IV radio bursts observed during 1998–2000 ...
68% of the catalogue events were associated with CMEs. – 67% of the type II events were associated with CMEs, in accordance with previous results.<|control11|><|separator|>
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The telescope - Institute for Solar Physics
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Inouye Solar Telescope: First Light - NSO
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The Daniel K. Inouye Solar Telescope – Observatory Overview
Dec 4, 2020 · The 4 m aperture provides sufficient resolution (0.08 arcsec) at these near infrared wavelengths. Similarly, modeling and numerical simulations ...
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[PDF] Infrared Observations with the 1.6 Meter New Solar Telescope in Big ...
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Jun 18, 2025 · The new telescope offers a significant incremental improvement in ground-based infrared and high angular resolution capabilities, and enhances ...
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The 85-electrode adaptive optics system of the Swedish 1-m Solar ...
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High-Resolution Observations of Fast Events in the Solar ...
We present new, high spatial and high temporal resolution observations of the Sun in Hα obtained with the Swedish 1 m Solar Telescope on La Palma.
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Harnessing AI for Space Weather Forecasting with NSF GONG Data
Oct 18, 2024 · The MAGFiLO dataset represents the convergence of solar physics and AI, with the promise to improve space weather prediction models.
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Solar Dynamics Observatory: SDO
SDO is designed to help us understand the Sun's influence on Earth and Near-Earth space by studying the solar atmosphere on small scales of space and time ...
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A Solar Cycle from Solar Dynamics Observatory - NASA SVS
This visualization is a series of graphic components to present nearly a full sunspot cycle observed by Solar Dynamics Observatory.
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The Active Sun from SDO: HMI Intensity - NASA SVS
The HMI on SDO takes images of the solar photosphere in visible light, showing how sunspots and active regions look in different wavelengths.
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NASA's Parker Solar Probe Makes History With Closest Pass to Sun
Dec 27, 2024 · NASA's Parker Solar Probe survived its record-breaking closest approach to the solar surface on Dec. 24, 2024.
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Switchbacks Science: Explaining Parker Solar Probe's Magnetic ...
traveling disturbances in the solar wind that caused the magnetic field to bend back on itself ...
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Solar Orbiter gets world-first views of the Sun's poles - ESA
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Mission - Solar Orbiter - ESA Cosmos - European Space Agency
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Top Solar Events of 2024 - The Sun Today with Dr. C. Alex Young
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Solar Variability and Terrestrial Climate - NASA Science
Jan 7, 2013 · Space-borne measurements of the total solar irradiance (TSI) show ~0.1 percent variations with solar activity on 11-year and shorter timescales.
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Solar observations from Earth have the same limit of about 1 arc-second as nighttime astronomy (1 arc-second = about 1/1920th the apparent size of the Sun or ...
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[PDF] NASA facts - Solar Dynamics Observatory
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Predicting the Appearance of Sunspots on the Solar Surface Using ...
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WSA-ENLIL Solar Wind Prediction | NOAA / NWS Space Weather ...
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NASA's TRACERS Launches Mission to Study Earth's Magnetic Field
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Vigil is planned for launch in 2031 ... point L5 in deep space. Looking at the Sun and Earth from L5, Vigil – once ...
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Solar Cycle Progression - Space Weather Prediction Center - NOAA
The predicted progression for the current solar cycle (Cycle 25) is given by the magenta line, with associated uncertainties shown by the shaded regions. This ...