Penetration of MeV electrons into the mesosphere accompanying pulsating aurorae

Pulsating aurorae (PsA) are caused by the intermittent precipitations of magnetospheric electrons (energies of a few keV to a few tens of keV) through wave-particle interactions, thereby depositing most of their energy at altitudes ~ 100 km. However, the maximum energy of precipitated electrons and its impacts on the atmosphere are unknown. Herein, we report unique observations by the European Incoherent Scatter (EISCAT) radar showing electron precipitations ranging from a few hundred keV to a few MeV during a PsA associated with a weak geomagnetic storm. Simultaneously, the Arase spacecraft has observed intense whistler-mode chorus waves at the conjugate location along magnetic field lines. A computer simulation based on the EISCAT observations shows immediate catalytic ozone depletion at the mesospheric altitudes. Since PsA occurs frequently, often in daily basis, and extends its impact over large MLT areas, we anticipate that the PsA possesses a significant forcing to the mesospheric ozone chemistry in high latitudes through high energy electron precipitations. Therefore, the generation of PsA results in the depletion of mesospheric ozone through high-energy electron precipitations caused by whistler-mode chorus waves, which are similar to the well-known effect due to solar energetic protons triggered by solar flares.

Observations of spectra of polar mesospheric summer echoes at 224 MHz using EISCAT radar during particle precipitation events – Some case studies from July 2019

<p>We present the initial results from investigation of polar mesospheric summer echoes (PMSE) spectra at 224 MHz observed by EISCAT VHF radar operated from Ramfjordmoen near Tromsø during July 2019. Since EISCAT UHF measurements were not available, we used the sudden enhancements in electron densities derived from the VHF observations above 90 km as indicators of particle precipitation. We note that the altitude extent of the PMSE increased along with an enhancement of the strength of the pre-existing PMSE. However, a closer examination reveals that the PMSE strengths vary significantly between different heights in the region of 80 to 90 km. Interestingly, the spectral widths show well separated regimes between the top and the bottom part of the PMSE layers following particle precipitation. In the altitudes where the maximum enhancement in PMSE backscatter occurred, there is no corresponding enhancement in the spectral widths. The frequency Doppler shifts showed alternating upward and downward motions without much difference before and after the particle precipitation. This indicates that the moderate levels of particle precipitation observed herein did not affect the vertical winds considerably. Further, after the particle precipitation subsided, the PMSE intensities continued to be stronger for a while.</p>

Middle atmosphere ionization from auroral particle precipitation as observed by the SSUSI satellite instruments

<p>Solar, auroral, and radiation belt electrons enter the atmosphere at polar regions leading to ionization and affecting its chemistry. Climate models usually parametrize this ionization and the related changes in chemistry based on satellite particle measurements. Precise measurements of the particle and energy influx into the upper atmosphere are difficult because they vary substantially in location and time. Widely used particle data are derived from the POES and GOES satellite measurements which provide electron and proton spectra.</p><p>We present electron energy and flux measurements from the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) satellite instruments on board the Defense Meteorological Satellite Program (DMSP) satellites. This formation of now four satellites observes the auroral zone in the UV from which electron energies and fluxes are inferred in the range from 2 keV to 20 keV. We use these observed electron energies and fluxes to calculate ionization rates and electron densities in the upper mesosphere and lower thermosphere (≈ 70–200 km). We present an initial comparison of these rates to other models and compare the electron densities to those measured by the EISCAT radar. This comparison shows that with the current standard parametrizations, the SSUSI inferred auroral (90–120 km) electron densities are larger than the ground-based measured ones by a factor of 2–5. It is still under investigation if this difference is due to collocation (in space and time) and EISCAT mode characteristics or caused by incompletely modelling the ionization and recombination in that energy range.</p>

Characteristics of a HSS-driven magnetic storm in the high-latitude ionosphere; A case study of 14th of March 2016 storm

<p> Solar wind High-Speed Streams (HSSs) affect the auroral ionosphere in many ways, and several separate studies have been conducted of the different effects seen e.g. on aurora, geomagnetic disturbances, F-region behavior, and energetic particle precipitation. In this work, we study an HSS event in the solar cycle (24), which was associated with a co-rotating interaction region (CIR) that hit the Earth’s magnetopause at about 17:20 UT on 14 March 2016. The associated magnetic storm lasted for seven days, and the Dst index reached -56 nT. We use a very comprehensive set of measurements to study the whole period of this storm, following day by day for the magnetic indices and solar wind parameters and relating its consequences on ionospheric plasma parameters. We use EISCAT radar data from Tromsø and Svalbard stations to see the response in plasma parameters at different altitudes, riometer data for cosmic noise absorption, and IMAGE magnetometers to see the intensities of auroral electrojets. TomoScand ionospheric tomography provides us with electron densities over a wide region in Scandinavia and AMPERE data the global field-aligned currents. We identified 13 local substorms in the Scandinavian sector from the IL (IMAGE lower) index. Altogether, there were 11 global substorms, for which the AE index reaches 1000 nT. We discuss the development of currents, as well as E and D region precipitation during the course of this long-duration storm and compare local versus global behavior.</p>

