Observations Of Aurora Research Articles

A nightside source of Saturn's kilometric radiation: Evidence for an inner magnetosphere energy driver

During Cassini's orbit insertion about Saturn, the spacecraft passed within 1.4 Rs of the planet passing from dayside into the nightside region. During this nightside passage, the onboard Radio and Plasma Wave (RPWS) instrument surprisingly detected Saturn kilometric radiation (SKR). Prior to this encounter, it was believed that SKR originated from a high‐latitude dayside source, and radio beams from such a source would not be viewable in this nearplanet night‐side location. Subsequent analysis presented here reveals that this SKR did indeed originate from the near‐midnight region on field lines near L ∼ 10–15. Such a radio source suggests the presence of an active region in the night‐side inner magnetosphere; this source possibly being near the outer edge of the icy‐moon created plasma torus surrounding the planet. The implication is that some of the SKR is driven by an internal energy source that may also account for recent UV aurora observations.

Aurora diaries

The 1771–1813 diary of Thomas Hughes from Stroud, Gloucestershire (51.75°N, 2.22°W), has provided a rich source of historical meteorological data. In addition, it lists 71 nights on which the aurora was seen, between 19 February 1771 and 13 October 1805 inclusive. 90% of Hughes' aurora observations occurred between 1771 and 1789. These observations provide information on the incidence of the aurora in the southern UK, before light pollution. Eleven of the auroral nights recorded by Hughes exactly coincide with central Europe auroral reports and a further nine coincide to within one day. In addition, Hughes' aurora observation of 22 October 1788 exactly coincides with a strong aurora seen in Europe and at Yagamata, Japan. Month by month variations (1771–1805) of aurorae at Stroud show, in common with 20th-century data, two equinoctal maxima, in March-April (19 nights) and October (15 nights). The seasonal variation at Stroud confirms theoretical predictions that the auroral zone has moved north since the 18th century.

World first complex optical instrumental observations of aurora in the Arctic in 1899−1900

Abstract. This report presents data and analysis of visual, photographic and auroral spectral data, obtained by the Russian astronomer J. Sykora from the Russian-Swedish expedition to Spitsbergen during the 1899–1900 winter season, which are historically significant for auroral studies. These data seem to be the first instrumental observations of auroral spectra in the Arctic and some of the emissions discovered have world priority. The second known photos in the world of aurora from the Arctic and undoubtedly the first ones for geomagnetic latitudes of about 75° in the Spitsbergen Archipelago were obtained. The results of the expedition are discussed from a modern point of view and compared with our knowledge of the 21st century. A description of the equipment and methods that were used by Russian astronomers is presented. Both photographic and spectral devices using registration by photographic plates were used, along with special methods of their development and enhancement. Some statistical analysis was done on the basis of the expedition reports and diaries. This analysis shows that by using Sykora's data it was possible to discover the auroral oval or instantaneous auroral distribution over the polar region. Analysis of photographic samples and sketches of the aurora demonstrate typical auroral form outlines as they are described today. Spectral plates exposed for several hours to auroral lights revealed not only the main auroral emissions, which were well-known at that time, but several other unidentified weak emissions, which were rediscovered and interpreted years later. Keywords. History of geophysics (Atmospheric sciences, instruments and techniques)

Two components of ionospheric plasma structuring at midlatitudes observed during the large magnetic storm of October 30, 2003

We consider VHF amplitude scintillations, GPS phase fluctuations, ionosonde measurements, maps of GPS total electron content (TEC), observations of daytime aurora and TIMED GUVI images during the large magnetic storms of October 29–31, 2003, and find two distinct classes of plasma processes that produce midlatitude ionospheric irregularities. One is associated with auroral plasma processes; the other, with storm enhanced density (SED) gradients, a part of which occur in close proximity to sub‐auroral polarization stream (SAPS) electric fields as discussed by J. C. Foster et al. (2002). We analyze in detail the storm event of October 30, 2003. The SAPS‐associated plasma structures may occur by an ion temperature gradient convective instability (M. J. Keskinen et al., 2004), but structuring by auroral processes requires elucidation.

