[1] The spatial distribution of electric fields, conductances, and currents of steadily drifting medium-scale (15-50 km) arcs in the evening sector (20-23 magnetic local time (MLT)) is obtained from European Incoherent Scatter Radar (EISCAT) and optical groundbased measurements. The current systems of stable arcs residing in the northward convection electric field region show a consistent pattern: currents flow downward on the equatorward side of the arcs, then poleward, and upward from the arcs. In one event where the arcs are located in a region of convection reversals, the current pattern is more complicated. Most of the arcs are associated with an enhanced northward-directed electric field region on the equatorward side of the arc, colocated with downward field-aligned currents (FACs) and suppressed E and F region electron densities. The width of the region of the enhanced electric field is one to four times the width of the arc. In some cases, the electron density reduction is so pronounced that the region can be described as an auroral ionospheric density cavity. The electrostatic magnetosphere-ionosphere coupling model of arcs predicts that the width L of an arc is related to the ionospheric Pedersen conductance. This study shows that stable medium-scale arcs in the evening sector obey this equation. A value of K = 2 Â 10 À8 S m À2is obtained for 15-35 km wide arcs. It is argued that the large value of the field-aligned conductance cannot be interpreted in terms of the adiabatic theory. Possibly the high value of K results from nonadiabatic processes acting on the current-carrying electrons.

Key Points:• Stream interaction regions and high-speed streams (SIR/HSSs) are 20-40% less geoeffective during solar cycle (SC) 24 than during SC23 • The most geoeffective SIR/HSSs in solar cycles 23 and 24 take place in the early declining phases • During the late declining phase of SC23, both SIR/HSS event number and maximum velocity are highest, yet their geoeffectiveness is low Corresponding author: Maxime Grandin, maxime.grandin@helsinki.fi -1-arXiv:2006.06302v1 [physics.space-ph] AbstractWe study the properties and geoeffectiveness of solar wind high-speed streams (HSSs) emanating from coronal holes and associated with stream interaction regions (SIRs). This paper presents a statistical study of 588 SIR/HSS events with solar wind speed at 1 AU exceeding 500 km/s during 1995-2017, encompassing the decline of solar cycle 22 to the decline of cycle 24. Events are detected using measurements of the solar wind speed and the interplanetary magnetic field (IMF). Events misidentified as or interacting with interplanetary coronal mass ejections (ICMEs) are removed by comparison with an existing ICME list. Using this SIR/HSS event catalog (list given in the supplementary material), a superposed epoch analysis of key solar wind parameters is carried out. It is found that the number of SIR/HSSs peaks during the late declining phase of solar cycle (SC) 23, as does their velocity, but that their geoeffectiveness in terms of the AE and SY M-H indices is low. This can be explained by the anomalously low values of magnetic field during the extended solar minimum. Within SC23 and SC24, the highest geoeffectiveness of SIR/HSSs takes place during the early declining phases. Geoeffectiveness of SIR/HSSs continues to be up to 40% lower during SC24 than SC23, which can be explained by the solar wind properties.

Two Rayleigh lidars were employed at a southern-hemisphere mid-latitude site in New Zealand (45 • S) and a northern-hemisphere high-latitude site in Finland (67 • N) in order to observe gravity waves between 30 and 85 km altitude under wintertime conditions. Two-dimensional wavelet analysis is used to analyze temperature perturbations caused by gravity waves and to determine their vertical wavelengths and phase progression. In both datasets, upward phase progression waves occur frequently between 30 and 85 km altitude. Six cases of large-amplitude wave packets are selected which exhibit upward phase progression in the stratosphere and/or mesosphere. We argue that these wave packets propagate downward and we discuss possible wave generation mechanisms. Spectral analysis reveals that superpositions of two or three wave packets are common. Furthermore, their characteristics often match those of upward-propagating waves which are observed at the same time or earlier. In the dataset means, the contribution of upward Preprint submitted to JASTP March 8, 2017 phase progression waves to the potential energy density E p is largest in the lower stratosphere above Finland. There, E p of upward and downward phase progression waves is comparable. At 85 km one third of the potential energy carried by propagating waves is attributed to upward phase progression waves. In some cases E p of upward phase progression waves far exceeds E p of downward phase progression waves. The downward-propagating waves might be generated in situ in the middle atmosphere or arise from reflection of upward-propagating waves.

Abstract. Long-term and continuous observations of mesospheric–lower thermospheric winds are rare, but they are important to investigate climatological changes at these altitudes on timescales of several years, covering a solar cycle and longer. Such long time series are a natural heritage of the mesosphere–lower thermosphere climate, and they are valuable to compare climate models or long-term runs of general circulation models (GCMs). Here we present a climatological comparison of wind observations from six meteor radars at two conjugate latitudes to validate the corresponding mean winds and atmospheric diurnal and semidiurnal tides from three GCMs, namely the Ground-to-Topside Model of Atmosphere and Ionosphere for Aeronomy (GAIA), the Whole Atmosphere Community Climate Model Extension (Specified Dynamics) (WACCM-X(SD)), and the Upper Atmosphere ICOsahedral Non-hydrostatic (UA-ICON) model. Our results indicate that there are interhemispheric differences in the seasonal characteristics of the diurnal and semidiurnal tide. There are also some differences in the mean wind climatologies of the models and the observations. Our results indicate that GAIA shows reasonable agreement with the meteor radar observations during the winter season, whereas WACCM-X(SD) shows better agreement with the radars for the hemispheric zonal summer wind reversal, which is more consistent with the meteor radar observations. The free-running UA-ICON tends to show similar winds and tides compared to WACCM-X(SD).

Abstract. In this paper we describe a new method to be used for the polar cap boundary (PCB) determination in the nightside ionosphere by using the EISCAT Svalbard radar (ESR) field-aligned measurements by the 42-m antenna and southward directed low-elevation measurements by the ESR 32m antenna or northward directed low-elevation measurements by the EISCAT VHF radar at Tromsø. The method is based on increased electron temperature (T e ) caused by precipitating particles on closed field lines. Since the Svalbard field-aligned measurement provides the reference polar cap T e height profile, the method can be utilised only when the PCB is located between Svalbard and the mainland. Comparison with the Polar UVI images shows that the radar-based method is generally in agreement with the PAE (poleward auroral emission) boundary from Polar UVI.The new technique to map the polar cap boundary was applied to a substorm event on 6 November 2002. Simultaneous measurements by the MIRACLE magnetometers enabled us to put the PCB location in the framework of ionospheric electrojets. During the substorm growth phase, the polar cap expands and the region of the westward electrojet shifts gradually more apart from the PCB. The substorm onset takes place deep within the region of closed magnetic field region, separated by about 6-7 • in latitude from the PCB in the ionosphere. We interpret the observations in the framework of the near-Earth neutral line (NENL) model of substorms. After the substorm onset, the reconnection at the NENL reaches within 3 min the open-closed field line boundary and then the PCB moves poleward together with the poleward boundary of the substorm current wedge. The poleward expansion occurs in the form of individual bursts, which are separated by 2-10 min, indicating that the reconnection in the magnetotail neutral line is impulsive. The poleward expansions of the PCB are followed by latitude dispersed intensifications inCorrespondence to: A. T. Aikio (anita.aikio@oulu.fi) the westward electrojet with high latitudes affected first and lower latitudes later. We suggest that reconnection bursts energize plasma and produce enhanced flows toward the Earth. While drifting earthward, part of the plasma population precipitates to the ionosphere producing latitude-dispersed enhancements in the WEJ.