K. Kauristie

Ionospheric current signatures of transient plasma sheet flows

The plasma flow in the central plasma sheet of the magnetospheric tail often includes short impulsive bursts. Here we investigate the ionospheric signatures of such bursts. The Wind satellite recorded several transient fast flows in the plasma sheet (at geocentric distances of ∼ 12 RE) on December 21–22, 1995. The data are compared with magnetic field observations made in the Scandinavian sector, at the ionospheric conjugate point of the satellite. Superposed epoch analysis of the satellite data suggests that most of the events are Earthward flow bursts accompanied by magnetic dipolarizations, increases in the convection electric field, and drops in the plasma pressure and density. Occasionally, also isolated tailward flow bursts within closed flux tubes can be observed. We demonstrate that in both cases the transient plasma sheet flows are systematically associated with distinct ground magnetic field variations which (after 90° rotation) have specific vortex-like spatial distributions. The vortex patterns have similar duration to that of the flows at Wind and their longitudinal extent (≤1 hour in local time) is consistent with the azimuthal scale sizes (∼3 RE) of the transient flows reported earlier. In many cases the sense of flow rotation observed at Wind and at Winds ionospheric footpoint agree with our expectation. Despite the care taken in accounting for the instantaneous and local currents that affect the mapping, uncertainties in the footpoint location may still be responsible of the absence of a higher degree for compliance with theory.

Understanding space weather to shield society : A global road map for 2015-2025 commissioned by COSPAR and ILWS

There is a growing appreciation that the environmental conditions that we call space weather impact the technological infrastructure that powers the coupled economies around the world. With that co ...

Size of the auroral oval: UV ovals and precipitation boundaries compared

The oval boundaries in 44 Viking UV images are compared with three critical boundaries as defined from simultaneous DMSP particle precipitation data. The particle boundaries are the equatorward boundary of the particle oval (often associated with the earthward edge of the main plasma sheet), the boundary between smooth and structured precipitation, and the poleward boundary of the particle oval (close to the open-closed field line separatrix). The UV oval is characterized by the latitude of maximum UV intensity, equatorward boundary, and poleward boundary which are the latitudes corresponding to the half values of the maximum intensity. Differences between the UV and particle boundaries are quantified in various magnetic local time sectors and at different activity levels. The study shows that the poleward boundary of the particle oval is often at ≥2° higher latitudes than the most intense UV luminosity. Large differences are typical especially in the midnight and morning sectors. The present results suggest that caution is needed in interpreting the dramatic poleward expansion of the oval in the UV images, or more generally in using UV images to compute changes in the amount of open flux under different states of substorm activity.

Performance study of the new EMCCD-based all-sky cameras for auroral imaging

The Magnetometers Ionospheric Radars All-sky Cameras Large Experiment (MIRACLE) network monitors auroral activity in the Fennoscandian sector. Network stations cover the range of 55° to 57° magnetic latitude north and span two hours in magnetic local time. Seven of the MIRACLE network stations include digital all-sky cameras (ASCs). Up to recent years, the type of ASC used in the network consisted of an optical system, a charged coupled device (CCD) and an image intensifier enabling short exposure times. This system is referred to as an intensified CCD (ICCD) camera. As image intensifiers degrade over time, it has become necessary to replace the MIRACLE network ASCs with newer technology. Since 2007 the Sodankylä (SOD) and Kilpisjärvi (KIL) stations have been equipped with electron multiplying CCD (EMCCD) cameras. Both ICCD and EMCCD cameras in the MIRACLE network operate at three different wavelengths: 427.8 nm, 557.7 nm and 630.0 nm. The signal-to-noise ratio and dynamic range of the EMCCD camera is greater than that of the ICCD camera by about a factor of 2. The goal of this study is to investigate the differences between the two types of camera placed under the same dome at the Sodankylä station. We also compare ASC images with in situ particle data (total electron energy flux) from low-altitude satellite NOAA18. Our analysis includes two days with different conditions (faint emission and substorm activity). This study shows that the EMCCD camera has greater sensitivity for blue and red wavelengths allowing for better measurement of auroral emission. The dynamic imaging range of the EMCCD camera is about 1.7 times that of the ICCD camera. Moreover, the EMCCD camera data are well correlated with the NOAA18 satellite total electron energy flux (cross-correlation coefficient ∼0.8) for the 557.7 nm emission.

