Low-energy Precipitation Research Articles

Correction to “Nanocrystal Preparation: Low-Energy Precipitation Method Revisited”

Correction to “Nanocrystal Preparation: Low-Energy Precipitation Method Revisited”

Improved Energy Resolution Measurements of Electron Precipitation Observed During an IPDP‐Type EMIC Event

AbstractHigh energy resolution DEMETER satellite observations from the Instrument for the Detection of Particle (IDP) are analyzed during an electromagnetic ion cyclotron (EMIC)‐induced electron precipitation event. Analysis of an Interval Pulsation with Diminishing Periods (IPDP)‐type EMIC wave event, using combined satellite observations to correct for incident proton contamination, detected an energy precipitation spectrum ranging from ∼150 keV to ∼1.5 MeV. While inconsistent with many theoretical predictions of >1 MeV EMIC‐induced electron precipitation, the finding is consistent with an increasing number of experimentally observed events detected using lower resolution integral channel measurements on the POES, FIREBIRD, and ELFIN satellites. Revised and improved DEMETER differential energy fluxes, after correction for incident proton contamination shows that they agree to within 40% in peak flux magnitude, and 85 keV (within 40%) for the energy at which the peak occurred as calculated from POES integral channel electron precipitation measurements. This work shows that a subset of EMIC waves found close to the plasmapause, that is, IPDP‐type rising tone events, can produce electron precipitation with peak energies substantially below 1 MeV. The rising tone features of IPDP EMIC waves, along with the association with the high cold plasma density regime, and the rapidly varying electron density gradients of the plasmapause may be an important factor in the generation of such low energy precipitation, co‐incident with a high energy tail. Our work highlights the importance of undertaking proton contamination correction when using the high‐resolution DEMETER particle measurements to investigate EMIC‐driven electron precipitation.

Robust structural stability and performance‐enhanced asymmetric supercapacitors based on CuMoO4/ZnMoO4 nanoflowers prepared via a simple and low-energy precipitation route

Robust structural stability and performance‐enhanced asymmetric supercapacitors based on CuMoO4/ZnMoO4 nanoflowers prepared via a simple and low-energy precipitation route

Suprathermal electron moments in the ionosphere

The ionospheric electron population is divided into two groups. The ambient electrons are thermalized. Their energy is usually smaller than one electron volt. Their densities and temperatures are the usual ones measured by incoherent scatter radars, or modeled by international codes such as International Reference Ionosphere (IRI). There is however a second population called the suprathermal electrons. This one is either due to photoionization or to electron impact between the thermosphere and the precipitation in the high latitude zone. In the frame of space weather, it may be the source of scintillations, plasma bulks and other physical phenomena. The suprathermal electron population can only indirectly be measured through the plasmaline and had never been modeled. Its modeling requires the computation of the electron stationary flux by solving the Boltzmann transport equation. This flux is multiplied by various powers of the velocity v and integrated to obtain the different order moments. By integrating f over v0dv, one deduces the suprathermal electron density. An integration of v1fdv allows the computation of their mean velocity. Higher moments give access to their temperature and finally to their heat flux. In this work, we demonstrate for the first time the full and rigorous calculation of the ionospheric electron moments up to three. As two case studies, we focus on high latitude in the auroral oval and low magnetic latitude over Algiers for different solar and geophysical conditions. We compare the suprathermal densities and temperatures to the thermal electron parameters. Our results highlight that – as expected – the suprathermal density is small compared to the thermal one. Although it is close to 3 × 103 m−3 at 180 km during the day, it drops drastically at night, to hardly reach 3 m−3. Contrarily to the density, the velocity is about 10 times more important during the nighttime when precipitation occurs than during the daytime under the electromagnetic solar flux. At 400 km, it varies during the day between 700,000 m s−1 (active solar conditions) and 900,000 m s−1 (quiet Sun). At night, the velocity varies between 3 × 106 m s−1 (low mean energy precipitation) and 3 × 107 m s−1 (high mean energy precipitation) at 400 km. The suprathermal temperature increases as the solar activity decreases or as the mean energy of the electron precipitation increases. It may reach values close to 3 × 108 K. The heat flux may be fully oriented downward or experiences a reversal with some flux going up depending on the forcing.

Nanocrystal Preparation: Low-Energy Precipitation Method Revisited

Nanocrystals have the potential to address the challenges of delivering drugs with low aqueous solubility. In this study, the use of low energy anti-solvent precipitation for producing nanocrystals has been investigated. Stable nanocrystals with uniform particle size were prepared for the three model compounds, glyburide, ibuprofen, and artemisinin, which are all practically insoluble in water and have diverse molecular structures and crystal packings. The choice of crystal growth inhibitors/stabilizers was found to be critical and specific for each drug. The effect of the process variables, temperature, stirring rate, and the solute solution infusion rate into the antisolvent, was rationalized in terms of how these factors influence the local supersaturation attained at the earliest stages of precipitation. The dissolution of the nanocrystals in aqueous media under physiological conditions was shown in all cases to occur almost instantaneously, being markedly more rapid than that observed for micronized suspensions of the model drugs and their marketed tablet formulations. Rationalization of the choice of optimum stabilizers in terms of molecular interaction with the exposed crystal surfaces proved to be difficult. The study demonstrates that standard crystallization technology is effective in producing nanocrystals with the primary challenge being physicochemical (rather than mechanical), involving the identification of molecule-specific crystal growth inhibitors and/or stabilizers.