Identification of possible intense historical geomagnetic storms using combined sunspot and auroral observations from East Asia

Abstract. Comprehensive catalogues of ancient sunspot and auroral observations from East Asia are used to identify possible intense historical geomagnetic storms in the interval 210 BC-AD 1918. There are about 270 entries in the sunspot catalogue and about 1150 entries in the auroral catalogue. Special databases have been constructed in which the scientific information in these two catalogues is placed in specified fields. For the purposes of this study, an historical geomagnetic storm is defined in terms of an auroral observation that is apparently associated with a particular sunspot observation, in the sense that the auroral observation occurred within several days of the sunspot observation. More precisely, a selection criterion is formulated for the automatic identification of such geomagnetic storms, using the oriental records stored in the sunspot and auroral databases. The selection criterion is based on specific assumptions about the duration of sunspot visibility with the unaided eye, the likely range of heliographic longitudes of an energetic solar feature, and the likely range of transit times for ejected solar plasma to travel from the Sun to the Earth. This selection criterion results in the identification of nineteen putative historical geomagnetic storms, although two of these storms are spurious in the sense that there are two examples of a single sunspot observation being associated with two different auroral observations separated by more than half a (synodic) solar rotation period. The literary and scientific reliabilities of the East Asian sunspot and auroral records that define the nineteen historical geomagnetic storms are discussed in detail in a set of appendices. A possible time sequence of events is presented for each geomagnetic storm, including possible dates for both the central meridian passage of the sunspot and the occurrence of the energetic solar feature, as well as likely transit times for the ejected solar plasma. European telescopic sunspot drawings from the seventeenth century are also used to assess the credibility of some of the later historical geomagnetic storms defined solely by the East Asian sunspot and auroral records. These drawings cast doubt on a few of the associations between sunspot and auroral observations based entirely on the oriental records, at least to the extent that the occidental drawings provide a more realistic date for central meridian passage of the sunspot actually associated with a particular auroral observation. Nevertheless, on those occasions for which European sunspot drawings are available, the dates of all the pertinent East Asian sunspot and auroral observations are corroborated, apart from just one Chinese sunspot observation. The ancient historical observations of sunspots and aurorae are discussed briefly in terms of modern observations of great geomagnetic storms.

Hemispheric asymmetry of the afternoon electron aurora

Using ultraviolet images from two global auroral imagers, IMAGE FUV in the northern hemisphere and Polar UVI in the south, we present the first synoptic scale conjugate observations of the dayside aurora. We find that the morphology of the afternoon aurora is significantly different in the two hemispheres. Multiple spots in a “string of pearls” configuration are seen in the southern hemisphere while the northern aurora is unstructured. We relate the observed asymmetry in the aurora to the Y GSM component of the IMF: a strong IMF BY modifies the ionospheric convection and field aligned current patterns, leading to the auroral asymmetry. Additionally, we suggest that the instability giving rise to the multiple spot morphology occurs at low altitude.

Search of temporal chaos in TV images of aurora

Television (TV) data obtained during the development of an auroral form containinformation about the spatial and temporal dynamics of the form generation. It is possibleto investigate the spatial distribution of auroral luminosity in each image. Also, onecan study the temporal dynamics of aurora using keograms or “virtual photometers.”In this report we investigate the temporal dynamics using information about the spatialdistribution of auroral luminosity to define the metric in “space of images.” The algorithmof Grassberger-Procaccia has been applied to obtain the correlation dimension as acharacteristic of the temporal chaos in sets of TV images of aurora. The approach has beenapplied to TV observations of pulsation aurora at Lovozero station.

Multi‐fluid simulations of Ganymede's magnetosphere

Comparative studies of 3D multi‐fluid simulations with Galileo magnetometer data are used to develop a quantitative model of the currents and fields within Ganymede's magnetosphere as well as its bulk plasma environment. The model includes contributions from Jupiter's magnetosphere and the flux of different ion species originating from Ganymede's ionosphere. Comparisons between the magnetometer data and the simulation demonstrate good agreement for the strength and structure of Ganymede's magnetosphere. An ionospheric outflow rate of ∼1026 ions/s was found for the simulation, which is well correlated to the sputtering rate determined for the surface of Ganymede that actively supplies the ionosphere. Qualitative comparisons are made with the Hubble Space Telescope observations of Ganymede's UV aurora. The size and location of regions of Jovian magnetospheric plasma precipitation are similar to the observed UV emissions. Plasma acceleration due to reconnection and the size of Ganymede's cusps are also examined.

