Local Dawn Research Articles

Main features of the geomagnetic effect of the October 14, 2023 annular solar eclipse in the Americas

The purpose of this paper is to investigate temporal variations in the northward, X, eastward, Y, and downward, Z, components of the geomagnetic field recorded during the October 14, 2023 annular solar eclipse, which main features include its annularity, the eclipse occurrence from local dawn to local dusk, its magnitude variation from 0.30 to 0.86, and the longest ever-observed path across the mainland of the Americas, covering latitudes from ∼65°N to 12°S. The analysis was made possible due to the data on temporal variations in the northward, X, eastward, Y, and downward, Z, components of the geomagnetic field collected at thirteen International Real-time Magnetic Observatory Network magnetometer stations (https://imag-data.bgs.ac.uk/GIN_V1/GINForms2). The solar eclipse acted to cause non-sinusoidal and quasi-sinusoidal perturbations having temporal durations of 180–240 min in all geomagnetic field components on a global scale (∼8000 km). The X-component experienced the largest perturbations attaining 10–20 nT, and the Z-component underwent the smallest disturbances. The quasi-sinusoidal perturbation amplitude did not exceed 5–6 nT, and the period most often showed variations within 15–40 min. The magnetic effect exhibited a tendency to increase with solar eclipse magnitude, while the magnitude of the effect has been shown to be significantly dependent on geographic coordinates, local time, ionospheric state, and the patterns of ionospheric currents as well. During the solar eclipse, the electron density depletion was estimated to be ∼0.10 to ∼0.40–0.60 when the eclipse obscuration Amax varied from 19% to 82%. The movement of the lunar shadow was accompanied by the generation of atmospheric gravity waves with period of ∼10–80 min and by electron density perturbations with amplitudes of the order of 0.01–0.03. The estimates made on the assumption that the magnetic effect is due to the ionospheric current disruptions show good agreement with the observations.

Equatorial Plasma Bubbles Observed at Dawn and After Sunrise Over South America During the 2015 St. Patrick's Day Storm

AbstractThis paper presents an analysis of Equatorial Plasma Bubble (EPB) events that occurred during pre‐ and post‐sunrise hours in the South America sector on 18 March 2015, when the St. Patrick's Day geomagnetic storm was in the recovery phase. The data from a set of multi‐diagnostic instruments, including Global Navigation Satellite Systems (GNSS) receivers, L‐band GNSS ionospheric scintillation monitors, ionosondes, and a Fabry‐Perot interferometer, are analyzed. Strong L‐band amplitude scintillation was observed with spread‐F and total electron content (TEC) fluctuations at times when the electron density and tTEC were minimum with respect to their diurnal variations. The EPBs remained observed for about 3:30 hr after sunrise on the east coast of the South American continent. The daytime ionospheric irregularities show a longitudinal dependency between the west and east coasts, separated by about 3,600 km. Our analysis of the observations suggests that a Disturbance Dynamo Electric Field effect caused by storm‐time wind perturbations might have triggered the Rayleigh‐Taylor instability near local dawn on the east and west coasts of the South American continent. The effects of ambient ionosphere‐thermosphere conditions on the evolution of irregularities are assessed to understand the longitudinal differences. The assessment includes the effect of the E region that appears in the morning hours and the effects of geomagnetic declination and tilting the solar‐terminator.

Illuminating the Motions of Jupiter's Auroral Dawn Storms

AbstractJupiter's auroral main emission (ME) has long been considered to be the result of currents keeping plasma corotating with the surrounding magnetosphere. As a result, the ME corotates with the planet, and individual auroral features making up the ME roughly follow suit. Jupiter's dawn storms, some of the rarest and brightest auroral features within the ME, are an exception, as they do not corotate but instead remain fixed near local dawn. The causes of this enigmatic motion are not fully understood. To test the significance of this motion, we have developed a process to identify auroral features and measure their degree of corotational motion, including dawn storms, in archival Hubble Space Telescope images of the Jovian ultraviolet aurorae. We compare motions of features inside and outside the dawn sector, characterizing the exact motions of dawn storms and providing context for these motions for the first time. In keeping with previous studies, we expected to identify features fixed near local dawn in 10% of observations; instead, we find that half of all features near local dawn lag corotation. We show that subcorotating dawn emissions are far more common than previously thought, and that the drivers of this motion must be similarly common. Corotational motion must be considered when identifying the processes driving all dawn aurorae, including the dawn storms. We explore the consistency of this result with various theories of dawn ME formation and propose that aspects of the known current system relating to the Sun‐Jupiter geometry can explain this behavior.