Dynamics and characteristics of black aurora as observed by high-resolution ground-based imagers and radar

High-resolution, multi-spectral data from the ground-based low-light auroral imager ASK (Auroral Structure and Kinetics) are used to characterize the fine structure of black aurora. Sixteen events comprising sheared and unsheared black arcs, as well as black patches and rings, constitute the analysed dataset. Simultaneous measurements of emissions caused by high- and low-energy precipitation make it possible to relate the characteristics of different black structures to the energy of the precipitating electrons. The reductions of high-energy particles versus low-energy particles in the black regions compared to the diffuse background are investigated for the different forms of black aurora. Two separate mechanisms have been suggested to cause black aurora. The larger reduction of high-energy precipitation within the fine-scale black structures discussed here favours a magnetospheric mechanism that blocks high-energy electrons from being scattered into the loss cone. European Incoherent SCATter radar (EISCAT) electron density profiles are available for one of the nights, and are compared to the optical measurements.

Simultaneous imaging of aurora on small scale in OI (777.4 nm) and N<sub>2</sub>1P to estimate energy and flux of precipitation

Abstract. Simultaneous images of the aurora in three emissions, N21P (673.0 nm), OII (732.0 nm) and OI (777.4 nm), have been analysed; the ratio of atomic oxygen to molecular nitrogen has been used to provide estimates of the changes in energy and flux of precipitation within scale sizes of 100 m, and with temporal resolution of 32 frames per second. The choice of filters for the imagers is discussed, with particular emphasis on the choice of the atomic oxygen line at 777.4 nm as one of the three emissions measured. The optical measurements have been combined with radar measurements and compared with the results of an auroral model, hence showing that the ratio of emission rates OI/N2 can be used to estimate the energy within the smallest auroral structures. In the event chosen, measurements were made from mainland Norway, near Troms\\o, (69.6 N, 19.2 E). The peak energies of precipitation were between 1–15 keV. In a narrow curling arc, it was found that the arc filaments resulted from energies in excess of 10 keV and fluxes of approximately 7 mW/m2. These filaments of the order of 100 m in width were embedded in a region of lower energies (about 5–10 keV) and fluxes of about 3 mW/m2. The modelling results show that the method promises to be most powerful for detecting low energy precipitation, more prevalent at the higher latitudes of Svalbard where the multispectral imager, known as ASK, is now installed.

Using spectral characteristics to interpret auroral imaging in the 731.9 nm O<sup>+</sup> line

Abstract. Simultaneous observations were made of dynamic aurora during substorm activity on 26 January 2006 with three high spatial and temporal resolution instruments: the ASK (Auroral Structure and Kinetics) instrument, SIF (Spectrographic Imaging Facility) and ESR (EISCAT Svalbard Radar), all located on Svalbard (78° N, 16.2° E). One of the narrow field of view ASK cameras is designed to detect O+ ion emission at 731.9 nm. From the spectrographic data we have been able to determine the amount of contaminating N2 and OH emission detected in the same filter. This is of great importance to further studies using the ASK instrument, when the O+ ion emission will be used to detect flows and afterglows in active aurora. The ratio of O+ to N2 emission is dependent on the energy spectra of electron precipitation, and was found to be related to changes in the morphology of the small-scale aurora. The ESR measured height profiles of electron densities, which allowed estimates to be made of the energy spectrum of the precipitation during the events studied with optical data from ASK and SIF. It was found that the higher energy precipitation corresponded to discrete and dynamic features, including curls, and low energy precipitation corresponded to auroral signatures that were dominated by rays. The evolution of these changes on time scales of seconds is of importance to theories of auroral acceleration mechanisms.

Multispectral observations of auroral rays and curls

Two cases of discrete aurora are presented, in which auroral curls and auroral rays, respectively, were seen. The aurora was imaged by two spatially separated imagers with a long‐pass filter (mainly sensitive to N2 and N2+ emissions), and another imager with a narrow‐band 7325 Å filter (sensitive to forbidden O+ doublet). Also, spectra of the aurora were recorded. Using the multispectral imaging and spectra we find that the curls occurred in aurora caused by precipitation of energetic electrons with a lack of low‐energy population, while in the rays both high and low energy precipitation were present simultaneously. These findings are confirmed by the altitude determination from triangulation.