Observations of the lower thermospheric neutral temperature and density in the DELTA campaign

The rotational temperature and number density of molecular nitrogen (N2) in the lower thermosphere were measured by the N2 temperature instrument onboard the S-310-35 sounding rocket, which was launched from Andøya at 0:33 UT on 13 December 2004, during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. The rotational temperature measured at altitudes between 95 and 140 km, which is expected to be equal to neutral temperature, is much higher than neutral temperature from the Mass Spectrometer Incoherent Scatter (MSIS) model. Neutral temperatures in the lower thermosphere were observed using the auroral green line at 557.7 nm by two Fabry-Perot Interferometers (FPIs) at Skibotn and the Kiruna Esrange Optical Platform System site. The neutral temperatures derived from the look directions closest to the rocket correspond to the rotational temperature measured at an altitude of 120 km. In addition, a combination of the all-sky camera images at 557.7 nm observed at two stations, Kilpisjärvi and Muonio, suggests that the effective altitude of the auroral arcs at the time of the launch is about 120 km. The FPI temperature observations are consistent with the in situ rocket observations rather than the MSIS model.

Temperature enhancements and vertical winds in the lower thermosphere associated with auroral heating during the DELTA campaign

[1] A coordinated observation of the atmospheric response to auroral energy input in the polar lower thermosphere was conducted during the Dynamics and Energetics of the Lower Thermosphere in Aurora (DELTA) campaign. N2 rotational temperature was measured with a rocket-borne instrument launched from the Andoya Rocket Range, neutral winds were measured from auroral emissions at 557.7 nm with a Fabry-Perot Interferometer (FPI) at Skibotn and the KEOPS, and ionospheric parameters were measured with the European Incoherent Scatter (EISCAT) UHF radar at Tromso. Altitude profiles of the passive energy deposition rate and the particle heating rate were estimated using data taken with the EISCAT radar. The local temperature enhancement derived from the difference between the observed N2 rotational temperature and the MSISE-90 model neutral temperature were 70–140 K at 110–140 km altitude. The temperature increase rate derived from the estimated heating rates, however, cannot account for the temperature enhancement below 120 km, even considering the contribution of the neutral density to the estimated heating rate. The observed upward winds up to 40 m s �1 seem to respond nearly instantaneously to changes in the heating rates. Although the wind speeds cannot be explained by the estimated heating rate and the thermal expansion hypothesis, the present study suggests that the generation mechanism of the large vertical winds must be responsible for the fast response of the vertical wind to the heating event.

Statistical study of auroral spirals

We present results of a statistical study of 189 auroral spiral forms. The data set was collected by a systematic examination of the images recorded at five all-sky camera (ASC) stations in northern Fennoscandia and Svalbard during the winters 1996–1997 and 1997–1998. For every spiral we defined the time of occurrence, diameter, direction of motion, arid distance between the adjacent spirals (wavelength) in cases of spiral streets. The magnetic activity level was determined by using the magnetic north component (Bx) recorded at the ASC station and the local auroral electrojet index (IL index) computed by using the data from the IMAGE magnetometer network. The typical values of the diameter, wavelength, and ratio between diameter and wavelength were 25–75 km, 125-175 km, and 2–6, respectively. According to our ASC data set, spirals are more common in the magnetic postmidnight sector than previous satellite observations suggest. Spirals drift predominantly in the same direction as the large-scale ionospheric convection. Our observations of the average drift speed of spirals of 4 km/s are comparable to the corresponding values of other auroral small-scale structures. Statistical analysis revealed that spirals occur most frequently under magnetically quiet (IL > −200 nT) conditions, and thus they cannot always be considered as precursors of substorm activity.

A search engine for auroral forms

Abstract The Finnish Meteorological Institute operates five digital all-sky cameras, which routinely monitor the auroral emissions in Northern Finland, Sweden, and Svalbard; each camera records an image of the full sky at 20-s interval at a resolution of 512 × 512 pixels. As a result, over 2.5 million images are recorded each winter. We demonstrate a syntactic pattern recognition algorithm for searching auroral arcs in the all-sky images. The algorithm operates in two phases: a quicksearch and a detection phase. The quicksearch is based on simple correlation to a model arc and is utilised to scan through the database to locate potential auroras. In the detection, an auroral image skeleton is determined, and, based on the shape skeleton, auroral forms known as “arcs” are located. A quality estimate is calculated for each arc. Trial runs indicate that the algorithms developed can be implemented in real-time at the auroral all-sky stations.

Auroral fading in ionosphere‐magnetosphere coupling model: Implications for possible mechanisms

The authors present results from a numerical model which includes coupling between the magnetosphere and ionosphere, and in which they observe indications similar to observations which occasionally show a fading of auroral arcs and electrojets during the later phases of substorm onset. The model shows that the magnetospheric plasma shifts toward the earth at this point, and at least the poleward part of the precipitation region is cooled. These effects combine to suppress the precipitation of hot dense plasma, for at least a short time.

Analysis of the substorm trigger phase using multiple ground-based instrumentation

The authors discuss in detail the observation of an event of auroral activity fading during the trigger, or growth phase of a magnetic storm. This event was observed by all-sky cameras, EISCAT radar and magnetometers, riometers, and pulsation magnetometers, from ground based stations in Finland and Scandanavia. Based on their detailed analysis, they present a possible cause for the observed fading.