J. Manninen

Proton aurora related to intervals of pulsations of diminishing periods

[1]xa0Geomagnetic pulsations in the Pc1 frequency range are believed to be an indicator of electromagnetic ion cyclotron waves arriving from the equatorial magnetosphere, where the waves are generated because of a cyclotron instability of the anisotropic distribution of ring current ions. Proton precipitation produced by the cyclotron instability can be responsible for proton aurora. Indeed, the relationship between some types of proton aurora (proton spots and proton flashes) and pulsations in the Pc1 range (quasi-monochromatic Pc1 and Pc1 bursts) has already been found. The aim of this study is to find the proton aurora pattern, which relates to the kind of geomagnetic pulsations in the Pc1 range called intervals of pulsation of diminishing periods (IPDP). This is done on the basis of 2 year observations of geomagnetic pulsations at the Finnish meridional network of search coil magnetometers and proton aurora from the IMAGE spacecraft. We found that during IPDP the proton arcs appear equatorward of the proton oval at the meridian of the ground magnetometers. The maximum intensity of the pulsations is observed at the ground station, which is closest to the proton arc. The proton arcs tend to appear at lower latitudes at later magnetic local times (MLTs). This agrees with the facts that the IPDP occurrence exhibits a similar behavior and that the IPDP end frequency tends to increase with increasing MLT. In addition, data from geosynchronous spacecraft showed that IPDP occur when clouds of energy-dispersed energetic protons pass through the meridian of the ground magnetometers. The spatial-temporal correlation of IPDP with proton aurora arcs confirms the expectation that the proton arcs, like the proton spots and flashes, are the ionospheric image of the region where the ion cyclotron instability develops in the equatorial magnetosphere. In the case of IPDP the instability develops when drifting proton clouds resulting from particle injections in the night sector contact the plasmaspheric plume onto which the proton arcs map.

Pc5 geomagnetic pulsations, pulsating particle precipitation, and VLF chorus: Case study on 24 November 2006

[1]xa0The event (24 November 2006, ∼0400–0500 UT) of the simultaneous observations of Pc5 ULF geomagnetic pulsations, electron precipitation (CNA riometer absorption), and whistler-mode chorus, as well as solar wind (SW) and IMF parameters have been analyzed based on the data from IMAGE magnetometers, Finnish riometer array, and temporal VLF station. The visible correlation between the simultaneous occurrence of several minutes scale patches of chorus and pulsating CNA enhancements was found. The dynamic spectra of the riometer data showed a maximum at ∼3.5 mHz in the first half-hour interval and at ∼2.0 mHz in the second one, while the ULF pulsation spectra exhibit these two maxima in both intervals simultaneously. In the first time interval, the Pc5 pulsations at ∼3.5 mHz demonstrated the typical FLR feature. The SW dynamic pressure fluctuations showed a broad (1.5–3.5 mHz) spectral maximum in the first interval; however, in the second one, the simultaneous oscillations at ∼2.0 mHz were observed in SW pressure and in IMF Bz. The similar ∼2.0 mHz peak has been found in the spectra of Pc5 pulsations from auroral zone to the equator, in riometer absorption, and in VLF chorus power. We suggest that the modulation of particle precipitation and whistler-mode chorus patches was caused by the 2.0 mHz compressional component of Pc5 poloidal geomagnetic pulsations driven in the magnetosphere by SW dynamic pressure and IMF Bz disturbances.