Daytime aurora observed from Spitsbergen

AbstractDaytime (or dayside cleft) aurora is almost a permanent feature of the midday skies over Spitsbergen during the continuous darkness of the polar night It was observed in one or other of its characteristic forms around geomagnetic noon on 58 of 59 clear day sduring the wintersof 1987/1988, 1990/1991, and 1992/1993. The three types of day time aurora were studied by visual, colour photographic, and interference-filter techniques to confirm the precise nature of the observed emissions. Prenoon aurora, which is characterised by diffuse, patchy, green aurora at 557.7 nm, was observed on 42 occasions. It is generated by low-energy electrons of less than 300 eV coming through the entry layer of the dayside cleft. Noontime aurora, which consists largely of pure red emissions at 630.0/636.4 nm, was observed on 50 occasions. It is generated by high-flux, very low-energy electrons of 10–50 eV flowing directly from the solar wind through the polar cusp. Postnoon aurora, which is characterised by discrete, green auroral arcs at 557.7 nm, was also observed on 42 occasions. Like prenoon aurora, it is generated by low-energy electrons of less than 300 eV derived from the entry layer of the cleft Occasionally, some background or diffuse aurora is also observed, generated by high fluxes of low-energy proton precipitation and characterised by the hydrogen lines Hα and Hβ. On the one exceptional day on which daytime aurora was not observed, magnetic activity was exceptionally low.These ground-based observations complement satellite studies of analogous auroral events. In particular, the visual characteristics of the different types of daytime aurora may be explained in terms of the flux rates and energy profiles of the electrons that have been mapped in the different regions of the dayside cleft by satellite-borne detectors.

F layer irregularity observations of the SAR arc event of March 5–6, 1981

Over several years, various observers have recorded irregularities in the F layer of the ionosphere at latitudes which were identified as the plasmapause region. During the stable auroral red arc of March 5–6, 1981, intense irregularities were recorded using amplitude fluctuations of satellite beacon signals. In the case of this SAR arc optical intensity was well correlated with irregularity intensity. The location of the small scale (∼1 km) irregularities was centered at plasmapause latitudes; scintillation levels tapered off in the same longitude sector at trough latitudes and were of lower than normal intensity and of different nature at auroral latitudes. Reported optical measurements indicated no visible aurora until areas four L shells poleward. Low energy (<12 eV) electron precipitation accompanied by elevated electron temperatures have been reported within the region covered by SAR arcs. High neutral gas temperatures have also been reported. Scintillation of moderate level is not uncommon for the plasmapause region. In March 1981, approximately 60% of the nights had scintillation levels > 6 dB at 137 MHz lasting over one hour. These irregularities, frequently of lower intensity than those accompanying the March 5–6 SAR arc, are probably initiated by low energy precipitation at these latitudes. However the role of convection is not as yet clear in this latitude region.

Investigation of electron precipitation during an auroral substorm by rocket-borne detectors

Results of auroral X ray observations from two rocket-borne detectors launched from Fort Churchill, Manitoba, Canada, into the breakup and postbreakup phases of an auroral substorm are discussed. The X ray intensity measurements obtained during the expansive phase of the substorm indicate the occurrence of widespread low-energy electron precipitation and the softening of the X ray spectrum during the northward movement of the auroral arcs. Electron data obtained by detectors carried on the same rocket indicate, in contrast, localized regions of high-energy precipitation along the leading edge of the northward propagating arcs. During the recovery phase of the substorm, however, the X ray detector registered very soft spectra which are best represented by two different values of the e folding energy, for the low- and high-energy ends. No significant spectral or temporal variation is observed in the low-energy precipitation. But the high-energy component shows large changes in the spectral shape with time. The electron detectors on board the same rocket exhibit similar variations, indicating that the spectral changes in the high-energy component are due to widespread rather than localized precipitation.

Considerations that the source of auroral energetic particles is not a parallel electrostatic field

It is shown that electrostatic fields parallel ( E 11) to the geomagnetic field cannot be the major mechanism that accelerates charged particles to auroral energies. Principal arguments are that electron and proton precipitation occur simultaneously, and also that precipitated electrons with energies less than 100 eV are found to accompany the electrons with energies of 1–10 keV that excite auroral luminosity. It is further shown that essentially all the ambient plasma in an entire tube of flux is required to sustain this intense low-energy precipitation, and this places a severe constraint on any replenishment process. It is found that generally the upper limit to ( E 11) throughout the auroral regions of the ionosphere and magnetosphere is of order 10 μV/m and it may be appreciably less. Relevant measurements are reviewed briefly. It is concluded that while there may occasionally be significant E 11 fields, they play only a minor role-if any-in auroral phenomena.

Effects of field-aligned currents on the structure of the ionosphere

The influence of electric currents along the magnetic field lines on the high-latitude F-layer ionosphere is investigated theoretically. It is shown that a current of either sign leads to a reduction of the total electron content and the maximum density, if the charge carriers have to be produced in the ionosphere. Low-energy precipitation leads to a corresponding increase. Extremely low densities in the topside ionosphere may be reached for ion currents of the order of 10-6 amp/m2 or electron currents of 10-3 amp/m2. Observational evidence exists for currents at least of the order of 10-6 amp/m2. It is suggested that the ionospheric trough, the F-layer storm, and other similar effects may be explained by field-aligned currents or plasma diffusion, which is also quantitatively accounted for by the present theory.