MESSENGER X‐Ray Observations of Electron Precipitation on the Dayside of Mercury

AbstractThe first maps of electron‐induced X‐ray emission from the dayside of Mercury's surface are presented, generated by the development of a solar X‐ray flux filter. This enables the isolation of the X‐ray fluorescence of calcium driven by probable electron precipitation. A catalog of such events has been generated and dayside maps of implied electron precipitation zones have been produced. We find that, similar to electron induced emission events on the nightside, these zones are strongly organized by latitude and magnetic local time. The majority of the dayside events appear in the southern hemisphere and there is a strong enhancement observed centered about local dawn (06:00 LT). There is apparent poleward continuation of emission in the north, but very few events were observed on the duskward hemisphere. These results carry implications for Mercury's magnetosphere by constraining zones of electron precipitation, for the exosphere as a potential source of exospheric species, and for surface science as an additional source of X‐ray fluorescence.

Methane on Mars is transient and local

Martian Atmosphere The Curiosity rover has detected seasonally varying levels of methane in Gale crater on Mars, which could have geological or astrobiological sources. The ExoMars Trace Gas Orbiter (TGO) spacecraft has detected no methane on Mars despite having better sensitivity than Curiosity, but had not yet examined the Gale location. Measurements from TGO were always for high altitudes during local dawn or dusk and those from Curiosity for the surface at midnight. Webster et al. used Curiosity to search for methane during the day, finding none and inferring a day–night cycle. Montmessin et al. performed TGO measurements at new locations, including close to Gale, ruling out methane with tight upper limits. Together, these two studies indicate that methane is released and trapped within Gale crater during the night and then rapidly dissipates or is destroyed during the day. Astron. Astrophys. 650 , A166, A140 (2021).

Different as night and day

Exoplanet Atmospheres The atmospheric compositions of exoplanets are usually determined by the technique of transit spectroscopy: When the planet transits between its host star and Earth, starlight passes through its atmosphere close to local dawn and dusk, imprinting additional absorption lines on the stellar spectrum. Pluriel et al. considered how the results are affected if the star heats the planet enough for atmospheric molecules to be dissociated on the planet's day side and recombine on the night side. They found that transit spectroscopy then produces highly biased atmospheric compositions differing from the true values by up to three orders of magnitude. More sophisticated methods are needed to determine the atmospheric compositions of hot exoplanets. Astron. Astrophys. 636 , A66 (2020).

Rosetta observes sublimating surface ices

Cometary Science Comets are “dirty snowballs” made of ice and dust, but they are dark because the ice sublimates away, leaving some of the dust behind on the surface. The Rosetta spacecraft has provided a close-up view of the comet 67P/Churyumov-Gerasimenko as it passes through its closest point to the Sun (see the Perspective by Dello Russo). Filacchione et al. detected the spectral signature of solid CO2 (dry ice) in small patches on the surface of the nucleus as they emerged from local winter. By modeling how the CO2 sublimates, they constrain the composition of comets and how ices generate the gaseous coma and tail. Fornasier et al. studied images of the comet and discovered bright patches on the surface where ice was exposed, which disappeared as the ice sublimated. They also saw frost emerging from receding shadows. The surface of the comet was noticeably less red just after local dawn, indicating that icy material is removed by sunlight during the local day. Science , this issue p. [1563][1], p. [1566][2]; see also p. [1536][3] [1]: /lookup/doi/10.1126/science.aag3161 [2]: /lookup/doi/10.1126/science.aag2671 [3]: /lookup/doi/10.1126/science.aal1964

Seasonal exposure of carbon dioxide ice on the nucleus of comet 67P/Churyumov-Gerasimenko.