Ground-based instruments of the PWING project to investigate dynamics of the inner magnetosphere at subauroral latitudes as a part of the ERG-ground coordinated observation network

The plasmas (electrons and ions) in the inner magnetosphere have wide energy ranges from electron volts to mega-electron volts (MeV). These plasmas rotate around the Earth longitudinally due to the gradient and curvature of the geomagnetic field and by the co-rotation motion with timescales from several tens of hours to less than 10xa0min. They interact with plasma waves at frequencies of mHz to kHz mainly in the equatorial plane of the magnetosphere, obtain energies up to MeV, and are lost into the ionosphere. In order to provide the global distribution and quantitative evaluation of the dynamical variation of these plasmas and waves in the inner magnetosphere, the PWING project (study of dynamical variation of particles and waves in the inner magnetosphere using ground-based network observations, http://www.isee.nagoya-u.ac.jp/dimr/PWING/) has been carried out since April 2016. This paper describes the stations and instrumentation of the PWING project. We operate all-sky airglow/aurora imagers, 64-Hz sampling induction magnetometers, 40-kHz sampling loop antennas, and 64-Hz sampling riometers at eight stations at subauroral latitudes (~xa060° geomagnetic latitude) in the northern hemisphere, as well as 100-Hz sampling EMCCD cameras at three stations. These stations are distributed longitudinally in Canada, Iceland, Finland, Russia, and Alaska to obtain the longitudinal distribution of plasmas and waves in the inner magnetosphere. This PWING longitudinal network has been developed as a part of the ERG (Arase)-ground coordinated observation network. The ERG (Arase) satellite was launched on December 20, 2016, and has been in full operation since March 2017. We will combine these ground network observations with the ERG (Arase) satellite and global modeling studies. These comprehensive datasets will contribute to the investigation of dynamical variation of particles and waves in the inner magnetosphere, which is one of the most important research topics in recent space physics, and the outcome of our research will improve safe and secure use of geospace around the Earth.

Ground‐based observations during the period between two strong November 2004 storms attributed to steady magnetospheric convection

[1]xa0Strong geomagnetic storms are of great scientific interest because they drive the magnetosphere to an extreme state and result in nontypical magnetosphere-ionosphere coupling. The present study examines the ground-based signatures of the magnetosphere-ionosphere disturbances during the recovery phase of the very intense storm on 7–8 November 2004. The recovery phase took place under steady and slightly negative (∼−5 nT) values of the IMF Bz. We compare this event with previously documented storm recovery phases occurring under positive IMF Bz and accompanied by morning Pc5 geomagnetic pulsations. During the period studied in this article the strongest pulsation activity was recorded in the evening and midnight sectors of the Earth. We analyze observations from the Scandinavian multipoint ground-based instrumentation: (1) geomagnetic variations and pulsations, (2) auroras in visual wavelengths, and (3) energetic particle precipitation (riometer data). We show that several enhancements in electrojet, auroral, and energetic precipitation activity were recorded at auroral latitudes. The activations lasted 0.5–2 hours, and the associated negative magnetic field deviations were often more than 1000 nT. Only one of these activations shows typical substorm behavior (poleward expansion and geostationary particle injection). We demonstrate remarkably good correlation between the magnetic variations and cosmic noise absorption variations (in pulsations and slower variations) as well as between the optical auroras and cosmic noise absorption (Imaging Riometer for Ionospheric Studies riometer system) both in time and in space. Thus the auroral precipitation revealed a very coherent behavior over a wide energy range (∼1–40 keV) during the analyzed period. The images acquired by the network of MIRACLE all-sky cameras show that the auroral distribution exhibited double oval configuration during our event. Double oval is often observed during so-called Steady Magnetospheric Convection (SMC) episodes. Furthermore, multiple auroral streamers were recorded, implying probable occurrence of bursty bulk flows (BBFs) in the magnetotail. The absence of recurrent geostationary injections, the wide oval configuration, and BBF signatures lead us to suggest that the intermediate period between the two November 2004 superstorms can be attributed to a SMC period. Mapping the motion of ionospheric signatures to the magnetotail with the Tsyganenko 96 models suggests BBF earthward speeds of ∼600–800 km/s. We suppose that the main drivers for the above described recurrent and intensive ionospheric phenomena are energy input from the solar wind due to slightly negative values of IMF Bz as well as huge energy storage in the magnetotail due to the previous storm main phase.