Research into connection between spectral resonance structures and harmonics of ionospheric Alfvén resonator

We present the results of modeling of the ionospheric Alfvén resonator (IAR) and compare the results of electromagnetic waves passing through IAR to Earth's surface with the dynamic spectrum of simultaneous observations of spectral resonance structures (SRS). IAR is simulated using ionospheric parameters obtained from measurements made with the CP-1 program of the Scandinavian EISCAT radar. The IAR model is employed to calculate coefficients of reflection RC(f) and transmission TC(f) of electromagnetic waves in the frequency range 0–5 Hz. The observed dynamic SRS spectrograms consist of spectral lines, in which frequencies, time variations of frequencies, and distances between adjacent resonant lines are confidently determined. The calculated frequencies of maxima of the signal transmission coefficient TC to Earth's surface correspond to the observed frequencies of the dynamic spectrum of SRS.

Research into connection between spectral resonance structures and harmonics of ionospheric Alfvén resonator

We present the results of modeling of the ionospheric Alfvén resonator (IAR) and compare the results of electromagnetic waves passing through IAR to Earth's surface with the dynamic spectrum of simultaneous observations of spectral resonance structures (SRS). IAR is simulated using ionospheric parameters obtained from measurements made with the CP-1 program of the Scandinavian EISCAT radar. The IAR model is employed to calculate coefficients of reflection RC(f) and transmission TC(f) of electromagnetic waves in the frequency range 0–5 Hz. The observed dynamic SRS spectrograms consist of spectral lines, in which frequencies, time variations of frequencies, and distanc-es between adjacent resonant lines are confidently de-termined. The calculated frequencies of maxima of the signal transmission coefficient TC to Earth's surface correspond to the observed frequencies of the dynamic spectrum of SRS.

Spatial and temporal variability in MLT turbulence inferred from in situ and ground-based observations during the WADIS-1 sounding rocket campaign

In summer 2013 the WADIS-1 sounding rocket campaign was conducted at the Andøya Space Center (ACS) in northern Norway (69° N, 16° E). Among other things, it addressed the question of the variability in mesosphere/lower thermosphere (MLT) turbulence, both in time and space. A unique feature of the WADIS project was multi-point turbulence sounding applying different measurement techniques including rocket-borne ionization gauges, VHF MAARSY radar, and VHF EISCAT radar near Tromsø. This allowed for horizontal variability to be observed in the turbulence field in the MLT at scales from a few to 100 km. We found that the turbulence dissipation rate, ε varied in space in a wavelike manner both horizontally and in the vertical direction. This wavelike modulation reveals the same vertical wavelengths as those seen in gravity waves. We also found that the vertical mean value of radar observations of ε agrees reasonably with rocket-borne measurements. In this way defined 〈εradar〉 value reveals clear tidal modulation and results in variation by up to 2 orders of magnitude with periods of 24 h. The 〈εradar〉 value also shows 12 h and shorter (1 to a few hours) modulations resulting in one decade of variation in 〈εradar〉 magnitude. The 24 h modulation appeared to be in phase with tidal change of horizontal wind observed by SAURA-MF radar. Such wavelike and, in particular, tidal modulation of the turbulence dissipation field in the MLT region inferred from our analysis is a new finding of this work.

Auroral ion acoustic wave enhancement observed with a radar interferometer system

Measurements of naturally enhanced ion acoustic line (NEIAL) echoes obtained with a five-antenna interferometric imaging radar system are presented. The observations were conducted with the European Incoherent SCATter (EISCAT) radar on Svalbard and the EISCAT Aperture Synthesis Imaging receivers (EASI) installed at the radar site. Four baselines of the interferometer are used in the analysis. Based on the coherence estimates derived from the measurements, we show that the enhanced backscattering region is of limited extent in the plane perpendicular to the geomagnetic field. Previously it has been argued that the enhanced backscatter region is limited in size; however, here the first unambiguous observations are presented. The size of the enhanced backscatter region is determined to be less than 900 × 500 m, and at times less than 160 m in the direction of the longest antenna separation, assuming the scattering region to have a Gaussian scattering cross section in the plane perpendicular to the geomagnetic field. Using aperture synthesis imaging methods volumetric images of the NEIAL echo are obtained showing the enhanced backscattering region to be aligned with the geomagnetic field. Although optical auroral emissions are observed outside the radar look direction, our observations are consistent with the NEIAL echo occurring on field lines with particle precipitation.