Carbon dioxide (CO2) is one of the most abundant species in cometary nuclei, but because of its high volatility, CO2 ice is generally only found beneath the surface. We report the infrared spectroscopic identification of a CO2 ice-rich surface area located in the Anhur region of comet 67P/Churyumov-Gerasimenko. Spectral modeling shows that about 0.1% of the 80- by 60-meter area is CO2 ice. This exposed ice was observed a short time after the comet exited local winter; following the increased illumination, the CO2 ice completely disappeared over about 3 weeks. We estimate the mass of the sublimated CO2 ice and the depth of the eroded surface layer. We interpret the presence of CO2 ice as the result of the extreme seasonal changes induced by the rotation and orbit of the comet.

Rosetta's comet 67P/Churyumov-Gerasimenko sheds its dusty mantle to reveal its icy nature.

The Rosetta spacecraft has investigated comet 67P/Churyumov-Gerasimenko from large heliocentric distances to its perihelion passage and beyond. We trace the seasonal and diurnal evolution of the colors of the 67P nucleus, finding changes driven by sublimation and recondensation of water ice. The whole nucleus became relatively bluer near perihelion, as increasing activity removed the surface dust, implying that water ice is widespread underneath the surface. We identified large (1500 square meters) ice-rich patches appearing and then vanishing in about 10 days, indicating small-scale heterogeneities on the nucleus. Thin frosts sublimating in a few minutes are observed close to receding shadows, and rapid variations in color are seen on extended areas close to the terminator. These cyclic processes are widespread and lead to continuously, slightly varying surface properties.

Density duct formation in the wake of a travelling ionospheric disturbance: Murchison Widefield Array observations

AbstractGeomagnetically aligned density structures with a range of sizes exist in the near‐Earth plasma environment, including 10–100 km wide VLF/HF wave‐ducting structures. Their small diameters and modest density enhancements make them difficult to observe, and there is limited evidence for any of the several formation mechanisms proposed to date. We present a case study of an event on 26 August 2014 where a travelling ionospheric disturbance (TID) shortly precedes the formation of a complex collection of field‐aligned ducts, using data obtained by the Murchison Widefield Array (MWA) radio telescope. Their spatiotemporal proximity leads us to suggest a causal interpretation. Geomagnetic conditions were quiet at the time, and no obvious triggers were noted. Growth of the structures proceeds rapidly, within 0.5 h of the passage of the TID, attaining their peak prominence 1–2 h later and persisting for several more hours until observations ended at local dawn. Analyses of the next 2 days show field‐aligned structures to be preferentially detectable under quiet rather than active geomagnetic conditions. We used a raster scanning strategy facilitated by the speed of electronic beamforming to expand the quasi‐instantaneous field of view of the MWA by a factor of 3. These observations represent the broadest angular coverage of the ionosphere by a radio telescope to date.

Observations of the F3-layer at equatorial region during 2005

The existence of the F3-layer has been observed at Brazilian equatorial stations. This paper reports on a 1-year observations at Parit Raja, Malaysia (Lat. 1°52′N, Long. 103°48′E). The greatest number of appearances of the F3-layer is around local noon while the least is after local dawn. Its occurrence is more pronounced during winter and the equinoxes. The mean height of reflection for the layer is about 600 km reaching a maximum of about 900 km during the winter of 2004/2005. It is seen that the critical frequency of the F2-layer decreases with the appearance of the F3-layer.

Latitudinal Variations of Diurnal Meteor Rates

The presence of a diurnal variation in meteor activity is well established. The sporadic meteor count rates are higher on the local dawn side and lower on the local dusk side. This phenomenon is caused by the Earth’s orbital motion and rotation. Meteor radar measurements have been compared from Esrange, Kiruna, Sweden, at 68° N, from Juliusruh, Germany, at 55° N, and from Ascension Island, at 8° S, to investigate how the diurnal variation depends on season at different latitudes. Data have been used from vernal and autumnal equinoxes and summer and winter solstices to locate the largest seasonal differences.