The location of the open-closed magnetic field line boundary in the dawn sector auroral ionosphere

Abstract. As a measure of the degree of coupling between the solar wind-magnetosphere-ionosphere systems, the rate at which the size of the polar cap (the region corresponding to ionospheric termini of open magnetic flux tubes) varies is of prime importance. However, a reliable technique by which the extent of the polar cap might be routinely monitored has yet to be developed. Current techniques provide particularly ambiguous indications of the polar cap boundary in the dawn sector. We present a case study of space- and ground-based observations of the dawn-sector auroral zone and attempt to determine the location of the polar cap boundary using multi-wavelength observations of the ultraviolet aurora (made by the IMAGE FUV imager), precipitating particle measurements (recorded by the FAST, DMSP, and Cluster 1 and 3 satellites), and SuperDARN HF radar observations of the ionospheric Doppler spectral width boundary. We conclude that in the dawn sector, during the interval presented, neither the poleward edge of the wideband auroral UV emission (140-180nm) nor the Doppler spectral width boundary were trustworthy indicators of the polar cap boundary location, while narrow band UV emissions in the range 130-140nm appear to be much more reliable.

Midlatitude auroras in the south of Eastern Siberia during strong geomagnetic storms on October 29?31, 2003 and November 20?21, 2003

Preliminary results of optical observations of midlatitude auroras at the Geophysical observatory of ISTP SD RAS in the south of Eastern Siberia (52°N, 103°E) during two strong magnetic storms on October 29–31, 2003 and November 20–21, 2003 are analyzed. The results of observations are compared to similar data on midlatitude auroras observed at the geophysical observatory in previous years. The recorded maximum intensities of atmospheric emissions and accompanying solar and geophysical conditions make it possible to conclude that the analyzed midlatitude auroras in their optical manifestation can be considered as extreme events observed at midlatitudes.

Solar Activity in the Past: From Different Proxies to Combined Reconstruction

Using proxy series that includes ancient observations of sunspots and auroras, concentrations of cosmogenic isotopes 14C and 10Be, we reconstruct sunspot activity level since 850 AD to the present. As a main reference index of solar activity we use the Wolf sunspot numbers, which, as we demonstrate, reflect true levels of the activity in the 18th and 19th centuries better than the Group sunspot numbers. We construct a set of linear and nonlinear inductive models, which are in a good agreement with each other and reproduce the known global solar activity extrema. According to our results, amplitudes of the global maxima are in intermediate positions in respect to estimations of other authors. It follows from our reconstructions that the global maximum of the 20th century is not much higher, if at all, than others.

Magnetic Storms in October 2003

Preliminary results of an analysis of satellite and ground-based measurements during extremely strong magnetic storms at the end of October 2003 are presented, including some numerical modeling. The geosynchronous satellites Ekspress-A2and Ekspress-A3, and the low-altitude polar satellites Coronas-F and Meteor-3M carried out measurements of charged particles (electrons, protons, and ions) of solar and magnetospheric origin in a wide energy range. Disturbances of the geomagnetic field caused by extremely high activity on the Sun were studied at more than twenty magnetic stations from Lovozero (Murmansk region) to Tixie (Sakha-Yakutia). Unique data on the dynamics of the ionosphere, riometric absorption, geomagnetic pulsations, and aurora observations at mid-latitudes are obtained.

Cassini observations of Io's visible aurorae

More than 500 images of Io in eclipse were acquired by the Cassini spacecraft in late 2000 and early 2001 as it passed through the jovian system en route to Saturn (Porco et al., 2003, Science 299, 1541–1547). Io's bright equatorial glows were detected in Cassini's near-ultraviolet filters, supporting the interpretation that the visible emissions are predominantly due to molecular SO 2. Detailed comparisons of laboratory SO 2 spectra with the Cassini observations indicate that a mixture of gases contribute to the equatorial emissions. Potassium is suggested by new detections of the equatorial glows at near-infrared wavelengths from 730 to 800 nm. Neutral atomic oxygen and sodium are required to explain the brightness of the glows at visible wavelengths. The molecule S 2 is postulated to emit most of the glow intensity in the wavelength interval from 390 to 500 nm. The locations of the visible emissions vary in response to the changing orientation of the external magnetic field, tracking the tangent points of the jovian magnetic field lines. Limb glows distinct from the equatorial emissions were observed at visible to near-infrared wavelengths from 500 to 850 nm, indicating that atomic O, Na, and K are distributed across Io's surface. Stratification of the atmosphere is demonstrated by differences in the altitudes of emissions at various wavelengths: SO 2 emissions are confined to a region close to Io's surface, whereas neutral oxygen emissions are seen at altitudes that reach up to 900 km, or half the radius of the satellite. Pre-egress brightening demonstrates that light scattered into Jupiter's shadow by gases or aerosols in the giant planet's upper atmosphere contaminates images of Io taken within 13 minutes of entry into or emergence from Jupiter's umbra. Although partial atmospheric collapse is suggested by the longer timescale for post-ingress dimming than pre-egress brightening, Io's atmosphere must be substantially supported by volcanism to retain auroral emissions throughout the duration of eclipse.