High-latitude geomagnetic pulsation response to the passage of the front edge of the interplanetary magnetic cloud of January 10, 1997

Abstract In this paper we studied the high-latitude long period geomagnetic pulsations excited during the first hours (0430– 0830 UT ) of the magnetic cloud impact on January 10, 1997. In this study we analysed the ground-based data of the IMAGE magnetometers and the Scandinavian network of ∼30 MHz riometers. Two time intervals were selected. The first interval (0430– 0600 UT ) corresponds to the passage of the compression region on the front edge of the magnetic cloud. During this interval bursts of geomagnetic and riometer data pulsations at frequency range of 1– 3 mHz as well as a magnetic substorm were observed at polar latitudes ( Λ >72°) with the strongest amplitudes near a footprint of the open/closed field lines boundary. The waves propagated rapidly eastward at ∼6– 7 km / s and had an azimuthal wave number m ∼12. This seems to be inconsistent with the main signature of a field line resonance. The green (557.7 nm ) optical emissions were observed at that time at Spitzbergen. The emissions moved rapidly eastward at about the same speed. We speculate that high-latitude geomagnetic pulsations under consideration were excited at the ionosphere altitudes, near the polar cusp footprint. The waves could be associated with quasi-periodic variations in the ionospheric conductivity produced by the particle precipitation oscillations. As the alternative hypothesis we could assume the direct penetration of solar wind compression or Alfven waves to the polar cap. These waves can modulate the particle precipitation produced the quasi-periodic variation of ionospheric conductivity. The second time interval (0600– 0830 UT ) corresponds to an abrupt increasing of the solar wind dynamic pressure, observed about 2 h after the passage of the leading edge of the magnetic cloud. Contrary to the first interval, the strongest amplitudes of geomagnetic and cosmic noise absorption pulsations were observed in closed magnetosphere, at Λ ∼66°, and showed the properties of the typical field line resonance.

Unusually high frequency natural VLF radio emissions observed during daytime in Northern Finland

Geomagnetic field variations and electromagnetic waves of different frequencies are ever present in the Earths environment in which the Earths fauna and flora have evolved and live. These waves are a very useful tool for studying and exploring the physics of plasma processes occurring in the magnetosphere and ionosphere. Here we present ground-based observations of natural electromagnetic emissions of magnetospheric origin at very low frequency (VLF, 3–30 kHz), which are neither heard nor seen in their spectrograms because they are hidden by strong impulsive signals (sferics) originating in lightning discharges. After filtering out the sferics, peculiar emissions are revealed in these digital recordings, made in Northern Finland, at unusually high frequencies in the VLF band. These recently revealed emissions, which are observed for several hours almost every day in winter, contain short (~1–3 min) burst-like structures at frequencies above 4–6 kHz, even up to 15 kHz; fine structure on the 1 s time scale is also prevalent. It seems that these whistler mode emissions are generated deep inside the magnetosphere, but the detailed nature, generation region and propagation behaviour of these newly discovered high latitude VLF emissions remain unknown; however, further research on them may shed new light on wave-particle interactions occurring in the Earths radiation belts.

Ground pulsation magnetometer observations conjugated with relativistic electron precipitation

In this report, we investigate the role of EMIC waves in production of relativistic electron precipitation (REP). Over a thousand REP events were detected from four NOAA POES satellites in July-December 2005. Of these, a total of 112 events were conjugated with a ground-based network of six Finnish induction coil magnetometers and one in Lovozero observatory at Kola Peninsula, Russia. The observation of geomagnetic pulsations during the conjugated events showed that about one third of them were accompanied by pulsations in the Pc1 range, which are the signature of EMIC waves. In fact, the sources of some of these EMIC waves were well outside the location of the REP event. This means that in such cases the REP events were not originated from scattering by EMIC waves. Finally, it is concluded that for this limited set of conjugated events only a quarter might be related to scattering by EMIC waves. The majority of the events are not correlated with EMIC wave signatures in ground-based observations; they were associated with either no pulsations or noise-like pulsations PiB and PiC.