Auroral optical emissions during the solar magnetic cloud event of October 1995

The solar magnetic cloud of October 18, 1995, swept by the Earth's magnetosphere, starting at about 2200 UT. This event triggered prolonged auroral displays in the terrestrial atmosphere. The optical emissions from these auroras were recorded with a CCD spectrograph, operating at the radar site in Sondrestromfjord, Greenland. Continuous spectroscopic measurements were made for about 10 hours, from 2200 UT on October 18, 1995, until local dawn set in at 0800 UT on October 19, 1995. These measurements show markedly different spectral characteristics from those of normal auroras observed at other times in various sectors of the auroral oval and in the polar cap region. The differences are most pronounced in the relative brightnesses of the ion and neutral line emissions from the thermospheric oxygen atoms as well as in the O2 At band emissions. Other differences between the normal auroras and the magnetic cloud induced auroras relate to the temperature of the thermospheric height regions where the N2 1PG and the N2+ M band emissions peak in these auroras. Least squares synthetic profiles fit to the rotational distributions of the N2 1PG and the N2+ M bands observed during the solar magnetic cloud event, coupled with auroral model calculations constrained to reproduce the measured emission brightnesses, indicate a higher than normal height for the emissions from the auroras associated with this event. Further evidence for the higher emission altitude is derived from the detailed comparison of the spectral distribution observed during this period with various other measurements of the relatively high‐altitude auroral emissions. All these observations point to the precipitation in the polar terrestrial atmosphere of electrons with an average energy of about 500eV ± 100eV during the interaction of the solar magnetic cloud with the near‐Earth space environment. Such an auroral electron precipitation event is unusual and occurs infrequently; it is similar to the rare type A global red aurora.

Survey of discrete hard X ray enhancements observed from low‐Earth orbit

Systematics of discrete hard X ray enhancements detected using the Army Background Experiment between April 1990 and October 1992 are presented. A total of 822 enhancement events were detected, most often within two bands of geomagnetic latitude that overlay (1) the inner trapped radiation belt and (2) the slot between inner and outer radiation belts. Local hour and seasonal variations are strongest for the innerzone events, which occur preferentially just after local dawn and during the northern summer months.

Ray-tracing analysis of multicomponent whistlers propagating in the asymmetric interhemispheric plasma

A ray-tracing model in which an interhemispheric plasma asymmetry exists, has been used to investigate the propagating characteristics of multicomponent whistlers recorded at Siple Station (L = 4.3), during local dawn on 4 Jul 1982, under very quiet geomagnetic conditions. The 3 kHz rays were started in the summer hemisphere at 300 km altitude, with vertical wave normals at 0.001 0 latitude intervals and over initial latitude ranges from which rays became trapped in eight ducts (L = 2.9 – 4.6). Final latitudes and final wave normal distributions at 300 km altitude in the winter hemisphere were calculated. The best results for trapping rays in each duct were obtained for a duct modeled ( δ = 0.15 and σ d = 50 km) to terminate at 300 km altitude and to reach full enhancement at 2100 km in both hemispheres, disregarding the seasonal variations. The oxygen ion, electron density ratio and ratio of temperature asymmetry, between summer and winter hemispheres were selected for each duct in such a way that electron density and oxygen ion concentrations were similar to data from the ISIS-2 satellite. Ray-tracing calculations were performed for 3 kHz ray that propagated in the magnetosphere in ducted mode or guided by gradient trapping.