Coordinated optical and EISCAT observations of black aurora

An image intensified video camera was used in conjunction with the EISCAT radar in Tromsø, Norway, for detailed studies of non‐sheared black aurora on March 5, 2002. While it is known that a downward field‐aligned current will decrease the ionospheric plasma density, any measurable effect will depend on the noise‐level in the density measurements and the background density. The results show that the plasma density is not reduced in the regions of black aurora, which shows that if a downward current existed within the black aurora, it must be less than ∼0.6 μA/m2. Combining this result with recent findings that small scale currents are generally strong, that the downward currents are associated with intense diverging electric fields, and that the observed black arcs show no signs of shear motion, it is concluded that the non‐sheared black aurora is not associated with any significant downward field‐aligned current.

Sunlit aurora and the N 2+ ion: a personal perspective

After a brief autobiographical summary, a description is given of observations of sunlit aurora and the corresponding twilight and dayglow effects, starting with the classic work of Størmer, which showed the phenomenon to extend as high as 1100 km . Spectroscopic studies found that the unusual intensity is caused by resonant scattering of sunlight by the N 2 + ion, and that this ion has a very short lifetime at altitudes below 160 km . At these altitudes an ion-atom reaction with the oxygen atom converts it to NO +. Independent work by others led to the suggestion that the same reaction is important in production of the ionosphere. The great extent of the sunlit aurora to high altitudes can only be understood if the ions are transported upwards from their site of production at lower altitudes. More recent observations from rockets, satellites, and the Space Shuttle have confirmed and extended the picture obtained from ground-based remote sensing.

High resolution FUV observations of proton aurora

We present new FUV observations of Doppler‐shifted Lyman‐α emissions from proton aurora obtained from the High Resolution Ionospheric and Thermospheric Spectrograph (HITS) aboard the Advanced Research and Global Observation Satellite (ARGOS). The HITS instrument observes the Doppler‐shifted H Lyman‐α emissions from proton precipitation with 1.5 Å resolution. The high spectral resolution of these measurements allows the auroral Lyman‐α emissions to be separated from the intense geocoronal background. The measured proton aurora Doppler spectra are modeled using a Monte Carlo simulation of proton flux transport. The forward modeling performed with this code allows us to infer the characteristic energies of the incident protons from the measured Lyman‐α Doppler spectra. The inferred characteristics of dayside versus nightside proton precipitation are compared and used to identify the magnetospheric regions from which the particles likely originated.

Io's auroral limb glow: Hubble Space Telescope FUV observations

Observations of Io's FUV aurora were obtained using the Space Telescope Imaging Spectrograph (STIS) on several occasions between 1997 and 2000. The STIS first‐order long‐slit spectroscopy mode and 2″ wide slit were used to produce nearly monochromatic images of Io. These images reveal several distinct auroral features, including limb brightened emissions at the poles, designated “limb glow”. A detailed study of the limb glow brightness in OI] 1356 Å images is reported. The limb glow on the hemisphere facing the plasma torus centrifugal equator is consistently brighter than on the other hemisphere, and is brighter by a factor of ∼2 when Io is farthest from the plasma torus centrifugal equator. We determine that this behavior is consistent with there being more electrons and more electron energy in the part of an intersecting plasma torus flux tube located above the brighter hemisphere. Since most of the electrons in an intersecting flux tube have time to travel along the field line and collide with Io before the flux tube moves downstream across Io's poles, more torus electron energy is transferred into the polar hemisphere with brighter aurora. We demonstrate that a Kappa distribution model of the plasma torus electron column density above each hemisphere predicts to first order the ratio of north and south limb glow brightnesses as a function of Io's distance from the plasma torus centrifugal equator. This finding illustrates the importance of field‐aligned torus electron energy transport for producing Io's aurora.

