62 research outputs found
Appearance and Precipitation Characteristics of High-Latitude Pulsating Aurora
Characteristics of pulsating aurora (PsA) at the equatorward part of the auroral oval have been well described in the literature by previous studies. We extend our knowledge on high-latitude PsA observations by analysing 68 PsA events from the optical observatory on Svalbard, at 75° magnetic latitude. We found that the pulsating emission structures are particularly large and transient, they do not experience drift motion, or their drift motion cannot be traced. Our results show that the high-latitude PsA events relate to lower geomagnetic activity and weaker solar wind driving than the lower latitude PsA. The high-latitude PsA events also occur less frequently, which is in agreement with their association to lower-than-average geomagnetic activity. We further show that the ionospheric electron density values during high-latitude PsA events are low compared to the lower latitude PsA. This, together with the non-traceable nature of the pulsating emission structures, suggests that these events are strongly dominated by a sub-type called Amorphous Pulsating Aurora (APA). We therefore conclude that, unlike the lower latitude PsA events, the high-latitude PsA events are not likely to cause direct changes in the chemical composition of the mesosphere
Observational Validation of Cutoff Models as Boundaries of Solar Proton Event Impact Area
High energy protons accelerated during solar proton events (SPEs) can access the Earth's middle atmosphere at high and middle latitudes causing large‐scale ionization and chemical changes. In this study, we have compared the performance of two cutoff latitude models that predict the limit of the SPE impact area in the atmosphere during 73 SPEs from 1997 to 2010. We use observations from 13 riometer stations and the D Region Absorption Prediction (DRAP) model to test the performance of the two cutoff latitude models by Dmitriev et al. (2010, https://doi.org/10.1029/2010JA015380) and Nesse Tyssøy and Stadsnes (2015, https://doi.org/10.1002/2014JA020508). We find similar performance from the two cutoff latitude models with respect to observations, but the Dmitriev et al. (2010, https://doi.org/10.1029/2010JA015380) model performs slightly better when observations are contrasted with the DRAP model results. The better performing model is also continuous with magnetic local time and particle energy, making it more suited for future use in climate model proton forcing. SPE forcing is currently included in climate models with a single static cutoff latitude limit at 60° geomagnetic latitude. In reality, the area that the solar protons can access is not static but varies with particle rigidity and geomagnetic conditions. We estimate that the SPE impact area is overestimated 90% of the time by this single static cutoff limit and the average overestimation of the impact area is about 15–25% for protons with energies <32 MeV
Appearance and Precipitation Characteristics of High-Latitude Pulsating Aurora
Characteristics of pulsating aurora (PsA) at the equatorward part of the auroral oval have been well described in the literature by previous studies. We extend our knowledge on high-latitude PsA observations by analysing 68 PsA events from the optical observatory on Svalbard, at 75 degrees magnetic latitude. We found that the pulsating emission structures are particularly large and transient, they do not experience drift motion, or their drift motion cannot be traced. Our results show that the high-latitude PsA events relate to lower geomagnetic activity and weaker solar wind driving than the lower latitude PsA. The high-latitude PsA events also occur less frequently, which is in agreement with their association to lower-than-average geomagnetic activity. We further show that the ionospheric electron density values during high-latitude PsA events are low compared to the lower latitude PsA. This, together with the non-traceable nature of the pulsating emission structures, suggests that these events are strongly dominated by a sub-type called Amorphous Pulsating Aurora (APA). We therefore conclude that, unlike the lower latitude PsA events, the high-latitude PsA events are not likely to cause direct changes in the chemical composition of the mesosphere.Peer reviewe
Auroral Morphological Changes to the Formation of Auroral Spiral during the Late Substorm Recovery Phase: Polar UVI and Ground All-Sky Camera Observations
The ultraviolet imager (UVI) of the Polar spacecraft and an all-sky camera at
Longyearbyen contemporaneously detected an auroral vortex structure (so-called
"auroral spiral") on 10 January 1997. From space, the auroral spiral was
observed as a "small spot" (one of an azimuthally-aligned chain of similar
spots) in the poleward region of the main auroral oval from 18 h to 24 h
magnetic local time. These auroral spots were formed while the
substorm-associated auroral bulge was subsiding and several poleward-elongated
auroral streak-like structures appeared during the late substorm recovery
phase. During the spiral interval, the geomagnetically north-south and
east-west components of the geomagnetic field, which were observed at several
ground magnetic stations around Svalbard island, showed significant negative
and positive bays caused by the field-aligned currents related with the aurora
spiral appearance. The negative bays were reflected in the variations of local
geomagnetic activity index (SML) which was provided from the SuperMAG
magnetometer network at high latitudes. To pursue the spiral source region in
the magnetotail, we trace each UVI image along field lines to the magnetic
equatorial plane of the nightside magnetosphere using an empirical magnetic
field model. Interestingly, the magnetotail region corresponding to the auroral
spiral covered a broad region from Xgsm ~ -40 to -70 RE at Ygsm ~ 8 to 12 RE.
The appearance of this auroral spiral suggests that extensive areas of the
magnetotail (but local regions in the ionosphere) remain active even when the
substorm almost ceases, and geomagnetic conditions are almost stable.Comment: 39 Pages, 6 Figures (8 pages), 1 Table, and Supporting Information
file (including 2 Figures (8 pages) and 1 Movie
On the determination of ionospheric electron density profiles using multi-frequency riometry
Radio wave absorption in the ionosphere is a function of electron density, collision frequency, radio wave polarisation, magnetic field and radio wave frequency. Several studies have used multi-frequency measurements of cosmic radio noise absorption to determine electron density profiles. Using the framework of statistical inverse problems, we investigated if an electron density altitude profile can be determined by using multi-frequency, dual-polarisation measurements. It was found that the altitude profile cannot be uniquely determined from a “complete” measurement of radio wave absorption for all frequencies and two polarisation modes. This implies that accurate electron density profile measurements cannot be ascertained using multi-frequency riometer data alone and that the reconstruction requires a strong additional a priori assumption of the electron density profile, such as a parameterised model for the ionisation source. Nevertheless, the spectral index of the absorption could be used to determine if there is a significant component of hard precipitation that ionises the lower part of the D region, but it is not possible to infer the altitude distribution uniquely with this technique alone.</p
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