The temporal variation of the frequency of high latitude field line resonances

The diurnal variation in the frequencies of the continuum of ULF field line resonances has been calculated by using the cross‐spectral phase of the north‐south components of data from latitudinally spaced ground magnetometers in the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) array. On most days the continuum is seen only during the local daytime, and only a single harmonic with an inverted U‐shaped temporal variation in frequency is seen. At 67° geomagnetic latitude (L = 6.6) the general trend is a resonant frequency around 2 mHz near local dawn, increasing up to ∼5 mHz by 0600‐0700 local time, followed by a decrease in frequency to 2 mHz by 1500–1600 local time. Near local noon, the fundamental resonant frequency is ∼3 mHz at 71° (L = 11.3), increasing monotonically to 7 mHz at 65° (L = 6.1). The waves appear to be a part of the resonant Alfvén mode continuum as opposed to the single‐frequency, driven magnetic field line resonances often seen at high latitudes. The cross‐phase spectra show evidence of impulsively driven resonances that energize the continuum over the latitudinal range of the CANOPUS magnetometers. The temporal variation in the resonant frequency is modeled by using the Tsyganenko (1987) magnetic field model and cold plasma MHD theory. With the use of the observed resonant frequencies, the plasma density for June 1, 1990, was 4.2 × 106 H+/m³ at L = 6.6 while the data for June 7, 1990, showed densities up to 100×106 H+/m³. These results suggest that observations of the magnetohydrodynamic continuum in the magnetometer data may give a very effective method for ground‐based time‐dependent mapping of the equatorial plasma density.

Plasma Observations at Venus with Galileo

Plasma measurements were obtained with the Galileo spacecraft during an approximately 3.5-hour interval in the vicinity of Venus on 10 February 1990. Several crossings of the bow shock in the local dawn sector were recorded before the spacecraft passed into the solar wind upstream from this planet. Although observations of ions of the solar wind and the postshock magnetosheath plasmas were not possible owing to the presence of a sunshade for thermal protection of the instrument, solar wind densities and bulk speeds were determined from the electron velocity distributions. A magnetic field-aligned distribution of hotter electrons or ;;strahl'' was also found in the solar wind. Ions streaming into the solar wind from the bow shock were detected. Electron heating at the bow shock, </=20%, was notably small, with substantial density increases by factors of 2 to 3 at the day side of the shock that decrease for shock crossings further downstream from the planet. A search for pickup ions from the hot hydrogen and oxygen planetary coronas yielded an upper limit for these densities in the range of 10(-3) ion per cubic centimeter, which is consistent with densities expected from current models of neutral gas densities.

Simultaneous energetic particle observations at geostationary orbit and in the upstream solar wind: Evidence for leakage during the magnetospheric compression event of November 1, 1984

Energetic ion (E ≳ 70 keV) and electron (E ≳ 30 keV) observations are presented for three fortuitously positioned geostationary spacecraft during a strong magnetospheric compression event which brought the subsolar magnetopause inward of the synchronous orbit (r = 6.6 RE) on November 1, 1984. Active Magnetospheric Particle Tracer Explorers (AMPTE) Charge Composition Explorer (CCE) field and particle data show that the CCE spacecraft (at r = 8.8 RE) was in the upstream solar wind during the compression event and several upstream particle bursts were observed between 0650 and 0930 UT. Spacecraft 1984‐037 near 2200 LT observed multiple, sharp energetic particle injection events indicative of substorm onsets occurring at ∼0650, 0720, and 0750 UT on November 1. Spacecraft 1981‐025 near local noon observed some brief magnetopause encounters as early as 0635 UT, and after 0650 UT it remained near (and often outside) the magnetopause for most of the ensuing 3 hours. Observations of energetic particle fluxes and anisotropies near the noon magnetopause suggest that magnetospheric particles escaped into the dayside magnetosheath. Spacecraft 1982‐019 (near local dawn) observed strong energetic electron flux enhancements consistent with azimuthal eastward drift following the substorm injections near local midnight. The flux of ions at spacecraft 1982‐019, however, was depleted, particularly at ∼90° pitch angles. This suggests that in their westward drift from local midnight the energetic ions failed to transit the outer dayside magnetosphere in the subsolar region but were instead lost to the magnetosheath, where their drift paths intercepted the postnoon magnetopause surface. Thus the pattern of magnetopause escape established by the synchronous orbit spacecraft together with excellent time association and spectral similarity with the CCE bursts upstream suggests that the process of magnetospheric particle escape is the most likely source for energetic particles in the magnetosheath and the upstream solar wind in this favorably observed case.