Proton aurora in the cusp during southward IMF

One of the most distinct aurorae in the high‐latitude dayside region occurs at the footprint of the cusp. The Spectrographic Imager SI‐12 channel on the IMAGE spacecraft allows for long‐duration observations of the proton aurora in this region and thus enables morphological and quantitative studies of the behavior of the proton aurora under different external solar wind conditions. Here we extend an earlier study of the proton aurora in the cusp during northward interplanetary magnetic field (IMF) conditions [Frey et al., 2002] to southward IMF conditions. We find a very similar dependence of the cusp aurora's location in magnetic local time (MLT) on the IMF By component, with locations at prenoon for negative By and postnoon for positive By. We also confirm the previously established dependence of the proton aurora brightness on the solar wind dynamic pressure. However, for similar solar wind pressures the proton aurora is brighter for the southward IMF case than for the northward IMF case. The other major difference between southward and northward IMF conditions is the very strong dependence of the latitude location on the magnitude of the IMF Bz for southward IMF. We discuss both differences in terms of the location of the reconnection site at the magnetopause that is connected to the ionosphere along cusp magnetic field lines.

Multi-frequency observations of E-region HF radar aurora

Abstract. Multi-frequency observations of E-region coherent backscatter from decametre waves reveal that auroral echoes tend to comprise two spectral components superimposed, one at low Doppler shifts, below 250 ms-1, and the other Doppler shifted to near the ion-acoustic speed or above, up to 800 ms-1. The low Doppler shift component occurs at all look directions; Doppler shifts near the ion acoustic speed occur when looking at low flow angles along the direction of the electron drift in the electrojet, and Doppler shifts in excess of the ion acoustic speed occur at intermediate flow angles. The latter population appears most commonly at radar frequencies near 10–12 MHz, with its occurrence decreasing dramatically at higher frequencies. The velocity of the high Doppler shift echoes increases with increasing radar frequency, or irregularity wave number k. The velocity of the low Doppler shift population appears to be suppressed significantly below the line-of-sight component of the electron drift. Initial estimates of the altitude from which scatter occurs suggest that the high Doppler shift echoes originate from higher in the E-region than the low Doppler shift echoes, certainly in the eastward electrojet. We discuss these observations with reference to the theories of de/stabilization of two-stream waves by electron density gradients and electrostatic ion cyclotron waves excited by field-parallel electron drifts.Key words. Ionosphere (ionospheric irregularities)

IMAGE FUV and in situ FAST particle observations of substorm aurorae

Images from the IMAGE Wide‐band Imaging Camera (WIC) and Spectrographic Imager (SI) channels SI‐12 and SI‐13 were compared to in situ data taken by FAST during several substorms. FAST spacecraft observations have shown that the high latitude auroral ionosphere has several distinct regions. Intense auroras are seen in regions of upward directed quasi static electric fields and of Alfven wave accelerated superthermal electrons. In two of the cases presented, the satellite passed through an active poleward propagating substorm surge on its duskward flank. Both show that the superthermal wave accelerated component to be on the polar cap boundary of the surge, and that it could be distinguished from quasi static field “inverted V” precipitation which occurred at the more equatorward parts of the auroral oval. In one of these cases, the surge was accompanied by intense ion outflow. Three cases showed the FAST satellite passing through the substorm aurora at midnight or the dawn side outside of the surge and the wave accelerated electrons were less clearly separated from the inverted V type precipitation. The wave accelerated electrons were seen to be part of very short‐lived transient events, i.e., bursts. The region of auroral forms, associated with Alfven waves exhibit relatively soft auroral precipitation with very intense electron fluxes. By comparing the intensity of the WIC and SI‐13 channels of IMAGE FUV, it is possible sometimes to distinguish optically between the two types of regions. Global imaging of the two regions would allow the separation of quasi‐static plasma convection regions containing inverted V‐s from regions of Alfven wave driven electrons signifying substorm related magnetic field dynamics.