PLASMON: Data assimilation of the Earth's plasmasphere

The principal source and loss mechanisms in the Earths radiation belts are currently not completely understood. Loss rates are important since they determine the duration of exposure of satellites to enhanced radiation conditions during a geomagnetic storm. The dominant loss process is relativistic electron precipitation via resonant interactions with a variety of wave modes. These interactions are governed by the characteristics of the plasmasphere. Current models provide an inadequate representation of the spatial and temporal evolution of the plasmasphere. In situ measurements of the plasmasphere provide only local characteristics and are thus unable to yield a complete global picture. Ground based measurements, based on the analysis of Very Low Frequency (VLF) whistlers and Field Line Resonances (FLRs), are able to describe large sections of the plasmasphere, extending over significant radial distances and many hours of local time. These measurements provide electron number and plasma mass densities. PLASMON is a funded FP7 project between 11 international partners. PLASMON intends to assimilate near real time measurements of plasmaspheric densities into a dynamic plasmasphere model. The VLF whistler analyses will be conducted by automatic retrieval of equatorial electron densities using data from AWDAnet. Equatorial mass densities will be constructed from FLR measurements along meridional magnetometer chains. The resulting model will facilitate the prediction of precipitation rates. The predicted rates will be compared to observations from the AARDDVARK network.

Simultaneous observations of magnetospheric ELF/VLF emissions in Canada, Finland, and Antarctica†

To investigate longitudinal extent of electromagnetic wave activity, we report the first simultaneous ground-based observations of magnetospheric ELF/VLF emissions at the following three longitudinally-separated stations at auroral and subauroral latitudes: Athabasca, Canada (ATH; magnetic latitude: 61.3∘N); Kannuslehto, Finland (KAN; 64.4∘N); and Syowa Station, Antarctica (SYO; 70.5∘S). The magnetic local time (MLT) separations of SYO-KAN, ATH-SYO, and ATH-KAN, are 3, 8, and 11 h, respectively. Simultaneous observation data at these stations are available for a total of 48 days in 2012-2014. The simultaneous occurrence rates of ELF/VLF emissions are 9.8%, 2.5%, and 3.6% for SYO-KAN, ATH-SYO, and ATH-KAN, respectively. We found that the simultaneous wave occurrence rate between two stations is higher in the morning-dayside sector, indicating that the longitudinal extent of the emissions exhibits MLT dependence. When emissions are simultaneously observed at two stations, the average AE and |Dst| indices tend to be higher. Similarly, if the two stations are more separated in MLT, the average |Dst| index increases. These results suggest that the longitudinal extent of ELF/VLF emissions increases with increasing geomagnetic activity.

A new type of daytime high-frequency VLF emissions at auroral latitudes (“bird emissions”)

This paper is concerned with a new, previously unknown type of high-frequency (above 4 kHz) VLF emissions that were detected during winter VLF campaigns in Kannuslehto (L ~ 5.5), Finland. These previously unknown emissions have been discovered as a result of the application of special digital filtering: it clears the VLF records from pulse signals of intensive atmospherics, which prevent other kinds of VLF emissions in the same frequency range from being seen on spectrograms. As it appears, aside from wellknown bursts of auroral hisses and discrete quasiperiodic emissions, a previously unknown type of daytime right-hand polarized VLF waves is also present at frequencies above 4 kHz. These emissions can persist for several hours as series of separate short discrete wideband (from 4 to 10 kHz and higher) signals, each with a duration between one and several minutes. It has been found that such signals can be observed almost daily in winter. These emissions sound like bird’s chirping to a human ear; for that reason, they were called “bird emissions.” The dynamic spectra of individual signals often resemble flying birds. The signals are observed during daytime, more often in magnetically quiet conditions preceded by geomagnetic disturbances. As a rule, the occurrence of these bird emissions is accompanied by a slight increase in electron density in the lower ionosphere, which is evidence of the precipitation of energetic (>30 keV) electrons. This raises a number of questions as to where and how the VLF bird emissions are generated and how such emissions, at frequencies greatly exceeding half the electron equatorial gyrofrequency at L ~ 5.5, can reach the Earth’s surface.