Rocket investigations of electron precipitation and VLF waves in the Antarctic upper atmosphere

The two Antarctic rocket campaigns conducted by American scientists during the past 10 years are described, and the results of these efforts are reviewed. Although the primary objective of both campaigns, the observation of precipitating electrons triggered by the Siple VLF transmitter, was not achieved, significant advancements in understanding upper atmosphere phenomena were made. Standing wave patterns in the ionosphere were observed for the first time by wave experiments flown aboard Nike‐Tomahawk rockets. The same detectors were able to monitor a continuous signal from the transmitter through the neutral atmosphere and into the ionosphere. This in situ observation of a radio wave traversing a neutral and a plasma environment provided unique data for comparison to theoretical studies of wave propagation. SuperArcas measurements provided two distinct types of energetic electron precipitation data: one type was continuous on the time scale of &gt; 1 min, and the other was in X ray microbursts created by bursts of energetic electrons stopping in the upper atmosphere with durations &lt; 1 s. The burst data were compared to theoretical models of wave‐particle interactions. The observations of continuous electron precipitation with sufficient energy to penetrate the atmosphere to an altitude of 60–80 km were found to be consistent with a model of electron transport from the trapped population to the atmosphere in which gradient and curvature drift in the drift cone plays a principal role. The model requires slow electron pitch angle diffusion of the geomagnetically trapped population and predicts that almost all energetic electron precipitation occurs in the South Atlantic Anomaly. The electron precipitation model is shown to organize all of the SuperArcas data, which include not only measurements at Siple Station, Antarctica, but also those at Kerguelen, another L = 4 location in the southern hemisphere, and at northern locations at higher latitudes. A prediction of the model which involves the effect of the dawn‐to‐dusk convection electric field on energetic electron precipitation is described: diurnal modulation is predicted with the maximum electron precipitation intensity when the South Atlantic Anomaly is at local dawn. A test of this prediction made by SuperArcas flights in 1981 provided support for the slow diffusion model. The SuperArcas results on energetic electron precipitation are shown to disagree with a satellite result at L = 4 which requires fast diffusion. Reasons for the discrepancy between the two data sets are discussed, and steps which could be taken to resolve the apparent controversy are suggested. Finally, the suggestion is made that the two rocket campaigns conducted by American scientists in Antarctica during the past decade be regarded as a feasibility study leading to an active rocket research program in Antarctica for the remainder of the century.

Electrodynamics of very high latitude arcs in the morning sector auroral zone

In November 1985 an experimental campaign was conducted at Sondre Stromfjord, Greenland, to study the electrodynamics of auroral arcs at very high latitudes. The experiments involved coordinated observations by the Sondrestrom radar and two polar orbiting spacecraft. For several hours around local dawn on November 17 a system of auroral arcs was observed over Sondrestrom. These arcs varied in intensity and width but were all extended generally in the magnetic east‐west direction. A Defense Meteorological Satellite Program photograph obtained during the observations showed that two types of arcs were present. In the south the arcs were localized enhancements or striations within a broad diffuse aurora. Poleward of the diffuse aurora was a system of arc segments that extended over about 5° of latitude. The radar measured enhanced ionospheric electron densities associated with each of these arcs, although the altitudes of the enhancements were different for the two types. In the diffuse aurora the ionization peaked at altitudes near 115 km, whereas in the arc segments the ionization peaked at altitudes above 180 km. Examination of electric field data from the radar indicated that the boundary in ionization associated with the two types of aurora occurred where the southward electric field maximized. The HILAT satellite passed in the vicinity of Sondrestrom during the experiment and measured precipitating electron fluxes and field‐aligned currents. The electron flux data showed that the low‐altitude ionization observed by the radar was produced by electrons with energies near 5 keV. The higher‐altitude ionization was produced by precipitation associated with enhancements in the fluxes of low‐energy electrons. The magnetometer data indicated that the boundary between the region 1 (downward) and region 2 (upward) current sheets in the morning sector is just poleward of the boundary separating the hard and soft precipitation regions. The latitudinal variation in field‐aligned current is consistent with the measured variations in electric field and conductivity.